https://www.galacticlibrary.net/mediawiki-1.41.1/api.php?action=feedcontributions&feedformat=atom&user=ROCKETGUYGalactic Library - User contributions [en]2026-05-02T17:58:07ZUser contributionsMediaWiki 1.41.1https://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=828Propulsion2021-12-28T16:49:47Z<p>ROCKETGUY: fixed a typo</p>
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<div>{{Quote|'''''"We look for things, things to make us go"'''''. <br/><br/>Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare}}<br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math alt=><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:Momentium_rescale.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological analysis" or a Zwicky box <ref>This technique as well as the entire classification approach used on this page derive from F. Zwicky, "Fundamentals of Propulsive Power", International Congress of Applied Mechanics, Paris, September 22-29, 1946 and many later works by the same author. The box above is a three dimensional box (energy internal/external, momentum source external/internal, beamed/harvested which has been 'flattened' for ease of use since 'internal' has no beamed/harvested classification</ref>)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the vehicle carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Conservation_Laws:_Limits_to_Cheating&diff=823Conservation Laws: Limits to Cheating2021-12-07T13:29:05Z<p>ROCKETGUY: </p>
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<div><blockquote><br />
This is a STUB ONLY at this time to provide the link structure on another page. Please don't edit until I can get a first draft up here -- Rocketguy<br />
</blockquote><br />
<br />
'''Conservation Laws'''<br />
<br />
The conservation laws (conservation of momentum, conservation of energy, and the second law of thermodynamics (Entropy doesn't decrease), are fundamental to our understanding of the physical universe. If working in the real world, one may dislike them, but obedience is strictly enforced by the Universe.<br />
<br />
In a work of fiction, of course, one can do anything; however, discarding these fundamentals is not to be done lightly or carelessly and doing so is one of the surest ways to make your work of science fiction not be 'hard' or 'tough' and to quickly cross the line in to fantasy. Fantasy of course is a genre of its own with its own conventions and literature and if that's what one intends to write, so be it. But if you want a work which is 'realistic' in the physical sense, there is usually some way, as an author, to get what you want without discarding the conservation laws.<br />
<br />
The conservation of energy -- the realization that energy may change from one form to another, but not be created or destroyed -- is in some ways the birth of all physics. The concept has proven so useful that with every new discovery, rather than treating it as a violation of the conservation law, we have instead found ways to add new forms of energy to the account. Objects in motion in the real world slow down -- but the energy shows up as frictional heat. Burning fuel appears to create energy -- but we now know it was there in the molecular bonds in the material in the first place. Even nuclear energy, which was quite mysterious when first discovered, we now consider an example of the same thing, in that we now (thanks to Einstein) appreciate that mass is just another kind of energy, and if one converts a tiny amount of mass in to energy, a large amount of energy is the result. If tomorrow, some new phenomenon were found that appeared to violate this law, we would almost certainly find a way to book-keep it as a new form of energy, rather than junking such a useful concept as conservation of energy.<br />
<br />
Momentum is much the same. If you push on a heavy object, you feel it pushing back on your hand. If you eject mass to the left, you are pushed to the right. If you push on the air to move forward, you create a wind going aft. People have theorized (but not yet achieved an accepted experimental validation) various drives which might 'push on nothing' -- so called 'reactionless' drives. For story purposes (and who knows, perhaps in reality), these can be re-cast as a drive which 'pushes on something big' -- the large-scale structure of space-time, for example -- without throwing away the conservation of momentum. And in any case, space is not empty; perfectly conventional real-world propulsion systems have been proposed which push on the thin plasma between the planets or between the stars, or which interact with the magnetic fields found between the stars. One might envision improving the performance of such systems to whatever degree desired for story purposes without throwing all of physics out the window. However, even if such a drive were to exist, if momentum and energy are still conserved, its usefulness would be limited -- pushing even on a large reaction mass still follows the same mechanics as propellers -- the power required scales with the product of thrust and velocity, which means any such drive, even if it existed, would be useful at '''low''' velocities rather than high.<br />
<br />
Thanks to the Nobel-winning work of Emmy Noether<ref>https://en.wikipedia.org/wiki/Noether%27s_theorem</ref> we now understand that every symmetry of the universe carries with it a corresponding conservation law. It can be shown that symmetry to time coordinate carries with it the conservation of energy, and symmetry with the space coordinate carries with it the conservation of momentum. In other words, violating these conservation laws means that if you do something tomorrow it doesn't necessarily produce the same result as today, and if you do something a meter to the left, it might turn out differently than it does a meter to the right. If the universe is not predictable, you don't need conservation laws -- but then it's hard to stay in the realm of 'science' fiction.<br />
<br />
The non-decrease of entropy is a bit more subtle, but everyday examples abound. If you put a bit of dye in a glass of water, you expect to see it spread through the water -- the reverse process, where the water spits out the drop of dye, doesn't take place. If you drop an ice cube in the glass of water you expect to end up with cool water; you'd be very surprised indeed to start with a glass of water and see that it had divided itself in to a bit of water and a bit of ice unless you move energy around (like putting the glass in to a freezer). This property of a system we call 'entropy'. You can think of it as a measure of how randomly arranged or ordered the system is. There's generally only a few ways for the atoms to be arranged that we think of as 'ordered', and many many ways we can think of them as 'disordered', and those can be treated with more rigor and math. For simple systems where all the atoms are mixed up together, we can describe that as "temperature" -- and one way to summarize the Second Law of Thermodynamics is "heat flows from high temperature to low temperature". This has major consequences for spacecraft propulsion, because we want to move a lot of energy about. In doing so, the energy flows from a high-temperature (low entropy) source to a lower-temperature sink. In some cases, that 'sink' can be the propellant being expelled -- well and good. In many cases, it can't be, and it has to go somewhere else. Space is short on good heat sinks -- vacuum, in spite of endless poetry about 'the cold of space' is an outstanding thermal insulator (as in a vacuum flask, Dewar flask, or Thermos bottle), and nearly the only way to get waste heat out through it is to radiate it. ((See 'Radiators' -- Page not created yet))<br />
<br />
'''Cheating'''<br />
If one wishes to 'appear' to violate these conservation laws, there are some tiny chinks in the structure of physics one can look to; but remember that centuries of experimentation on these laws will still be 'right' and there must be some very good reason why any 'cheating' is restricted to very special circumstances.<br />
<br />
In highly curved spacetime, space and time really aren't quite the same from place to place or time to time. Therefore, defining just what is meant by 'energy' and 'momentum' can get a bit tricky. This is, however, much more likely to be a question of doing the bookkeeping properly than 'violating' the conservation laws.<br />
<br />
It is much easier to invent (and in the real world, to seek) new forms of energy, or to find new things to push on. That can produce much the same effect for story purposes without loosing magic in the world.<br />
<br />
The Second Law is a bit fuzzier. It applies to a closed system without energy flow in or out of the system. Those energy flows are not always obvious. There have been very small microscopic systems in which it appears that room-temperature energy flowing in to the system is producing useful work without a low-temperature reservoir (so far, only at tiny levels). The 'randomness' character also means that when looking at small fluctuations, what this might mean at all is somewhat debatable. Still, handle with care; unless you think through all the implications, ignoring the Second Law can lead you in to pure fantasy very quickly without intending to do so. Ignoring the implications of this law -- in particular, neglecting radiators on your ships where they really should be there -- is one of the current hallmarks of how 'hard' your science fiction is meant to be.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=822Propulsion Performance2021-12-06T18:16:13Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (<math>1.5 \cdot 10^{11}</math> meters), light years (ly) (<math>9.5 \cdot 10^{15}</math> meters), or parsecs (pc) (<math>3.1 \cdot 10^{16}</math> meters). Though not in common use, a useful metric unit for interstellar distances is the Petameter (Pm), <math>1 \cdot 10^{15}</math> neters; a light year is then 9.5 Pm.<br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s <math>1 \cdot 10^{12}</math> meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (<math>1 \cdot 10^4</math>) m/s, then the time is (<math>10^{12}</math> m) / (<math>10^4</math> m/s), or <math>1 \cdot 10^8</math> seconds. (1 year is <math>3.16 \cdot 10^7</math> seconds, or very close to <math>\pi \cdot 10^7</math> seconds), so the trip takes about 3.16 years (<math>1 \cdot 10^8</math>/<math>3.16 \cdot 10^7</math> = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (<math>1 \cdot 10^{-4}</math> m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). The use of Psp as a way to compare dissimiliar propulsion systems emerged in a study on that subject.<ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=821Propulsion Performance2021-12-06T18:10:35Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (<math>1.5 \cdot 10^{11}</math> meters), light years (ly) (<math>9.5 \cdot 10^{15}</math> meters), or parsecs (pc) (<math>3.1 \cdot 10^{16}</math> meters). Though not in common use, a useful metric unit for interstellar distances is the Petameter (Pm), <math>1 \cdot 10^{15}</math> neters; a light year is then 9.5 Pm.