Section 2.6 - Ion and Plasma Engines

Combustion type engines are limited in temperature, and thus gas exhaust velocity, either by the chemical energy of the fuel, or the melting point of the engine materials. Ion and plasma type engines bypass these limits and reach much higher velocities by one or more of: using external energy sources, lower energy density to limit engine heating, or using non-material containment such as magnetic fields to direct the flow. These methods also tend to make ion and plasma engines heavy compared to combustion engines, so they are not used for launch from high gravity bodies. Rather they operate where there is already a low-g environment in orbit or away from gravity wells.

The ion and plasma types both involve high energy particles. The distinction is one of density. In the former the particles act individually, while in the latter they are dense enough to require treating them as a flowing medium.

Ion Engines

48 Electrostatic Ion Engine

Alternate Names:

Type:

Description: Electrostatic ion thrusters generally work by first ionizing the propellant, then using a voltage gradient to accelerate the ions. They are capable of high exhaust velocities (30-50 km/s), but relatively low energy density and thrust in order to prevent overheating and erosion of the screens creating the voltage difference. Since the voltage gradient is uniform between two flat screens, the ion beam is well directed without a nozzle. Ejecting only positive ions would produce a net charge on the engine, so ion engines also have an electron gun to balance the charges emitted.

All ion engines require an external power source. A number of variations for the power source have been proposed, but only Solar-Electric has actually been used. Since ionization merely removes an electron, but does not accelerate the ion, it represents an efficiency loss for the engine. Therefore ion engines tend to prefer fuels with high atomic weight relative to ionization energy. Since most high atomic weight elements are solids, Mercury and Xenon have often been chosen, and sometimes Argon if operating in a cold environment or reaching a very high exhaust velocity is desired.

Status: Ion thrusters have been used in recent years on some communications satellites and interplanetary spacecraft.

Variations:

• 48a Solar-Electric Ion - Sunlight is converted to electricity by a photovoltaic array. The electricity is used to ionize and accelerate the propellant by electrostatic voltage.
• 48b Thermoelectric Ion - Radioactive isotope decay produces heat. Heat is converted to electricity by semiconductors. Electricity ionizes and accelerates atoms in the engine.
• 48c Laser-Electric Ion - A laser tuned to the optimum absorption wavelength of photovoltaic cells supplies power. The cells convert laser light to electricity, which is used to power the ion engine. The ion engine then ionizes and accelerates the propellant. The reason for using a laser is to get higher power levels than natural sunlight provides.
• 48d Microwave-Electric Ion - A microwave receiving antenna, or Rectenna, on the spacecraft converts microwaves to electricity. Electricity is used to ionize and accelerate atoms. Microwave antennas can be low mass, but the operating distance is limited by the ability to focus the long wavelength.
• 48e Nuclear-Electric Ion - A nuclear reactor generates heat, which is converted to electricity in thermoelectric or turbine/generator cycles. Electricity is used to ionize propellant and accelerate it by electrostatic voltage. This is more suited to high power applications, and outer Solar System locations where sunlight is weak.
• 48f Electrospray Thruster - An ionic liquid is emitted from a capillary nozzle similar to an inkjet printer. A high voltage electrode then accelerates the charged droplets. This is suited to very small thrust levels for small spacecraft, or for steering, since lithography can be used to create microscopic nozzles.
• 48g Colloid Thruster - Rather than individual ions, this method accelerates relatively massive colloidal charged particles.

References:

Solar-Electric, 1970s

• Loeb, H. W. "Electric Propulsion Technology Status and Development Plans - European Programs (Space Vehicles)", J. Spacecraft and Rockets , vol 11 no 12 pp 821-8, Dec. 1974.
• Mutin, J.; Tatry, B. "Electric Propulsion in the Field of Space", Acta Electron. (France) vol 17 no 4 pp 357-70, Oct. 1974 (in French).
• Byers, D. C.; Rawlin, V. K. "Critical Elements of Electron- Bombardment Propulsion for Large Space Systems", J. Spacecraft and Rockets vol 14 no 11 pp 648-54, Nov. 1977.
• Parkash, D. M. "Electric Propulsion for Space Missions", Electr. India vol 19 no 7 pp 5-15, 15 April 1979.

