Ion Engine

Ion Engine Summary Quick Facts Ion engines are a form of electric propulsion using ions (and more) as propellant. They produce low, but constant, thrust for kilowatts of electrical power. The low particular mass of their propellant results in high exhaust velocities, and high Specific Impulse, giving them impressive amounts of delta-v for a given mass of propellant. Some more speculative designs can achieve higher thrust.

They are suited to long-duration missions and are ideal for Ward-spiral type orbital transitions. They are a common propulsion choice for satellites and as interplanetary transit engines for spacecraft. They are a poor choice for any use that requires high thrust and short impulsive burns, and cannot make Hohmann orbit transfers efficiently. A Example Table For Engine Performance for Busek RF Gridded Electrostatic Ion Thrusters:

(table)

Introduction Most ion engines’ mechanisms involve knocking electrons out from a neutral gas, ionizing it, and using electromagnetic repulsion to accelerate it out of the engine.

Ionizing neutral gas requires significant power - a typical ratio requires kilowatts of power for a few hundred millinewtons of thrust. 100 millinewtons of thrust is comparable to the force exerted by about three grapes (10 grams) sitting on a table. Even small chemical rockets, such as the 30 kg wet-mass Super Loki sounding rocket’s motor produces a force of 9,550 Newtons, huge in comparison.

Ion drives have an advantage in exhaust velocity (20,000 to 50,000 meters per second is a useful range), which translates into specific impulse (Isp) of 2039-5099 seconds. By way of comparison, the Raptor engine used by SpaceX’s Starship has an exhaust velocity of 3,200 m/sec, and an Isp of 330 seconds. Since ion drives provide higher exhaust velocities, this allows for higher delta-v from the same mass-ratio, or the same delta-v for a smaller mass-ratio.

The tradeoff of high exhaust velocity and low thrust makes ion engines ideally suited to orbit-to-orbit missions; for anything that requires higher thrust, like getting things out of even a modest gravity well, you’ll need a second rocket with higher thrust. Ion engines are well-suited for satellite orbital maneuvering, and long duration spacecraft missions between planets. Ion drives work best for Ward-spiral style interplanetary transits. Lowest delta-V Hohmann transfers require a short burn of high thrust to work; Ion drives will get your payload or passengers to their destination with a smaller fuel fraction and/or less time. Ion engines do not provide enough thrust for brachistochrone transfers.

D Ion Engine Design There are three main types of ion engines, electromagnetic thrusters [or plasma propulsion engines] (using the Lorentz force), electrostatic thrusters (using the Coulomb force) and radioisotope ion thrusters.

[links here to some education articles about the lorentz and coulomb forces?] Electrostatic Ion Thruster: Electrostatic thrusters accelerate ions through the Coulomb force along an electric field. Electrons are stored temporarily and then re-injected into the ionized gas through a neutralizer, so that the gas can disperse freely through space without interacting with the thruster itself, though some designs forego the neutralizer in favor of something else.

There are 4 main types of electrostatic ion thrusters in summary:

The gridded electrostatic ion thruster [link], which uses an electron gun to ionize the propellant and electrostatic grids to accelerate the propellant.

The electrostatic colloid thruster [link], which uses droplets of ionic liquid produced by an electrospray ionization process. The radioisotope electrostatic colloid thruster [link], which uses a beta emitter to generate huge voltages for the electrostatic colloid thruster without power conversion. (when the radioisotope ion thruster article is made, make sure this is still conformant with it, and use it as context for the link to encourage someone to go read that article.) Further reading on radioisotope ion thrusters: Bookmark

The hall effect ion thruster [link], which uses a voltage between a cylindrical anode and a negatively charged plasma cathode. Propellant is introduced and ionized at the anode, and accelerates after reaching the cathode, picking up electrons and neutralizing.

The field emission electric propulsion thruster [link], which extracts ions from the tips of conical cusps on a liquid metal (usually caesium or iridium), forming due to the electric field in the thruster. The ions are accelerated by the field and are neutralized by an external source of electrons. Electromagnetic Ion Thruster / Plasma Propulsion Engine: Perhaps the largest family of ion thrusters, the main principles here are that both free electrons and ions are accelerated in contrast to the electrostatic ion thruster. Some use the Lorentz force (the kind that propels projectiles in a railgun) to accelerate all particles in the same direction, though other designs do not use the Lorentz force (ponderomotive force, magnetic nozzles, confinement escape and heating).