<br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s <math>1 \cdot 10^{12}</math> meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (<math>1 \cdot 10^4</math>) m/s, then the time is (<math>10^{12}</math> m) / (<math>10^4</math> m/s), or <math>1 \cdot 10^8</math> seconds. (1 year is <math>3.16 \cdot 10^7</math> seconds, or very close to <math>\pi \cdot 10^7</math> seconds), so the trip takes about 3.16 years (<math>1 \cdot 10^8</math>/<math>3.16 \cdot 10^7</math> = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (<math>1 \cdot 10^{-4}</math> m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=820Propulsion Performance2021-12-06T18:07:01Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (<math>1.5 \cdot 10^{11}</math> meters), light years (ly) (<math>9.5 \cdot 10^{15}</math> meters), or parsecs (pc) (<math>3.1 \cdot 10^{16}</math> meters). Though not in common use, a useful metric unit for interstellar distances is the Petameter (Pm), <math>1 \cdot 10^{15}</math> neters; a light year is then 9.5 Pm.<br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s <math>1 \cdot 10^{12}</math> meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (<math>1 \cdot 10^4</math>) m/s, then the time is (<math>10^{12}</math> m) / (<math>10^4</math> m/s), or <math>1 \cdot 10^8</math> seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=819Propulsion Performance2021-12-06T18:06:04Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (<math>1.5 \cdot 10^{11}</math> meters), light years (ly) (<math>9.5 \cdot 10^{15}</math> meters), or parsecs (pc) (<math>3.1 \cdot 10^{16}</math> meters). Though not in common use, a useful metric unit for interstellar distances is the Petameter (Pm), <math>1 \cdot 10^{15}</math> neters; a light year is then 9.5 Pm.<br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s <math>1 \cdot 10^{12}</math> meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (<math>1 \cdot 10^4</math>) m/s, then the time is <math>10^{12}</math> m / <math>10^4</math> m/s, or <math>1 \cdot 10^8</math> seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=818Propulsion Performance2021-12-06T17:56:59Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (<math>1.5 \cdot 10^11</math> meters), light years (ly) (<math>9.5 \cdot 10^15</math> meters), or parsecs (pc) (<math>3.1 \cdot 10^16</math> meters). Though not in common use, a useful metric unit for interstellar distances is the Petameter (Pm), <math>1 \cdot 10^15</math> neters; a light year is then 9.5 Pm.<br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (<math>1 \cdot 10^4</math>) m/s, then the time is <math>10^12</math> m / <math>10^4</math> m/s, or <math>1 \cdot 10^8</math> seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=817Propulsion Performance2021-12-06T17:54:29Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (<math>1.5 \cdot 10^11</math> meters), light years (ly) (<math>9.5 \cdot 10^15</math> meters), or parsecs (pc) (<math>3.1 \cdot 10^16</math> meters). Though not in common use, a useful metric unit for interstellar distances is the Petameter (Pm), <math>1 \cdot 10^15</math> neters; a light year is then 9.5 Pm.<br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (<math>1 \cdot 10^4</math>) m/s, then the time is <math>1 \cdot 10^12</math> m /<math>1 \cdot 10^4</math> m/s, or <math>1 \cdot 10^18</math> seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=816Propulsion Performance2021-12-06T17:54:05Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (<math>1.5 \cdot 10^11</math> meters), light years (ly) (<math>9.5 \cdot 10^15</math> meters), or parsecs (pc) (<math>3.1 \cdot 10^16</math> meters). Though not in common use, a useful metric unit for interstellar distances is the Petameter (Pm), <math>1 \cdot 10^15</math> neters; a light year is then 9.5 Pm.<br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (<math>1 \cdot 10^4</math>) m/s, then the time is <math>1 \cdot 10^12</math>1E12 m /<math>1 \cdot 10^4</math> m/s, or <math>1 \cdot 10^18</math> seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Scientific_notation&diff=815Scientific notation2021-12-06T17:44:35Z<p>ROCKETGUY: Added (after discussion on the Discord) a standard style guide</p>
<hr />
<div>A good writeup on scientific notation is at https://en.wikipedia.org/wiki/Scientific_notation<br />
<br />
Stylistically, on the Galactic Library, the preferred style for writing 1.5 million in scientific notation is<br />
<br />
<math>1.5 \cdot 10^6</math></div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Scientific_notation&diff=814Scientific notation2021-12-06T14:53:25Z<p>ROCKETGUY: Created page with "A good writeup on scientific notation is at https://en.wikipedia.org/wiki/Scientific_notation ((NOTE: we really ought to decide on whether we're going to use the "1E9" or the..."</p>
<hr />
<div>A good writeup on scientific notation is at https://en.wikipedia.org/wiki/Scientific_notation<br />
<br />
((NOTE: we really ought to decide on whether we're going to use the "1E9" or the "1x10^9" or a typeset notation as our Wiki standard>></div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=813Propulsion Performance2021-12-06T14:33:26Z<p>ROCKETGUY: Isp discussion</p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on [[Basics Of Calculus]]))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.<br />
<br />
Looking back at the classification box of propulsion methods, many of them involve something that is carried aboard the ship and expended during the propulsion process, whether that be a source of energy, a source of reaction mass, or both (any of the propulsion methods that have an 'internal' source fall in to this category). If that's so, some part of the ship mass is given over to carrying the "thing that gets used up for propulsion" -- generically 'propellant' or 'fuel' or 'reaction mass'. In this case, obviously, there is only so much of the consumable supply onboard and this limits the achievable Δv. It will turn out that the systems that do not carry consumables have their own limitations. In the case of propellant using an internal consumable, it obviously matters how much propulsion we can get out of a given mass of the propellant or fuel or reaction mass.<br />
<br />
Δv = acceleration * time<br />
<br />
So:<br />
<br />
Δv = (force / (ship mass)) * time<br />
<br />
The quantity "force times time" is called "Impulse", so we can write this as:<br />
<br />
Δv = Impulse / (ship mass)<br />
<br />
Now if we are expending propellant, ship mass during the maneuver is '''not constant'''. So we can't just ask "how much impulse" did we get; we have to look at slices of time and ask how much impulse we get during each little slice of time. The math to work that out is a bit too complicated to include here (see https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation )). But the important thing is that there is a relationship between how much impulse we got, and how much consumable (reaction mass, fuel, propellant) we had to use to get it. Again, we call something 'specific' when it is 'per unit mass', so we can define a performance parameter which applies to all propulsion systems using an onboard consumable:<br />
<br />
Impulse / (mass of propellant or other consumable) = Specific Impulse = Isp<br />
<br />
From the definition of Impulse, we can write:<br />
<br />
Isp = Impulse / (propellant mass) = force * time / (propellant mass)<br />
<br />
This has units of '''velocity''' -- there's a lot of confusion on this point for historical reasons, but it is clear in the physics. In SI units, Newton-seconds/kg simplifies to meters/second. In various 'English/American Provincial' units, 'if used properly' one would write force in poundals and mass in pounds, and get feet/second, or one would write force in pounds and mass in slugs, and again get feet/second. Sadly, early workers in the rocket field were not so careful about units, and they wrote force in 'pounds force' and mass in 'pounds mass' and cancelled the two different kinds of pound and got specific impulse in 'seconds'. The usage is widespread. We will try at least to clear up the confusion by writing "Isp/g0" when we want to talk about 'seconds' (where g0 is a standard Earth gravity, the difference between a 'pound of force' and a 'pound of mass'). Airbreathing engines define a different unit of "specific fuel consumption" based on lb/hour rather than lb/s of fuel, which is even more confusing, but it can be converted to Isp.<br />
<br />
Just what physically corresponds to that 'characteristic velocity' we call Isp is different in different propulsion systems -- but for performance purposes, it doesn't matter. Whether it be fuel in an airbreathing engine, propellant in a rocket, or reaction mass being expelled from some other energy supply onboard, if you're using something up to get your propulsion, Isp defines how much propulsion you get for each unit of consumed mass.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=812Propulsion Performance2021-12-06T14:02:32Z<p>ROCKETGUY: add Psp ref</p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power" (in metric, that is watts/kilogram)<br />
<br />
Psp = acceleration * (Relevant Velocity)<br />
<br />
This is '''not''' the most common way of talking about this concept; each type of propulsion system has its own vocabulary. However the parameter usually appears in some form or another -- rocket people talk about "thrust to weight" ratio, and nuclear-electric people talk about "alpha" which is the inverse of specific power (kg/W). This emerged in study on how to compare dissimilar propulsion systems <ref>Millis, Marc G., Jeff Greason, and Rhonda Stevenson. Breakthrough Propulsion Study: Assessing Interstellar Flight Challenges and Prospects. No. HQ-E-DAA-TN60290. 2018. <br />
retrieved from https://ntrs.nasa.gov/api/citations/20180006480/downloads/20180006480.pdf </ref> as a metric that could apply to all of them.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=811Propulsion Performance2021-12-06T13:51:55Z<p>ROCKETGUY: introduce Psp</p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards. This is one of the reasons why it is very common to use a different system to accelerate than to brake. For example, all Mars missions to date have used rockets to accelerate from the neighborhood of Earth out towards Mars, but have used braking in the Martian atmosphere for at least some portion of the deceleration. That doesn't add much complication; you can just keep track of how much Δv is required for each propulsion system.<br />
<br />
Now before getting in to the details of how one estimates performance for each different kind of propulsion system, two general principles are clear -- propulsion systems have limits on how much Δv you can get, and they have limits on how much acceleration you can get, and '''both''' parameters matter for working how how long a trip takes and what kind of propulsion system matters. <br />
<br />
There is a little bit more we can say about that which applies to all propulsion systems; acceleration capability of a propulsion system and achievable Δv for a propulsion system aren't really independent parameters. One can see this from basic mechanics:<br />