Solar-Electric, 1980s

• Kaufman, H. R. "Performance of Large Inert-Gas Thrusters", AIAA paper number 81-0720 presented at 15th International Electric Propulsion Conference, Las Vegas, Nevada, 21-23 April 1981.
• Clark, K. E.; Kaufman, H. B. "Aerospace Highlights 1981: Electric Propulsion", Astronautics and Aeronautics, v 19 no 12 pp 58-59, 1981.
• Zafran, S. et al "Aerospace Highlights 1982: Electric Propulsion", Astronautics and Aeronautics, v 20 no 12 pp 71-72, 1982.
• James, E.; Ramsey, W., Sr.; Steiner, G. "Developing a Scaleable Inert Gas Ion Thruster", AIAA paper number 82-1275 presented at AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, OH, 21- 23 June 1982.
• Anon. "Ion Propulsion Engine Tests Scheduled", Aviation Week and Space Technology, v 116 no 26 pp 144-5, 1982.
• Brophy, J. R.; Wilbur, P. J. "Recent Developments in Ion Sources for Space Propulsion", Proceedings of the Intl. Ion Engineering Congress vol 1 pp 411-22, 1983.
• Bartoli, C. et al "Recent Developments in High Current Liquid Metal Ion Sources for Space Propulsion", Vacuum vol 34 no 1-2 pp 43-6, Jan. -Feb. 1984.
• Jones, R. M.; Poeschel, R. L. "Primary Space Propulsion for 1995-2000 - Electrostatic Technology Applications" AIAA/SAE/ASME 20th Joint Propulsion Conference, AIAA paper number 84-1450, 1984.
• Imai, R.; Kitamura, S. "Space Operation of Engineering Test Satellite -III Ion Engine", Proceedings of JSASS/AIAA/DGLR 17th Intl. Electric Propulsion Conf. pp 103-8, 1984.
• Bartoli, C. et al "A Liquid Caesium Field Ion Source for Space Propulsion", J. Phys. D vol 17 no 12 pp 2473-83, 14 Dec. 1984.
• Voulelikas, G. D. "Electric Propulsion: A Review of Future Space Propulsion Technology" Communications Research Centre, Ottawa, Ontario, report number CRC-396, October 1985.
• Nakamura, Y.; Kuricki, K. "Electric Propulsion Test Onboard the Space Station", Space Solar Power Review vol 5 no 2 pp 213-9, 1985.
• Rawlin, Vincent K; Patterson, Michael J. "High Power Ion Thruster Performance", NASA Technical Memorandum 100127, 1987.
• Mitterauer, J. "Liquid Metal Ion Sources as Thrusters for Electric Space Propulsion", J. Phys. Colloq. (France) vol 48, no C-6, pp 171-6, Nov. 1987.
• Mitterauer, J. "Field Emission Electric Propulsion - Emission Site Distribution of Slit Emitters", IEEE Trans. on Plasma Sci. vol PS-15, pp 593-8, Oct. 1987.
• Stuhlinger, E. et al "Solar-Electric Propulsion for a Comet Nucleus Sample Return Mission" presented at 38th Congress of the International Astronautical Federation, Brighton, England, 10 October 1987.

Laser-Electric Ion:

• Maeno, K. "Advanced Scheme of CO2 Laser for Space Propulsion", Space Solar Power Review vol 5 no 2 pp 207-11, 1985.

Microwave-Electric Ion:

• Nordley, G. D.; Brown, W. C. "Space Based Nuclear-Microwave Electric Propulsion", 3rd Symposium on Space Nuclear Power Systems, Albuquerque, New Mexico, 13 January 1986, pp 383-95, 1987.