Unlike electrostatic ion thrusters, the electric field doesn’t have to be in the direction of the acceleration. Since free electrons are part of the exhaust, they are also called plasma propulsion engines.

There are 8 main types of electromagnetic ion thrusters in summary:

The pulsed plasma thruster [link], which uses an electric arc to vaporize propellant (usually a block of teflon or PTFE) into plasma. Due to the force of the vaporization, the propellant plasma moves between an anode and cathode, conducting electricity between them and thus is accelerated through the Lorentz force.

The ELF pulsed plasmoid thruster [link], which generates a plasmoid (smoke torus of plasma) from propellant and expands it down an electrically conducting nozzle, with directed kinetic energy (KE) generated from image currents in the nozzle due to interactions with the plasmoid. It is also electrodeless. The alfvenic pulsed plasmoid thruster [link], which uses magnetic reconnection (the same kind that causes solar flares) to spontaneously generate low-temperature plasmoids. The outgoing plasmoid is an alfvenic outflow from the reconnection site, and so depends on the magnetic field strength rather than the propellant ion species mass for thrust.

The pulsed inductive thruster [link], which uses pulses instead of continuous thrust and can run on high power levels (on the order of megawatts). Using a coil and conical propellant emitter, large built up charges are released behind the coil. This creates an electric current, and induces a magnetic field in the propellant. Together, the electric and magnetic fields accelerate the propellant through the Lorentz force.

The magnetoplasmadynamic thruster [link], which ionizes the propellant via the electric field between the anode and the cathode. The plasma conducts electricity between the anode and cathode, closing the circuit, and creating a new magnetic field, which with the electric field, accelerates the propellant through the Lorentz force.

The electrodeless plasma thruster [link], which does not use anode or cathode electrodes. Propellant is ionized through electromagnetic waves, and oscillating electric and magnetic fields in another chamber accelerate the propellant due to the ponderomotive force. The separation of ionization and acceleration allows for throttling.

The helicon double-layer thruster [link], which uses radio waves emitted from an antenna to ionize the propellant into a plasma and excite a helicon wave (low frequency EM waves in bounded plasma) in the plasma, further heating it. A special kind of magnetic nozzle using the propellant and solenoids accelerates the propellant.

The VASIMR [link], which functions similarly to the helicon double-layer thruster, however it uses a second step to heat the plasma to 1 million K through ion cyclotron resonance heating. It uses superconductors to magnetically confine the plasma, and thrust is generated when the plasma escapes confinement at the exhaust. VASIMR has mechanisms that allow it to trade lower exhaust velocity and Isp for higher thrust.

The microwave electrothermal thruster [link], which uses microwaves to ionize a volume of the propellant. As the ionized propellant gets closer to the nozzle, it’s mixed with neutral propellant reserves, which allows the remaining energy to be absorbed, and converted into thrust. By using different types of neutral propellant, thrust levels can be adjusted, giving different ‘gears’ of thrust, though this is still very low by chemical rocket standards.

E Basic Electrostatic Ion Thruster Mathematics The energy of a charged particle gains from moving through a potential difference of V voltage is given by: E = qV Where q is the charge in coulombs, V the voltage

With the mass of the charged particle known, one can calculate the exhaust velocity of the electrostatic ion thruster with: v = (sqrt(2)*sqrt(E))/sqrt(M) Where v is the velocity and M is the mass of the charged particle. This is the E=½Mv^2 equation rearranged to solve for velocity.

The thrust of the electrostatic ion thruster is given by: N = vṁ Where N is the thrust and ṁ is the propellant mass flow rate. ṁ = N/v is the equation rearranged to solve for propellant mass flow rate.

The drive power of the electrostatic ion thruster can be calculated two ways, most simply with 1: P = vN½ P = AV Where A is the amperage

The amperage needed is given by: A = ṁ/nq Where n is the molar mass of the propellant. Ion Thruster Propellant Choice Considerations: Ionizing the propellant can be a significant source of power consumption. Therefore, the ideal propellant choice is easy to ionize and has a high mass to ionization energy ratio. Another consideration is erosion, which limits the life-time of the ion thruster. Some otherwise excellent propellants are toxic (mercury) or toxic and cancerous (cadmium).

Another consideration is being able to source propellants without significant processing overhead. An advantage of the colloid thrusters is that finely ground regolith can be used as a propellant.