<br />
Energy = Force * Distance --- this is the definition of energy<br />
<br />
So:<br />
<br />
d/dt (Energy) = Power = Force * d/dt (Distance) = Force * (Relevant Velocity)<br />
<br />
We haven't said '''what''' velocity is involved here, and it's different for different propulsion systems, but clearly, as we talk about more force, or higher velocity changes, more '''power''' is involved. And there's a tradeoff between force and velocity -- for a given power, if we want more force (thrust), we have to accept less velocity, and vice versa.<br />
<br />
In physics "specific" is a term that means 'per unit mass'. So we can divide both side of this equation by some scaled mass. Let's use the mass of the ship. Then we get:<br />
<br />
Power/Mass = "Specific Power" = (Force/mass) * (Relevant Velocity) = acceleration * (Relevant Velocity)<br />
<br />
Or, simply, writing Psp for "Specific Power"<br />
<br />
Psp = acceleration * (Relevant Velocity)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=810Propulsion Performance2021-12-06T13:38:33Z<p>ROCKETGUY: added brachistochrone example</p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
Now obviously, the lower the acceleration, the more time it takes to build up to cruise velocity and the more distance covered while still accelerating. If you keep lowering the acceleration, there comes a point where you barely have enough distance to even get to a given cruise velocity; you accelerate to the midpoint of your voyage and then turn around and decelerate the rest of the way. That particular trajectory has a fancy name ("brachistochrone"), and it essentially defines the lowest acceleration you can use and still hit a given peak velocity. If the acceleration is the same during the acceleration and braking maneuver, that is:<br />
<br />
minimum_acceleration = (peak velocity)^2 / (voyage distance)<br />
<br />
And in this case, the trip takes just twice as long as it would for the same peak velocity but an instantaneous impulse. For the example given, that is an acceleration of 0.0001 m/s^2 (1E-4 m/s^2), and you get velocity and distance curves like this:<br />
<br />
[[File:V_vs_t_brachistochrone.jpg]]<br />
The velocity ramps up to a peak then immediately back down as we start braking<br />
<br />
[[File:D_vs_t_brachistochrone.jpg]]<br />
The distance passes more and more quickly to peak velocity and then more slowly as we brake<br />
<br />
Three graphs:<br />
distance vs. time<br />
velocity vs. time<br />
acceleration vs. time<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16).<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards.<br />
<br />
Now if the acceleration time is small compared to the total trip time, (and gravity is negligible) you can get away with treating the problem with this simple calculation (if acceleration is more than about 100*(Δv)^2/d, and if acceleration is more than about 5 times the local gravity, you can pretty much treat things with this “instantaneous impulse” assumption with small errors). For the example we’re using, that’s acceleration more than about 0.04 m/s^2. But note that the higher the delta-v, the higher the acceleration has to be to treat it as infinite impulse.<br />
<br />
When accelerating, velocity is constantly increasing. If acceleration is constant, you get:<br />
v=v_inital + a*t<br />
d=v_initial*t + 0.5*a*t^2<br />
<br />
If you accelerate, then coast, then brake, you get a case like:<br />
(three graphs of distance vs. time, velocity vs. time, acceleration vs. time)<br />
<br />
<br />
<br />
<br />
I’m going to work the math out here, just so the reader can see that this kind of thing isn’t actually requiring more then high school algebra, but you can skip down to the answer below:<br />
<br />
(derive time = d/vmax + vmax/4a)<br />
<br />
If you keep turning the acceleration down, eventually, you just finish accelerating at the midpoint of the flight, and you immediately have to start decelerating (if your deceleration system has the same rate as your acceleration system). That leads to the ‘minimum time’ trajectory for a given acceleration, which has the fancy name ‘brachistochrone’ which shows up in SF books but rarely in real life, because you simply don’t fly that way in real life).<br />
<br />
Graphs of brachistochrone.<br />
<br />
Therefore, there’s a point where trying to go faster won’t work – you don’t have enough “runway” to get up to speed. Therefore, for advanced drives, it’s the acceleration, not must the maximum attainable delta-v, that starts to limit what you can do.<br />
<br />
((from this work up towards PSP and then delta-V limits and then ISP))</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=809Propulsion Performance2021-12-06T13:29:36Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
<br />
Three graphs:<br />
distance vs. time<br />
velocity vs. time<br />
acceleration vs. time<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16).<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards.<br />
<br />
Now if the acceleration time is small compared to the total trip time, (and gravity is negligible) you can get away with treating the problem with this simple calculation (if acceleration is more than about 100*(Δv)^2/d, and if acceleration is more than about 5 times the local gravity, you can pretty much treat things with this “instantaneous impulse” assumption with small errors). For the example we’re using, that’s acceleration more than about 0.04 m/s^2. But note that the higher the delta-v, the higher the acceleration has to be to treat it as infinite impulse.<br />
<br />
When accelerating, velocity is constantly increasing. If acceleration is constant, you get:<br />
v=v_inital + a*t<br />
d=v_initial*t + 0.5*a*t^2<br />
<br />
If you accelerate, then coast, then brake, you get a case like:<br />
(three graphs of distance vs. time, velocity vs. time, acceleration vs. time)<br />
<br />
<br />
<br />
<br />
I’m going to work the math out here, just so the reader can see that this kind of thing isn’t actually requiring more then high school algebra, but you can skip down to the answer below:<br />
<br />
(derive time = d/vmax + vmax/4a)<br />
<br />
If you keep turning the acceleration down, eventually, you just finish accelerating at the midpoint of the flight, and you immediately have to start decelerating (if your deceleration system has the same rate as your acceleration system). That leads to the ‘minimum time’ trajectory for a given acceleration, which has the fancy name ‘brachistochrone’ which shows up in SF books but rarely in real life, because you simply don’t fly that way in real life).<br />
<br />
Graphs of brachistochrone.<br />
<br />
Therefore, there’s a point where trying to go faster won’t work – you don’t have enough “runway” to get up to speed. Therefore, for advanced drives, it’s the acceleration, not must the maximum attainable delta-v, that starts to limit what you can do.<br />
<br />
((from this work up towards PSP and then delta-V limits and then ISP))</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=808Propulsion Performance2021-12-06T13:29:05Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
The velocity jumps almost at once to cruise velocity and comes back to zero during braking<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
<br />
The distance changes at a constant rate defined by the cruise velocity<br />
<br />
<br />
Three graphs:<br />
distance vs. time<br />
velocity vs. time<br />
acceleration vs. time<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16).<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards.<br />
<br />
Now if the acceleration time is small compared to the total trip time, (and gravity is negligible) you can get away with treating the problem with this simple calculation (if acceleration is more than about 100*(Δv)^2/d, and if acceleration is more than about 5 times the local gravity, you can pretty much treat things with this “instantaneous impulse” assumption with small errors). For the example we’re using, that’s acceleration more than about 0.04 m/s^2. But note that the higher the delta-v, the higher the acceleration has to be to treat it as infinite impulse.<br />
<br />
When accelerating, velocity is constantly increasing. If acceleration is constant, you get:<br />
v=v_inital + a*t<br />
d=v_initial*t + 0.5*a*t^2<br />
<br />
If you accelerate, then coast, then brake, you get a case like:<br />
(three graphs of distance vs. time, velocity vs. time, acceleration vs. time)<br />
<br />
<br />
<br />
<br />
I’m going to work the math out here, just so the reader can see that this kind of thing isn’t actually requiring more then high school algebra, but you can skip down to the answer below:<br />
<br />
(derive time = d/vmax + vmax/4a)<br />
<br />
If you keep turning the acceleration down, eventually, you just finish accelerating at the midpoint of the flight, and you immediately have to start decelerating (if your deceleration system has the same rate as your acceleration system). That leads to the ‘minimum time’ trajectory for a given acceleration, which has the fancy name ‘brachistochrone’ which shows up in SF books but rarely in real life, because you simply don’t fly that way in real life).<br />
<br />
Graphs of brachistochrone.<br />
<br />
Therefore, there’s a point where trying to go faster won’t work – you don’t have enough “runway” to get up to speed. Therefore, for advanced drives, it’s the acceleration, not must the maximum attainable delta-v, that starts to limit what you can do.<br />
<br />
((from this work up towards PSP and then delta-V limits and then ISP))</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=807Propulsion Performance2021-12-06T13:27:48Z<p>ROCKETGUY: Added graphs for instantaneous impulse</p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16). The graphs below are for 0.01 m/s^2 acceleration on that voyage during both acceleration and braking, which, over that distance, is very nearly an instant jump to cruise velocity. Missions where you can neglect the distance and time spent accelerating and decelerating are called 'instantaneous impulse' trajectories.<br />
<br />
[[File:V_vs_t_highaccel.jpg]]<br />
<br />
[[File:D_vs_t_highaccel.jpg]]<br />
<br />
<br />
Three graphs:<br />
distance vs. time<br />
velocity vs. time<br />
acceleration vs. time<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16).<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards.<br />
<br />
Now if the acceleration time is small compared to the total trip time, (and gravity is negligible) you can get away with treating the problem with this simple calculation (if acceleration is more than about 100*(Δv)^2/d, and if acceleration is more than about 5 times the local gravity, you can pretty much treat things with this “instantaneous impulse” assumption with small errors). For the example we’re using, that’s acceleration more than about 0.04 m/s^2. But note that the higher the delta-v, the higher the acceleration has to be to treat it as infinite impulse.<br />
