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"We look for things, things to make us go".

Pakled Captain, Star Trek: The Next Generation -- Samaritan Snare

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.

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.

To understand the basics of propulsion, you have to take three basic laws of physics as a given:

  • Conservation of Energy (First law of thermodynamics)
  • Conservation of Momentum (action = reaction)
  • Energy flows from high temperature (low entropy) sources to low temperature (high entropy) states (Second law of thermodynamics)

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.

The kinetic energy of a moving spacecraft is . 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.

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.

Momentium rescale.gif

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.

A table with some examples of each type:

Morphological Classification of Propulsion Systems
Energy Source Source of Reaction Mass
Internal External,
Internal Chemical rockets,
Nuclear rockets
Propellers 'seeded' ramjet with
onboard antimatter
solar rocket
Magnetic sails,
Wind-Pellet Shear Sailing
Laser-driven rocket Laser-driven ramjet photon beam sails,
particle beam magsail

(A map of all the possibilities of important properties of a system like this is called a "morphological analysis" or a Zwicky box [1])

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).

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.

Some examples of each type, to guide the reader:

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.

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.

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

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.

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.

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.

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.

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.

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

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

  1. 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