Balancing these considerations (except the in situ refueling one) have led to xenon and krypton being common choices for electrostatic ion thrusters. They are high Z (proton number, therefore having more mass per particle), inert and easy to ionize. For gridless ion thrusters, like the Hall effect ion thrusters, bismuth and iodine are also available.

Plasma propulsion engines are quite flexible in their choice of propellants, more so than electrostatic ion thrusters. Pulsed inductive thrusters commonly use ammonia gas. Magnetoplasmadynamic thrusters can use hydrogen, argon, ammonia or nitrogen as propellant. Ammonia is a common volatile, making in-situ refueling possible. History / Development [This section should be in its own specific history article, so a link would be provided here instead of the text]

The history of the concept of ion engines dates all the way back to 1911, by Konstantin Tsiolkovsky (yes that guy), the first person to publish a paper on the subject. He recommended it for near-vacuum conditions at altitude, however thrust was also seen with ionized air streams at atmospheric pressure.

Hermann Oberth later reignited conversation with his “Ways to Spaceflight” book in 1923, talking about its advantages in mass ratios and mass savings for payloads. He predicted its usage in spacecraft propulsion and attitude control, as well advocated for the electrostatic kind of ion thruster.

By 1959, a gridded electrostatic ion thruster had been built by Harold R. Kaufman at NASA Glenn Research Facilities, using mercury as its propellant. Suborbital tests were conducted throughout the 1960s, and an orbital test was done in 1970, the SERT-1 and SERT-2 demonstration missions.

The Hall effect thruster design, studied by both the United States and the Soviet Union, had been used by the Soviets from 1972 to late 1990s for attitude control. It is estimated that 100-200 engines were used on Soviet and Russian missions. By 1992, the designs had been brought to the West, thanks to a team of electric propulsion specialists from the Ballistic Missile Defense Organization (BMDO) visiting Soviet labs.

G Worked Examples Electrostatic Ion Thruster: Gridded Electrostatic Ion Thruster - NSTAR, NeXT

Colloid Thruster - Busek Electrospray Thruster Bookmark

Hall Effect Ion Thruster - PPS-1350, on the SMART-1 mission. AEPS

Field Emission Electric Propulsion Thruster - IFM Nano Electromagnetic Ion Thruster / Plasma Propulsion Engine: Pulsed Inductive Thruster - Several laboratory examples built by NGST.

Magnetoplasmadynamic Thruster - MPD thrusters flown on EPEX (Japan) mission

Helicon Double-Layer Thruster - Prototype built by Dr Christine Charles

VASIMR - VX-10, VX-50, VX-100, VX-200. Built by Ad Astra. (not flown)

H Additional Sections I Additional Reading Links from ion engine design section Links from history section

https://cdn.discordapp.com/attachments/886253956777533450/887757110501318757/POTENTIALITIES_OF_THE_RADIOISQTOPE_ELECTROSTATIC_PROPULSION_SYSTEM.pdf Further reading for the radioisotope ion thruster.

J Additional References https://satsearch.co/products/busek-bit-3 http://www.projectrho.com/public_html/rocket/enginelist.php https://www.busek.com/bet-300p

https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/NASA_Solar_Technology_Application_Readiness https://www1.grc.nasa.gov/space/sep/gridded-ion-thrusters-next-c/

https://en.wikipedia.org/wiki/Gridded_ion_thruster https://en.wikipedia.org/wiki/Hall-effect_thruster https://en.wikipedia.org/wiki/Field-emission_electric_propulsion

https://en.wikipedia.org/wiki/Pulsed_inductive_thruster https://en.wikipedia.org/wiki/Magnetoplasmadynamic_thruster https://en.wikipedia.org/wiki/Electrodeless_plasma_thruster https://en.wikipedia.org/wiki/Helicon_double-layer_thruster https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magnetoplasma_Rocket https://en.wikipedia.org/wiki/Microwave_electrothermal_thruster

https://en.wikipedia.org/wiki/SERT-1 https://spacemath.gsfc.nasa.gov/weekly/5Page64.pdf Electrostatic Ion Engine Math

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Credit: To Tshhmon for writing the article To SOPHONT SIMP for test-reading To KRKIIIIII for test-reading To AdAstraGames for test-reading and some editing To MatterbeamToughSF for checking sources and details