<br />
When accelerating, velocity is constantly increasing. If acceleration is constant, you get:<br />
v=v_inital + a*t<br />
d=v_initial*t + 0.5*a*t^2<br />
<br />
If you accelerate, then coast, then brake, you get a case like:<br />
(three graphs of distance vs. time, velocity vs. time, acceleration vs. time)<br />
<br />
<br />
<br />
<br />
I’m going to work the math out here, just so the reader can see that this kind of thing isn’t actually requiring more then high school algebra, but you can skip down to the answer below:<br />
<br />
(derive time = d/vmax + vmax/4a)<br />
<br />
If you keep turning the acceleration down, eventually, you just finish accelerating at the midpoint of the flight, and you immediately have to start decelerating (if your deceleration system has the same rate as your acceleration system). That leads to the ‘minimum time’ trajectory for a given acceleration, which has the fancy name ‘brachistochrone’ which shows up in SF books but rarely in real life, because you simply don’t fly that way in real life).<br />
<br />
Graphs of brachistochrone.<br />
<br />
Therefore, there’s a point where trying to go faster won’t work – you don’t have enough “runway” to get up to speed. Therefore, for advanced drives, it’s the acceleration, not must the maximum attainable delta-v, that starts to limit what you can do.<br />
<br />
((from this work up towards PSP and then delta-V limits and then ISP))</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=File:D_vs_t_brachistochrone.jpg&diff=806File:D vs t brachistochrone.jpg2021-12-06T13:20:49Z<p>ROCKETGUY: example of brachistochrone distance</p>
<hr />
<div>== Summary ==<br />
example of brachistochrone distance</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=File:V_vs_t_brachistochrone.jpg&diff=805File:V vs t brachistochrone.jpg2021-12-06T13:20:09Z<p>ROCKETGUY: example of brachistochrone velocity</p>
<hr />
<div>== Summary ==<br />
example of brachistochrone velocity</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=File:D_vs_t_highaccel.jpg&diff=804File:D vs t highaccel.jpg2021-12-06T13:19:32Z<p>ROCKETGUY: example of instant impulse distance</p>
<hr />
<div>== Summary ==<br />
example of instant impulse distance</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=File:V_vs_t_highaccel.jpg&diff=803File:V vs t highaccel.jpg2021-12-06T13:18:59Z<p>ROCKETGUY: Example of instant impulse velocity</p>
<hr />
<div>== Summary ==<br />
Example of instant impulse velocity</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=802Propulsion Performance2021-12-06T13:18:10Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16).<br />
<br />
<br />
<br />
<br />
Three graphs:<br />
distance vs. time<br />
velocity vs. time<br />
acceleration vs. time<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16).<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards.<br />
<br />
Now if the acceleration time is small compared to the total trip time, (and gravity is negligible) you can get away with treating the problem with this simple calculation (if acceleration is more than about 100*(Δv)^2/d, and if acceleration is more than about 5 times the local gravity, you can pretty much treat things with this “instantaneous impulse” assumption with small errors). For the example we’re using, that’s acceleration more than about 0.04 m/s^2. But note that the higher the delta-v, the higher the acceleration has to be to treat it as infinite impulse.<br />
<br />
When accelerating, velocity is constantly increasing. If acceleration is constant, you get:<br />
v=v_inital + a*t<br />
d=v_initial*t + 0.5*a*t^2<br />
<br />
If you accelerate, then coast, then brake, you get a case like:<br />
(three graphs of distance vs. time, velocity vs. time, acceleration vs. time)<br />
<br />
<br />
<br />
<br />
I’m going to work the math out here, just so the reader can see that this kind of thing isn’t actually requiring more then high school algebra, but you can skip down to the answer below:<br />
<br />
(derive time = d/vmax + vmax/4a)<br />
<br />
If you keep turning the acceleration down, eventually, you just finish accelerating at the midpoint of the flight, and you immediately have to start decelerating (if your deceleration system has the same rate as your acceleration system). That leads to the ‘minimum time’ trajectory for a given acceleration, which has the fancy name ‘brachistochrone’ which shows up in SF books but rarely in real life, because you simply don’t fly that way in real life).<br />
<br />
Graphs of brachistochrone.<br />
<br />
Therefore, there’s a point where trying to go faster won’t work – you don’t have enough “runway” to get up to speed. Therefore, for advanced drives, it’s the acceleration, not must the maximum attainable delta-v, that starts to limit what you can do.<br />
<br />
((from this work up towards PSP and then delta-V limits and then ISP))</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_Performance&diff=801Propulsion Performance2021-12-06T01:13:17Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY -- there is a page in draft for this, I put the blank page up to create a place to link to from other pages -- stay tuned (Rocketguy)<br />
</blockquote><br />
<br />
Almost the first question anyone asks about propulsion systems is “What’s the best one?”. If that question had an answer, everyone would already know the answer. You have to add a little more detail and ask “best for doing what?” to even start the conversation. This is a subject that can get pretty complicated, but we’ll start with the basics and work up. <br />
<br />
Usually, what you care about in a transportation system is “how long does a trip take” and “how much does it cost”. Let’s start with “how long”.<br />
<br />
The simplest case is a trip which is out in space, far enough away from planets and stars that the effect of gravity can be ignored. Even inside a Solar system, this comes pretty close for very fast trips because gravity doesn’t have a lot of time to affect the course. When your cruise velocity is more than about 1.5 times the local escape velocity from the Sun, and the start and end point are on the same side of the Sun rather than crossing near the Sun during the trip, the straight-line approximation works fairly well.<br />
<br />
We will work everything in metric units (SI), meters, kilograms, and seconds. One of the challenges in explaining propulsion is that it is easy to get tangled up in the units. That requires familiarity with [[scientific notation]], '((NOTE: do we want to typeset scientific notation or use computer notation, 1.2E23 and so on? Typesetting is a pain but more formal, we should probably make that style decision for use throughout))'. Distances in spaceflight are usually measured in kilometers (1000 meters), miles (1609 meters), nautical miles (1852 meters), astronomical units (AU) (1.5E11 meters), light years (ly) (9.5E15 meters), or parsecs (pc) (3.1E16 meters). <br />
<br />
Consider a trip of about 6.6 AU … a little further than the closest distance from Earth to Jupiter, a little closer than Earth to Saturn. That’s 1E12 meters. We’ll want to go there and come to a stop, rather than just passing.<br />
<br />
A few terms: velocity is the rate of change of distance (meters/second), and acceleration is the rate of change of velocity (meters/second^2).<br />
<br />
If your propulsion system provides a high acceleration for a short time, you can treat this very simply: you pick up a velocity change (usually written Δv, pronounced ‘delta-vee’), coast at constant velocity, and make a similar quick braking maneuver at the end.<br />
<br />
Three graphs:<br />
distance vs. time<br />
velocity vs. time<br />
acceleration vs. time<br />
((link to a side page on ‘the basics of calculus’))<br />
<br />
In that case, the time is mostly spent in the coast, and that time comes from the simple high-school physics equation:<br />
<br />
d = v * t<br />
<br />
Which can also be written:<br />
<br />
t = d/v<br />
<br />
If the coast velocity is 10000 (1E4) m/s, then the time is 1E12m /1E4 m/s, or 1E8 seconds. (1 year is 3.16E7 seconds, or very close to pi*1e7 seconds), so the trip takes about 3.16 years (1E8/3.16E7 = 3.16).<br />
<br />
Now to make this trip, your propulsion system has to deliver a velocity change (Δv) of 20000 m/s, or twice the Δv just to accelerate. Even that modest velocity is more than a chemical rocket can realistically provide, and yet the trip is pretty slow by most standards.<br />
<br />
Now if the acceleration time is small compared to the total trip time, (and gravity is negligible) you can get away with treating the problem with this simple calculation (if acceleration is more than about 100*(Δv)^2/d, and if acceleration is more than about 5 times the local gravity, you can pretty much treat things with this “instantaneous impulse” assumption with small errors). For the example we’re using, that’s acceleration more than about 0.04 m/s^2. But note that the higher the delta-v, the higher the acceleration has to be to treat it as infinite impulse.<br />
<br />
When accelerating, velocity is constantly increasing. If acceleration is constant, you get:<br />
v=v_inital + a*t<br />
d=v_initial*t + 0.5*a*t^2<br />
<br />
If you accelerate, then coast, then brake, you get a case like:<br />
(three graphs of distance vs. time, velocity vs. time, acceleration vs. time)<br />
<br />
<br />
<br />
<br />
I’m going to work the math out here, just so the reader can see that this kind of thing isn’t actually requiring more then high school algebra, but you can skip down to the answer below:<br />
<br />
(derive time = d/vmax + vmax/4a)<br />
<br />
If you keep turning the acceleration down, eventually, you just finish accelerating at the midpoint of the flight, and you immediately have to start decelerating (if your deceleration system has the same rate as your acceleration system). That leads to the ‘minimum time’ trajectory for a given acceleration, which has the fancy name ‘brachistochrone’ which shows up in SF books but rarely in real life, because you simply don’t fly that way in real life).<br />
<br />
Graphs of brachistochrone.<br />
<br />
Therefore, there’s a point where trying to go faster won’t work – you don’t have enough “runway” to get up to speed. Therefore, for advanced drives, it’s the acceleration, not must the maximum attainable delta-v, that starts to limit what you can do.<br />
<br />
((from this work up towards PSP and then delta-V limits and then ISP))</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=800Propulsion2021-12-05T03:05:27Z<p>ROCKETGUY: rescaled bomb graphic</p>
<hr />