Nuclear-Electric Ion:

• Reichel, R. H. "The Air-Scooping Nuclear-Electric Propulsion Concept for Advanced Orbital Space Transportation Missions", J. British Interplanetary Soc. vol 31 no 2 pp 62-6, Feb. 1978.
• Hsieh, T. M.; Phillips, W. M. "An Improved Thermionic Power Conversion System for Space Propulsion", Proceedings of the 13th Intersociety Energy Conversion Engineering Conference pp 1917-1923, 1978.
• Ray, P. K. "Solar Electric versus Nuclear Electric Propulsion in Geocentric Space", Trans. Am. Nucl. Soc. vol 39 pp 358-9, Nov.-Dec. 1981.
• Powell, J. R.; Botts, T. E.; Myrabo, L. N. "Annular Bed Nuclear Power Source for Electric Thrusters", AIAA paper number 82-1278 presented at AIAA/SAE/ ASME 18th Joint Propulsion Conference, Cleveland, Ohio, 21-23 June 1982.
• Powell, J. R.; Boots, T. E. "Integrated Nuclear Propulsion/Prime Power Systems", AIAA paper number 82-1215 presented at AIAA/SAE/ASME 18th Joint Propulsion Conference, Cleveland, Ohio, 21-23 June 1982.
• Buden, D.; Garrison, P. W. "Space Nuclear Power Systems and the Design of the Nuclear Electric Propulsion OTV", presented at AIAA/SAE/ASME 20th Joint Propulsion Conference, AIAA paper number 84-1447, 1984.
• Cutler, A. H. "Power Demands for Space Resource Utilization", Space Nuclear Power Systems 1986 pp 25-42.

Electrospray Thruster:

• Micromachined propulsion systems for very small satellites, EFPL Microsystems for Space Technologies Lab.

Plasma Engines

49 Arc Jet Engine

Alternate Names:

Type:

Description: Sunlight is converted to electricity by a photovoltaic array. The electricity is arced through a propellant stream, heating it to a plasma. The propellant is then expanded through a nozzle. No specific protection from heating is used in the nozzle. The arc jet method uses relatively low energy density and thrust levels, and a thick-walled refractory metal for the chamber and nozzle. Exhaust velocities of about 6-8 km/s are reached with liquid fuels like Hydrazine or Ammonia.

Status: Arc Jets have been used on spacecraft, and are commonly used on Earth as plasma torches for cutting metals.

Variations:

• Pulsed Plasma Thruster - operates by ablating Teflon material between two electrodes by means of an electric arc. The Lorentz force accelerates the material out as thrust.
• Vacuum Arc Thruster - operates by vaporizing and ionizing cathode material by means of a vacuum arc. The plasma accelerates outward as thrust.
• Magnetoplasmadynamic Thruster - operates via electric arc between central cathode and shell anode. Flowing gas between them is ionized and accelerated by the Lorentz force from current and field interactions.

References:

• Hardy, Terry L.; Curran, Francis M. "Low Power DC Arcjet Operation with Hydrogen/Nitrogen/Ammoinia Mixtures", NASA Technical Memorandum 89876, 1987.
• Stone, James R.; Huston, Edward S. "NASA/USAF Arcjet Research and Technology Program", NASA Technical Memorandum 100112, 1987.
• Kagaya, Y. et al "Quasi-steady MPD Arc-jet for Space Propulsion", Symposium for Space Technology and Science, Tokyo, Japan, 19 May 1986, pp 145-154, 1986.
• Manago, Masata et al "Fast Acting Valve for MPD Arcjet", IHI Engineering Review, v 19 no 2 pp 99-100, April 1986.
• Pivirotto, T. J.; King, D. Q. "Thermal Arcjet Technology for Space Propulsion", Chemical Propulsion Information Agency, Laurel, Maryland, 1985.

50 Electron Beam Heated Plasma Engine

Alternate Names:

Type:

Description: A high voltage (hundreds of keV) electron beam is injected axially into a propellant flow. The electron beam heats the flow to plasma temperatures, which produces high specific impulse. Cool gas is injected along the chamber walls to provide film cooling and protect the chamber from the very high temperature plasma.