<div>{{Quote|'''''"We look for things, things to make us go"'''''. <br/><br/>Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare}}<br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math alt=><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:Momentium_rescale.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological analysis" or a Zwicky box <ref>This technique as well as the entire classification approach used on this page derive from F. Zwicky, "Fundamentals of Propulsive Power", International Congress of Applied Mechanics, Paris, September 22-29, 1946 and many later works by the same author. The box above is a three dimensional box (energy internal/external, momentum source external/internal, beamed/harvested which has been 'flattened' for ease of use since 'internal' has no beamed/harvested classification</ref>)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=File:Momentium_rescale.gif&diff=799File:Momentium rescale.gif2021-12-05T03:01:31Z<p>ROCKETGUY: smaller version of momentum.gif</p>
<hr />
<div>== Summary ==<br />
smaller version of momentum.gif</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=795Propulsion2021-12-04T18:42:10Z<p>ROCKETGUY: Updated opening quote from 'blockquote' to the wiki quote format</p>
<hr />
<div>{{Quote|'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare}}<br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:momentum.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological analysis" or a Zwicky box <ref>This technique as well as the entire classification approach used on this page derive from F. Zwicky, "Fundamentals of Propulsive Power", International Congress of Applied Mechanics, Paris, September 22-29, 1946 and many later works by the same author. The box above is a three dimensional box (energy internal/external, momentum source external/internal, beamed/harvested which has been 'flattened' for ease of use since 'internal' has no beamed/harvested classification</ref>)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Conservation_Laws:_Limits_to_Cheating&diff=794Conservation Laws: Limits to Cheating2021-12-04T18:39:03Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
This is a STUB ONLY at this time to provide the link structure on another page. Please don't edit until I can get a first draft up here -- Rocketguy<br />
</blockquote><br />
<br />
'''Conservation Laws'''<br />
<br />
The conservation laws (conservation of momentum, conservation of energy, and the second law of thermodynamics (Entropy doesn't decrease), are fundamental to our understanding of the physical universe. If working in the real world, one may dislike them, but obedience is strictly enforced by the Universe.<br />
<br />
In a work of fiction, of course, one can do anything; however, discarding these fundamentals is not to be done lightly or carelessly and doing so is one of the surest ways to make your work of science fiction not be 'hard' or 'tough' and to quickly cross the line in to fantasy. Fantasy of course is a genre of its own with its own conventions and literature and if that's what one intends to write, so be it. But if you want a work which is 'realistic' in the physical sense, there is usually some way, as an author, to get what you want without discarding the conservation laws.<br />
<br />
The conservation of energy -- the realization that energy may change from one form to another, but not be created or destroyed -- is in some ways the birth of all physics. The concept has proven so useful that with every new discovery, rather than treating it as a violation of the conservation law, we have instead found ways to add new forms of energy to the account. Objects in motion in the real world slow down -- but the energy shows up as frictional heat. Burning fuel appears to create energy -- but we now know it was there in the molecular bonds in the material in the first place. Even nuclear energy, which was quite mysterious when first discovered, we now consider an example of the same thing, in that we now (thanks to Einstein) appreciate that mass is just another kind of energy, and if one converts a tiny amount of mass in to energy, a large amount of energy is the result. If tomorrow, some new phenomenon were found that appeared to violate this law, we would almost certainly find a way to book-keep it as a new form of energy, rather than junking such a useful concept as conservation of energy.<br />
<br />
Momentum is much the same. If you push on a heavy object, you feel it pushing back on your hand. If you eject mass to the left, you are pushed to the right. If you push on the air to move forward, you create a wind going aft. People have theorized (but not yet achieved an accepted experimental validation) various drives which might 'push on nothing' -- so called 'reactionless' drives. For story purposes (and who knows, perhaps in reality), these can be re-cast as a drive which 'pushes on something big' -- the large-scale structure of space-time, for example -- without throwing away the conservation of momentum. And in any case, space is not empty; perfectly conventional real-world propulsion systems have been proposed which push on the thin plasma between the planets or between the stars, or which interact with the magnetic fields found between the stars. One might envision improving the performance of such systems to whatever degree desired for story purposes without throwing all of physics out the window.<br />
<br />
Thanks to the Nobel-winning work of Emmy Noether<ref>https://en.wikipedia.org/wiki/Noether%27s_theorem</ref> we now understand that every symmetry of the universe carries with it a corresponding conservation law. It can be shown that symmetry to time coordinate carries with it the conservation of energy, and symmetry with the space coordinate carries with it the conservation of momentum. In other words, violating these conservation laws means that if you do something tomorrow it doesn't necessarily produce the same result as today, and if you do something a meter to the left, it might turn out differently than it does a meter to the right. If the universe is not predictable, you don't need conservation laws -- but then it's hard to stay in the realm of 'science' fiction.<br />
<br />
The non-decrease of entropy is a bit more subtle, but everyday examples abound. If you put a bit of dye in a glass of water, you expect to see it spread through the water -- the reverse process, where the water spits out the drop of dye, doesn't take place. If you drop an ice cube in the glass of water you expect to end up with cool water; you'd be very surprised indeed to start with a glass of water and see that it had divided itself in to a bit of water and a bit of ice unless you move energy around (like putting the glass in to a freezer). This property of a system we call 'entropy'. You can think of it as a measure of how randomly arranged or ordered the system is. There's generally only a few ways for the atoms to be arranged that we think of as 'ordered', and many many ways we can think of them as 'disordered', and those can be treated with more rigor and math. For simple systems where all the atoms are mixed up together, we can describe that as "temperature" -- and one way to summarize the Second Law of Thermodynamics is "heat flows from high temperature to low temperature". This has major consequences for spacecraft propulsion, because we want to move a lot of energy about. In doing so, the energy flows from a high-temperature (low entropy) source to a lower-temperature sink. In some cases, that 'sink' can be the propellant being expelled -- well and good. In many cases, it can't be, and it has to go somewhere else. Space is short on good heat sinks -- vacuum, in spite of endless poetry about 'the cold of space' is an outstanding thermal insulator (as in a vacuum flask, Dewar flask, or Thermos bottle), and nearly the only way to get waste heat out through it is to radiate it. ((See 'Radiators' -- Page not created yet))<br />
<br />
'''Cheating'''<br />
If one wishes to 'appear' to violate these conservation laws, there are some tiny chinks in the structure of physics one can look to; but remember that centuries of experimentation on these laws will still be 'right' and there must be some very good reason why any 'cheating' is restricted to very special circumstances.<br />
<br />
In highly curved spacetime, space and time really aren't quite the same from place to place or time to time. Therefore, defining just what is meant by 'energy' and 'momentum' can get a bit tricky. This is, however, much more likely to be a question of doing the bookkeeping properly than 'violating' the conservation laws.<br />
<br />
It is much easier to invent (and in the real world, to seek) new forms of energy, or to find new things to push on. That can produce much the same effect for story purposes without loosing magic in the world.<br />
<br />
The Second Law is a bit fuzzier. It applies to a closed system without energy flow in or out of the system. Those energy flows are not always obvious. There have been very small microscopic systems in which it appears that room-temperature energy flowing in to the system is producing useful work without a low-temperature reservoir (so far, only at tiny levels). The 'randomness' character also means that when looking at small fluctuations, what this might mean at all is somewhat debatable. Still, handle with care; unless you think through all the implications, ignoring the Second Law can lead you in to pure fantasy very quickly without intending to do so. Ignoring the implications of this law -- in particular, neglecting radiators on your ships where they really should be there -- is one of the current hallmarks of how 'hard' your science fiction is meant to be.</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=793Propulsion2021-12-04T17:57:29Z<p>ROCKETGUY: First draft complete</p>
<hr />
<div><blockquote><br />
'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare<br />
</blockquote><br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:momentum.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological analysis" or a Zwicky box <ref>This technique as well as the entire classification approach used on this page derive from F. Zwicky, "Fundamentals of Propulsive Power", International Congress of Applied Mechanics, Paris, September 22-29, 1946 and many later works by the same author. The box above is a three dimensional box (energy internal/external, momentum source external/internal, beamed/harvested which has been 'flattened' for ease of use since 'internal' has no beamed/harvested classification</ref>)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=792Propulsion2021-12-04T17:50:30Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote><br />
'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare<br />
</blockquote><br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:momentum.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological analysis" or a Zwicky box <ref>This technique as well as the entire classification approach used on this page derive from F. Zwicky, "Fundamentals of Propulsive Power", International Congress of Applied Mechanics, Paris, September 22-29, 1946 and many later works by the same author. The box above is a three dimensional box (energy internal/external, momentum source external/internal, beamed/harvested which has been 'flattened' for ease of use since 'internal' has no beamed/harvested classification</ref>)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
'''NOTE IN PROGRESS: I EXPECT EACH OF THESE BOLD HEADINGS TO BECOME A PAGE WHERE WE WILL HAVE A LIST OF LINKS TO THE ARTICLES ON RELEVANT PROPULSION SYSTEMS'''<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=791Propulsion2021-12-04T17:49:47Z<p>ROCKETGUY: zwicky box reference</p>
<hr />
<div><blockquote> <br />
'''This is VERY MUCH A WORK IN PROGRESS; request you give me a few days before I can call it even a first draft -- Rocketguy'''<br />
</blockquote><br />
<br />
<blockquote><br />
'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare<br />
</blockquote><br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:momentum.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological analysis" or a Zwicky box <ref>This technique as well as this entire classification approach used on this page derive from F. Zwicky, "Fundamentals of Propulsive Power", International Congress of Applied Mechanics, Paris, September 22-29, 1946 and many later works by the same author. The box above is a three dimensional box (energy internal/external, momentum source external/internal, beamed/harvested which has been 'flattened' for ease of use since 'internal' has no beamed/harvested classification</ref>)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
'''NOTE IN PROGRESS: I EXPECT EACH OF THESE BOLD HEADINGS TO BECOME A PAGE WHERE WE WILL HAVE A LIST OF LINKS TO THE ARTICLES ON RELEVANT PROPULSION SYSTEMS'''<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=790Propulsion2021-12-04T17:34:16Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote> <br />
'''This is VERY MUCH A WORK IN PROGRESS; request you give me a few days before I can call it even a first draft -- Rocketguy'''<br />
</blockquote><br />
<br />
<blockquote><br />
'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare<br />
</blockquote><br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:momentum.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological box" or a Zwicky box, after its popularizer, Fritz Zwicky)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
'''NOTE IN PROGRESS: I EXPECT EACH OF THESE BOLD HEADINGS TO BECOME A PAGE WHERE WE WILL HAVE A LIST OF LINKS TO THE ARTICLES ON RELEVANT PROPULSION SYSTEMS'''<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=789Propulsion2021-12-04T17:33:38Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote> <br />
'''This is VERY MUCH A WORK IN PROGRESS; request you give me a few days before I can call it even a first draft -- Rocketguy'''<br />
</blockquote><br />
<br />
<blockquote><br />
'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare<br />
</blockquote><br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:momentum.gif|200px]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological box" or a Zwicky box, after its popularizer, Fritz Zwicky)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
'''NOTE IN PROGRESS: I EXPECT EACH OF THESE BOLD HEADINGS TO BECOME A PAGE WHERE WE WILL HAVE A LIST OF LINKS TO THE ARTICLES ON RELEVANT PROPULSION SYSTEMS'''<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=788Propulsion2021-12-04T17:31:05Z<p>ROCKETGUY: added 'bomb' illustration for momentum conservation.</p>
<hr />
<div><blockquote> <br />
'''This is VERY MUCH A WORK IN PROGRESS; request you give me a few days before I can call it even a first draft -- Rocketguy'''<br />
</blockquote><br />
<br />
<blockquote><br />
'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare<br />
</blockquote><br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel. If you think about a bomb exploding, chemical energy (which can be measured by one number) is converted to kinetic energy of all the pieces (which is still the SAME amount of energy, when you add them all up). But the center of mass of the system of pieces doesn't change its velocity -- because the momentum is a vector with direction -- the product of mass and velocity of the pieces going left is balanced by those going right, those going up are balanced by those going down, and so on.<br />
<br />
[[File:momentum.gif]]<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological box" or a Zwicky box, after its popularizer, Fritz Zwicky)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
'''NOTE IN PROGRESS: I EXPECT EACH OF THESE BOLD HEADINGS TO BECOME A PAGE WHERE WE WILL HAVE A LIST OF LINKS TO THE ARTICLES ON RELEVANT PROPULSION SYSTEMS'''<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
<br />
'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
<br />
The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=File:Momentum.gif&diff=787File:Momentum.gif2021-12-04T17:25:40Z<p>ROCKETGUY: Illustration of a bomb breaking apart, showing how even though the pieces move, the center of mass stays in the same place. Contributed via the Discord server by Dæmoria#0914 to the Galactic Library</p>
<hr />
<div>== Summary ==<br />
Illustration of a bomb breaking apart, showing how even though the pieces move, the center of mass stays in the same place. Contributed via the Discord server by Dæmoria#0914 to the Galactic Library</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion&diff=786Propulsion2021-12-04T17:21:50Z<p>ROCKETGUY: </p>
<hr />
<div><blockquote> <br />
'''This is VERY MUCH A WORK IN PROGRESS; request you give me a few days before I can call it even a first draft -- Rocketguy'''<br />
</blockquote><br />
<br />
<blockquote><br />
'''''"We look for things, things to make us go"'''''. Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare<br />
</blockquote><br />
<br />
In real space activities, and in science fiction, we face the need to move people and things from place to place. Space, by definition, is 'out there', and right now, we are 'here'. To get from 'here' to 'there' you have to move. In fiction, unless the protagonist spends the entire story sitting in an armchair, the characters have to move to get where the action is. Space opera could hardly exist without the 'Cool Ship' at the center of the action, both as character and setting. The technology of moving things around is called 'propulsion', and the thing that does it is, generically, a 'propulsion system', though it may be called many things, such as 'engine', 'sail', 'rocket', 'drive', etc.<br />
<br />
Propulsion is not the only technology that matters in spaceflight, however beloved that assumption is by propulsion engineers. However, it underpins all the others. Improve propulsion, and you improve all the missions; improve instruments, or communications, or life support, and you improve only some. The technology of propulsion very much defines the scope of a setting, the distances practical to travel, how long it takes to get from place to place, and how much it costs.<br />
<br />
To understand the basics of propulsion, you have to take three basic laws of physics as a given:<br />
* Conservation of Energy (First law of thermodynamics)<br />
* Conservation of Momentum (action = reaction)<br />
* Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)<br />
<br />
The respect for these laws in fiction is one of the clearer indication that a work of SF is "hard" -- and in the real world, of course, obedience to the laws of physics is not at all optional. Since these laws are so fundamental, underpinning our understanding of the world around us, it is rather unlikely that they will be abandoned as our understanding improves. See '''[[Conservation Laws: Limits to Cheating]]''' for more discussion.<br />
<br />
The kinetic energy of a moving spacecraft is <math><br />
{E} = \frac{1}{2} \cdot {mass} \cdot {velocity}^2<br />
</math>. A propulsion system might use more energy than that, but at a minimum, the kinetic energy of the ship has to come from somewhere -- and the faster the ship goes, the more energy is required. <br />
<br />
<br />
The basics of momentum conservation are simply Newton's "every action has an equal and opposite reaction". If you want to push a ship to the right, something else has to be pushed to the left. Momentum is "mass * velocity", and it is a vector quantity (one that has a direction). To push a ship to the right, you can push a lot of mass to the left slowly, or a little mass to the left quickly, so long as the vectors cancel.<br />
<br />
'''(Insert picture of momentum conservation by arrows, and another, of the exploding bomb, showing how a lot of kinetic energy can be in a system without net momentum)'''<br />
<br />
There are a whole range of propulsion systems in both reality and fiction. Broadly speaking, they fall in to different categories based on where the energy comes from, and what they push on (how momentum is conserved). Generically, the mass you push against to get a force on the ship is called the "reaction mass", so where the reaction mass comes from is another factor. We classify propulsion systems in this work with the source of energy being internal to the ship, harvested from natural sources around the ship, or transmitted (beamed) to the ship, and likewise, that the reaction mass can be carried internal to the ship and expelled (called 'propellant' in that case), or harvested from natural sources around the ship, or transmitted (beamed) to the ship.<br />
<br />
A table with some examples of each type:<br />
<br />
{| class="wikitable"<br />
|+ Morphological Classification of Propulsion Systems<br />
!rowspan="2"|Energy Source <br />
!colspan="3"|Source of Reaction Mass<br />
|-<br />
! Internal !! External,<br/>Harvested !! External,<br/>Beamed<br />
|-<br />
!Internal <br />
| Chemical rockets,<br/>Nuclear rockets <br />
| Propellers <br />
| 'seeded' ramjet with<br/>onboard antimatter<br />
|- <br />
! External,<br/>Harvested <br />
| 'q-drive',<br/>solar rocket <br />
| Magnetic sails,<br/>e-sails <br />
| Wind-Pellet Shear Sailing<br />
|- <br />
! External,<br/>Beamed <br />
| Laser-driven rocket <br />
| Laser-driven ramjet <br />
| photon beam sails,<br/>particle beam magsail<br />
|}<br />
<br />
<br />
(A map of all the possibilities of important properties of a system like this is called a "morphological box" or a Zwicky box, after its popularizer, Fritz Zwicky)<br />
<br />
In space, where friction is usually negligible unless a ship is deliberately doing something create it, a vehicle usually has to accelerate to cruising speed and then decelerate at the destination. Both maneuvers are equally important and both take some kind of propulsion system (although in some cases, it's easier to use different systems to slow down than were used to speed up).<br />
<br />
Many real life systems incorporate features that blend properties; for example, a turbojet engine in an atmosphere is mostly 'internal energy, external reaction mass', using the air, but part of the energy supply comes from the air gathered (to burn with the onboard fuel), and a small part of the reaction mass comes from the combustion products (internal reaction mass). Still, they are usually classed as 'internal energy, external reaction mass' because that's where the dominant effects come from. Some cases will be so borderline they might appear in either case, in which case the practice in the Galactic Library should be to include a cross-reference in the descriptive pages.<br />