Status:

Variations:

References:

51 Microwave Heated Plasma Engine

Alternate Names: Electron-Cyclotron Absorption Rocket, Variable Specific Impulse Magnetoplasma Rocket (VASIMR), Helicon plasma thruster

Type:

Description: Partially ionized gas directly absorbs microwaves, becoming hot, then expands through rocket nozzle. To keep the hot plasma from damaging the engine, a magnetic field is used for confinement, often using superconductors for efficiency. Current versions use Argon as propellant, but other gases should function with tuning. It may be possible to use unprocessed rock as fuel.

Status: VASIMR is in ground testing

Variations:

References:

• Personal communication from Ad Astra Rocket Company in reference to alternate fuels, 27 Mar 2012. Extensive publications about their thruster are linked to the main description page.

52 Fusion Heated Plasma Engine

Alternate Names:

Type:

Description: The exhaust of a pure fusion rocket is a thin, extremely hot plasma which gives very high performance. If higher thrust is needed, Hydrogen can be mixed with the plasma. This increases thrust at the expense of performance. By varying the mixture ratio, the performance vs thrust can be adjusted as needed during a mission.

Status: Fusion engines await practical fusion reactors, which are still in the research stage as of 2012.

Variations:

• Reactor Leakage Mixed - Some of the fusion reaction plasma leaks past the containment fields. This may be a mix of reacted particles, and un-reacted fuel. The leakage can be directly used as engine exhaust, or further mixed with additional material for higher mass flow/thrust at lower performance.
• Plasma Kernel Mixed - The fusion core plasma may be intentionally seeded with non-reacting material, which gets heated as part of the reaction. A certain percentage is directed out of the core for thrust, balanced by new fuel added to the core. The possible advantage of this method is eliminating the mixing problem of adding mass after the reactor. Fusion engines produce very high exhaust velocities, so trying to mix more matter into the stream may require very large engine components. Otherwise the stream may simply be gone before it has a chance to mix.

References:

53 Antimatter-Heated Plasma Engine

Alternate Names:

Type:

Description: The exhaust of a pure antimatter rocket is charged particles and gamma rays. This gives an extraordinarily high exhaust velocity, but relatively low thrust. If higher thrust is needed, hydrogen can be mixed with the plasma, at at the expense of performance. This likely requires a large magnetic bottle to contain the particles and Hydrogen ions (protons) long enough to mix.

Status:

Variations:

References:

53 Electrodeless Lorentz Force (ELF) Thruster

Alternate Names:

Type:

Description: The ELF-160A thruster creates a high-density, magnetized plasmoid known as a Field Reversed Configuration (FRC) employing a Rotating Magnetic Field (RMF). The RMF driven azimuthal currents, coupled with the enhanced axial magnetic field gradient produced by the FRC inside the flux preserving conical thruster, produce a large axial JθxBr force that accelerates the plasmoid to high velocity. The ELF-160A is completely electrodeless, the propellant is magnetically isolated from the thruster body, quasi-neutral, and there is zero contact between high temperature propellant and the thruster.

Status: Active Development under Department of Defense contract.

Variations: One variation is the ElectroMagnetic Plasmoid Thruster (EMPT), an electrodeless pulsed plasma thruster that generates and accelerates a Field Reversed Configuration (FRC) to produce thrust.

References:

• Slough, J.; Kirtley, D.; Weber T. "Pulsed Plasmoid Propulsion: The ELF Thruster", presented at the 31st International Electric Propulsion Conference, September 20-24 2009
• Pancotti, A. P.; Little, J. M.; Neuhoff, J. S.; Cornella, B. M.; Kirtley D. E.; Slough J. T. "Elecrodeless Lorentz Force (ELF) Thruster for ISRU and Sample Return Mission", presented at Joint Conference of 30th International Symposium on Space Technology and Science, 34th International Electric Propulsion Conference and 6th Nano-satellite Symposium, July 4-10 2015

Further Reading

• Wikipedia: plasma propulsion engine