<br />
Some examples of each type, to guide the reader:<br />
'''NOTE IN PROGRESS: I EXPECT EACH OF THESE BOLD HEADINGS TO BECOME A PAGE WHERE WE WILL HAVE A LIST OF LINKS TO THE ARTICLES ON RELEVANT PROPULSION SYSTEMS'''<br />
<br />
'''[[Propulsion - Internal Energy, Internal Reaction Mass]]:''' This is the classic "rocket" that opened space for the first time. Because everything is carried onboard the vehicle, it works outside the atmosphere of the Earth. The archetypical chemical rocket relies on the combustion of a fuel and an oxidizer (both carried aboard), which supply the energy, and also, together, form the propellant reaction mass. Nuclear rockets, both fission and fusion, fall in this class as well since the propellant carries both the energy and the reaction mass with it.<br />
<br />
'''[[Propulsion - Internal Energy, Harvested Reaction Mass]]:''' Most familiar in propulsion systems that push on the air or water (the rowboat, where the energy of the rower pushes oars to push on the water around the vehicle, or a battery-powered propeller aircraft, where energy stored aboard the aircraft pushes on the air as the reaction mass. Airbreathing propulsion systems of all types, even where the air is used as an oxidizer, tend to be best classified within this category as they use the same performance equations.<br />
<br />
'''[[Propulsion - Internal Energy, Beamed Reaction Mass]]:''' A "seeded" ramjet that sends pellets ahead of the vehicle to be scooped up as reaction mass, but uses an onboard energy supply, such as antimatter, to accelerate the reaction mass scooped by the ramjet<br />
<br />
'''[[Propulsion - Harvested Energy, Internal Reaction Mass]]:''' Solar-powered electric rockets used in modern satellites and some recent deep-space missions. Also, the "q-drive" system recently proposed which harvests energy from the passing solar wind to drive the expulsion of stored reaction mass. Both of these have properties quite different from self-contained chemical rockets. <br />
<br />
'''[[Propulsion - Harvested Energy, Harvested Reaction Mass]]:''' on Earth, the "square rigged" sailing vessel that runs only downwind is an example of gaining speed from an external flow. In space, parachutes are often used to decelerate in this way during atmospheric entry (in a parachute, which slows down the vehicle, the "external energy" is a *sink* of energy rather than a *source*, since you are subtracting kinetic energy from the decelerating ship. Plasma sails of all types interacting with the solar wind or interstellar medium are further examples. The Bussard Ramjet concept would have been an example of using this to speed up. Plasma soaring uses external gradients in wind speed to accelerate using this principle.<br />
<br />
'''[[Propulsion - Harvested Energy, Beamed Reaction Mass]]:''' The 'wind-pellet shear sailing' concept, in which plasma wind energy is used to interact with pellets laid down ahead of the ship that provide the reaction mass falls in this category.<br />
<br />
'''[[Propulsion - Beamed Energy, Internal Reaction Mass]]:''' A laser or microwave powered rocket, where the power supply is left on the ground but used to expel propellant stored on the ship.<br />
<br />
'''[[Propulsion - Beamed Energy, Harvested Reaction Mass]]:''' A beam-powered, propeller-driven aircraft would be an example available today; there are also drives that push against the solar wind or the interstellar plasma that can be powered by beamed energy.<br />
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'''[[Propulsion - Beamed Energy, Beamed Reaction Mass]]:''' a classic photon or particle 'beamrider' in which the beam provides both the propulsive energy and the propulsive momentum<br />
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The performance characteristics of these systems vary widely, not only in the technical details but even in what kinds of equations govern the performance -- see the page on '''[[Propulsion Performance]]'''</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Beamed_Energy,_Beamed_Reaction_Mass&diff=785Propulsion - Beamed Energy, Beamed Reaction Mass2021-12-03T02:38:13Z<p>ROCKETGUY: </p>
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<div>{{WIPNotice}}</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Interstellar_Medium_Shielding&diff=783Interstellar Medium Shielding2021-12-02T02:02:38Z<p>ROCKETGUY: Added a paragraph and references for interstellar dust</p>
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<div>It might surprise you that you need to shield your ship from the interstellar medium, specifically at velocities greater than 40% of c. This is a result of interstellar space being filled with a diffuse medium of mostly hydrogen, which when relative to a ship at high enough velocities, comes to increasingly resemble ionizing radiation. <br />
<br />
The main danger is heating and not erosion – erosion is insignificant enough that a 1 cm thick carbon shield can go 25,000 light-years (at a speed regime of 30% of c), ignoring that not all particles displaced will be lost to space, instead landing back on the shield.<br />
<br />
=Interstellar Medium Density=<br />
To begin with, the interstellar medium density varies greatly, ranging from 10<sup>-4</sup> particles per cubic centimeter in the coronal gas component of the galactic halo of the Milky Way, to 10<sup>6</sup> particles per cubic centimeter in molecular clouds. <ref>https://en.wikipedia.org/wiki/Interstellar_medium<br />
<br/> Reference for ISM density</ref><br />
<br />
This is important in calculating the flux that the forward portion of the ship will receive at a particular velocity.<br />
<br />
==Particle Density Table==<br />
(In units of particles per cubic centimeter)<br />
{| class="wikitable sortable"<br />
|-<br />
! Component !! Particle Density<br />
|-<br />
| Molecular clouds || 10<sup>2</sup>-10<sup>6</sup> <br />
|-<br />
| H II regions || 10<sup>2</sup>-10<sup>4</sup><br />
|-<br />
| Cold neutral medium || 20-50<br />
|-<br />
| Warm neutral medium || 0.2-0.5<br />
|-<br />
| Warm ionized medium || 0.2-0.5<br />
|-<br />
| Coronal gas <br/>(Hot ionized medium) || 10<sup>-4</sup>-10<sup>-2</sup><br />
|}<br />
<br />
The local neighborhood around the sun is assumed to have a particle density of 1 particle per cubic centimeter on average.<br />
<br />
=Interstellar Medium Composition=<br />
<br />
By mass, the interstellar medium is 70% hydrogen, 28% helium and 2% heavier elements.<br />
<br/>By number of atoms, the interstellar medium is 91% hydrogen, 8.9% helium and 0.1% heavier elements. <ref>https://en.wikipedia.org/wiki/Interstellar_medium<br />
<br/> Reference for ISM composition</ref><br />
<br />
There is also a dust component to the interstellar medium; the dust is considerably more dangerous than the diffuse gases as the particles are much larger. In the interstellar medium immediately around the Solar System, the mass of dust is ~0.5% of the mass of the gas, with the bulk of the particles ranging from 1E-18 to 1E-14 kg; however, the population of less-numerous but larger particles which pose the greatest hazard is not yet well known. <ref>H. Kruger et. al., "Sixteen Years of Ulysses Interstellar Dust Measurements in the Solar System. I. Mass Distribution and Gas-to-Dust Mass Ratio", Astrophysical Journal, October 20, 2015. https://ui.adsabs.harvard.edu/link_gateway/2015ApJ...812..139K/PUB_PDF </ref><br />
<br />
=Erosion=<br />
Erosion is not taken to be a significant component of the danger in interstellar shielding. <br />
For example, at 30% of c a ship's forward shield will encounter 1E+18 ISM particles per square centimeter per light-year traveled (ignoring differences in ISM density through the journey). <br />
<br/>A light year contains 946.1 quadrillion centimeters. In that length, there are thus 946.1 quadrillion cubic centimeters, and assuming a particle density of one per cubic centimeter, there are 9.467E+17 particles in that volume, rounding up to 1E+18.<br />
<br />
If each impact displaces 2 atoms from the shield, every light year traveled will cause the loss of 2E+18 atoms per square centimeter. For carbon shields, this is a loss of 40 micrograms per light year per square centimeter, ignoring that not all particles displaced will be lost to space, instead landing back on the shield.<br />
<br />
To get the mass loss rate, 2E+18 times the atomic mass of carbon gives 40 micrograms.<br />
<br />
This means that a 1 cm thick shield, can survive a trip of 56,250 light-years before being worn through. At high relativistic velocities however, space-time contraction is significant enough that the effective ISM density increases.<br />
<br />
The density of carbon, times the length, divided by mass loss rate gives the max trip length due to erosion.<br />
<br />
Note: In the Daedalus report, a number of other mass loss factors and average of a variety of material choices gave a mass loss rate of 80 milligrams per cubic centimeter per light year at a speed of 25% of c. <ref>https://bis-space.com/shop/product/project-daedalus-demonstrating-the-engineering-feasibility-of-interstellar-travel/<br />
<br/> Reference for the Daedalus report figure.</ref><br />
<br />
=Calculating the Heat Flux=<br />
Before we can begin calculating the flux, the mass density of the interstellar medium first be known. <br />
<br/>The mass density is given by:<br />
<br />
<math alt=>\rho=mp</math><br />
* where <math alt=>\rho</math> is the mass density of the interstellar medium<br />
* where <math alt=>m</math> is the mass of the particle<br />
* where <math alt=>p</math> is the particle density of the interstellar medium<br />
<br />
Since the interstellar medium is not homogeneous, a weighted average must be done per the composition of the interstellar medium. We can assume that all of the heavier elements are iron atoms as an approximation.<br />
<br />
<math alt=>1.475 \cdot 10^{-27} kg\,(average\,\,mass) = (0.7\cdot H+0.28\cdot He+0.015\cdot Fe)/3 </math><br />
* <math alt=>H,\,\,He,\,\,Fe</math> respectively refer to the atomic masses of hydrogen, helium and iron.<br />
<br />
Now we can finally calculate the flux with the relativistic flux equation <sup>[[https://www.galacticlibrary.net/mediawiki-1.36.1/index.php?title=Interstellar_Medium_Shielding&action=submit#Derivation_of_the_Relativistic_Flux_Equation 7]]:<br />
<br />
<math alt=>\phi=\gamma \rho vc(\gamma-1)c^2</math><br />
* where <math alt=>\phi</math> is the interstellar medium flux<br />
* where <math alt=>\rho</math> is the mass density of the interstellar medium<br />
* where <math alt=>v</math> is the velocity of the ship<br />
* where <math alt=>c</math> is the speed of light<br />
* where <math alt=>\gamma</math> is gamma, calculated with:<br />
: <math alt=>\gamma = 1/\sqrt{1-v^2}</math><br />
<br />
Assuming a particle density of 1 particle per cubic centimeter, at 41% of the speed of light, the flux is comparable to what Earth receives from the sun, already enough for ice to begin melting. At 80% of the speed of the light, the flux is 35,327 W/m<sup>2</sup>.<br />
<br />
=Calculating the Temperature of the Forward Shield=<br />
The temperature of the forward portion is given by the Stefan Boltzmann Law <ref> https://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law <br/>Stefan Boltzmann Law </ref>:<br />
<br />
<math alt=> \phi_e = A_d \epsilon \sigma_{sb} T^4 </math><br />
* where <math alt=> \phi_e </math> is the radiant power<br />
* where <math alt=>A_d</math> is the radiating/absorbing surface area<br />
* where <math alt=>\epsilon</math> is the emissivity of the radiating/absorbing material<br />
* where <math alt=>\sigma_{sb}</math> is the stefan boltzmann constant<br />
* where <math alt=>T</math> is the temperature of the material<br />
<br />
Now we rearrange the equation to solve for temperature:<br />
<br />
<math alt=> T= \sqrt[4]{\phi_e} / (\sqrt[4]{A_d} \sqrt[4]{\epsilon} \sqrt[4]{\sigma_{sb}})</math><br />
<br />
Before we can solve the equation for temperature, the radiant power must be obtained from the interstellar medium flux, given by:<br />
<br />
<math alt=> \phi_e = IA</math><br />
* where <math alt=>I</math> is the interstellar medium flux<br />
* where <math alt=>A</math> is the area exposed to the interstellar medium flux<br />
<br />
=Conclusions=<br />
<br />
A calculator for interstellar medium shielding is provided here:<br />
<br/>'''[link?]'''<br />
<br />
Below are two tables:<br />
<br />
==Velocity and Flux Table==<br />
(assuming particle density of 1 particle per cubic centimeter)<br />
{| class="wikitable sortable"<br />
|-<br />
! Ship Velocity !! Interstellar Medium <br/>Heat Flux<br />
|-<br />
| 0.1c || 20.122 W/m<sup>2</sup><br />
|-<br />
| 0.2c || 167.283 W/m<sup>2</sup><br />
|-<br />
| 0.3c || 603.483 W/m<sup>2</sup><br />
|-<br />
| 0.4c || 1579.947 W/m<sup>2</sup><br />
|-<br />
| 0.5c || 3549.648 W/m<sup>2</sup><br />
|-<br />
| 0.6c || 7451.7 W/m<sup>2</sup><br />
|-<br />
| 0.7c || 15,593.049 W/m<sup>2</sup><br />
|-<br />
| 0.8c || 35,326.58 W/m<sup>2</sup><br />
|-<br />
| 0.9c || 106,195.696 W/m<sup>2</sup><br />
|-<br />
| 0.99c || 1,698,225.484 W/m<sup>2</sup><br />
|-<br />
| 0.999c || 18,973,260.691 W/m<sup>2</sup><br />
|}<br />
<br />
==Example Shield Temperature Table==<br />
Assuming a cylindrical shape, radius of 10 meters and thickness of 1 meter<br />
{| class="wikitable sortable"<br />
|-<br />
! Ship Velocity !! Interstellar Medium <br/>Heat Flux !! Ice <br/>Temperature !! Graphite <br/>Temperature<br />
|-<br />
| 0.1c || 20.122 W/m<sup>2</sup> || 113.558 K || 123.207 K<br />
|-<br />
| 0.2c || 167.283 W/m<sup>2</sup> || 192.824 K <br/> Beyond <br/> sublimation point <ref> https://en.wikipedia.org/wiki/Frost_line_%28astrophysics%29<br />
<br/> Reference for ice sublimation </ref> || 209.208 K<br />
|-<br />
| 0.3c || 603.483 W/m<sup>2</sup> || 265.744 K || 288.325 K<br />
|-<br />
| 0.4c || 1579.947 W/m<sup>2</sup> || 338.033 K <br/> Too hot even at <br/>standard pressure|| 366.756 K<br />
|-<br />
| 0.5c || 3549.648 W/m<sup>2</sup> || || 449.017 K <br />
|-<br />
| 0.6c || 7451.7 W/<sup>2</sup> || || 540.481 K<br />
|-<br />
| 0.7c || 15,593.049 W/m<sup>2</sup> || || 650.054 K<br />
|-<br />
| 0.8c || 35,326.58 W/m<sup>2</sup> || || 797.521 K<br />
|-<br />
| 0.9c || 106,195.696 W/m<sup>2</sup> || || 1050.131 K<br />
|-<br />
| 0.99c || 1,698,225.484 W/m<sup>2</sup> || || 2099.982 K<br />
|-<br />
| 0.999c || 18,973,260.691 W/m<sup>2</sup> || || 3839.303 K <br/> Beyond <br/> sublimation point <ref> https://en.wikipedia.org/wiki/Carbon<br />
<br/> Reference for graphite sublimation. I assume the point occurs at a lower temperature due to lower pressure. </ref><br />
|}<br />
Notes: Ice has an emissivity of 0.97, while Graphite has an emissivity of 0.7<br />
<br />
The parameters vary with changing exposed area, area and emissivity, flux. What is clear here is that the interstellar medium flux can present a significant danger at high enough velocities as to sublimate (in the vacuum of space) ice, and at ever increasing velocity, even graphite. <br />
<br />
Therefore, care must be taken to shield your interstellar spacecraft from the flux if it is moving at a velocity high enough to heat the spacecraft with disastrous consequences.<br />
<br />
=Additional Reading=<br />
<br />
=Additional References=<br />
<references/><br />
<br />
==Derivation of the Relativistic Flux Equation==<br />
<ref group="RFE">https://en.wikipedia.org/wiki/Kinetic_energy#Relativistic_kinetic_energy_of_rigid_bodies<br />
<br/> Reference for the relativistic kinetic energy equation.</ref><br />
# The kinetic energy of an amount of mass is given by <math alt=>(\gamma -1)mc^2</math>. To get the power, the kinetic energy is differentiated against time and thus assuming constant velocity, obtain that with <math alt=>\dot{m}</math> (the mass flow rate); the power is given by <math alt=>(\gamma -1)\dot{m} c^2</math>.<br />
# The mass flow is given by the mass per volume encountered every second, doing this in the reference frame of the ship, the ISM density is length contracted to to <math alt=>\gamma \rho</math> and multiply by <math alt=>Av</math> where <math alt=>A</math> is area.<br />
# This yields <math alt=>P = \gamma \rho Av(\gamma -1)c^2</math>, to obtain the flux per unit area divide by <math alt=>A</math> and thereby cancel the <math alt=> A</math> in the earlier expression.<br />
<br />
'''Reference for the Derivation of the Relativistic Flux Equation'''<br />
<references group="RFE"/><br />
<br />
=Credit=<br />
To Tshhmon for writing the article<br />
* To lwcamp for helping with erosion calculation<br />
* To Kerr for the relativistic flux equation and derivation</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Q-drive&diff=782Q-drive2021-12-02T01:45:47Z<p>ROCKETGUY: Created page with "((STUB to get the link structure to work; will add shortly -- rocketguy ))"</p>
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<div>((STUB to get the link structure to work; will add shortly -- rocketguy ))</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Harvested_Energy,_Internal_Reaction_Mass&diff=781Propulsion - Harvested Energy, Internal Reaction Mass2021-12-02T01:45:23Z<p>ROCKETGUY: </p>
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<div>Energy is gathered from the surrounding environment, and used to expel reaction mass for thrust.<br />
<br />
If the power supply is solar panels gathering electricity, you have a solar-electric rocket system. The power supply dominates vehicle performance in this case, and any of a variety of electric thrusters may be used, such as:<br />
<br />
[[Ion Engine]]<br />
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If the power supply is a 'windmill', extracting energy from the flow of surrounding space plasma past the vehicle, you have:<br />
<br />
[[q-drive]]</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Internal_Energy,_Internal_Reaction_Mass&diff=780Propulsion - Internal Energy, Internal Reaction Mass2021-12-02T01:42:42Z<p>ROCKETGUY: </p>
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<div>The classic rocket. The vehicle carries a supply of one or more substances that, singly or in combination, release energy (the 'fuel') and the energy is used to push a stream of mass (the 'reaction mass') out of the vehicle -- exhaust goes one way, vehicle accelerates the other way. It is a common case for the fuel and the reaction mass to actually be the same thing (as in a chemical rocket, where you burn two chemicals and the reaction products form the reaction mass) in which case they are more commonly called 'propellant'. But they may be separate things; in a nuclear-thermal rocket, the 'fuel' is the fissionable material in the reaction, and the 'reaction mass' is pushed through the reactor, made hot, and ejected through a nozzle.<br />
<br />
Examples include:<br />
<br />
[[Hydrolox Engine]]<br />
<br />
Nuclear-Thermal Rocket<br />
<br />
[[Photon Rocket]]<br />
<br />
Onboard power such as a nuclear reactor can also be used to supply electricity, which can be used to power various forms of electric rocket such as:<br />
<br />
[Ion Engine]</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Internal_Energy,_Internal_Reaction_Mass&diff=779Propulsion - Internal Energy, Internal Reaction Mass2021-12-02T01:42:18Z<p>ROCKETGUY: </p>
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<div>The classic rocket. The vehicle carries a supply of one or more substances that, singly or in combination, release energy (the 'fuel') and the energy is used to push a stream of mass (the 'reaction mass') out of the vehicle -- exhaust goes one way, vehicle accelerates the other way. It is a common case for the fuel and the reaction mass to actually be the same thing (as in a chemical rocket, where you burn two chemicals and the reaction products form the reaction mass) in which case they are more commonly called 'propellant'. But they may be separate things; in a nuclear-thermal rocket, the 'fuel' is the fissionable material in the reaction, and the 'reaction mass' is pushed through the reactor, made hot, and ejected through a nozzle.<br />
<br />
Examples include:<br />
[[Hydrolox Engine]]<br />
Nuclear-Thermal Rocket<br />
[[Photon Rocket]]<br />
<br />
Onboard power such as a nuclear reactor can also be used to supply electricity, which can be used to power various forms of electric rocket such as:<br />
[Ion Engine]</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Beamed_Energy,_Beamed_Reaction_Mass&diff=778Propulsion - Beamed Energy, Beamed Reaction Mass2021-12-02T01:33:27Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Beamed_Energy,_Harvested_Reaction_Mass&diff=777Propulsion - Beamed Energy, Harvested Reaction Mass2021-12-02T01:33:06Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Beamed_Energy,_Internal_Reaction_Mass&diff=776Propulsion - Beamed Energy, Internal Reaction Mass2021-12-02T01:32:43Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Harvested_Energy,_Beamed_Reaction_Mass&diff=775Propulsion - Harvested Energy, Beamed Reaction Mass2021-12-02T01:32:26Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Harvested_Energy,_Harvested_Reaction_Mass&diff=774Propulsion - Harvested Energy, Harvested Reaction Mass2021-12-02T01:32:07Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Harvested_Energy,_Internal_Reaction_Mass&diff=773Propulsion - Harvested Energy, Internal Reaction Mass2021-12-02T01:31:43Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Internal_Energy,_Beamed_Reaction_Mass&diff=772Propulsion - Internal Energy, Beamed Reaction Mass2021-12-02T01:31:29Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUYhttps://www.galacticlibrary.net/mediawiki-1.41.1/index.php?title=Propulsion_-_Internal_Energy,_Harvested_Reaction_Mass&diff=771Propulsion - Internal Energy, Harvested Reaction Mass2021-12-02T01:31:13Z<p>ROCKETGUY: Created page with "(STUB: Description of type and links to pages of example propulsion systems)"</p>
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<div>(STUB: Description of type and links to pages of example propulsion systems)</div>ROCKETGUY