Electrically powered spacecraft propulsion

An electrically-powered spacecraft propulsion system uses electrical, and possibly also magnetic fields, to change the velocity of a spacecraft. Most of these kinds of spacecraft propulsion systems work by electrically expelling propellant (reaction mass) at high speed.[1]

6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory

Electric thrusters typically use much less propellant than chemical rockets because they have a higher exhaust speed (operate at a higher specific impulse) than chemical rockets.[2] Due to limited electric power the thrust is much weaker compared to chemical rockets, but electric propulsion can provide a small thrust for a long duration of time.[3] Electric propulsion can achieve high speeds over long periods and thus can work better than chemical rockets for some deep space missions.[2]

Electric propulsion is now a mature and widely used technology on spacecraft. Russian satellites have used electric propulsion for decades[4] and it is predicted that by 2020, half of all new satellites will carry full electric propulsion.[5] As of 2019, over 500 spacecraft operated throughout the Solar System use electric propulsion for station keeping, orbit raising, or primary propulsion.[6] In the future, the most advanced electric thrusters may be able to impart a Delta-v of 100 km/s, which is enough to take a spacecraft to the outer planets of the Solar System (with nuclear power), but is insufficient for interstellar travel.[2][7] An electric rocket with an external power source (transmissible through laser on the photovoltaic panels) has a theoretical possibility for interstellar flight.[8][9] However, electric propulsion is not a method suitable for launches from the Earth's surface, as the thrust for such systems is too weak.

History

The idea of electric propulsion for spacecraft dates back to 1911, introduced in a publication by Konstantin Tsiolkovsky.[10] Earlier, Robert Goddard had noted such a possibility in his personal notebook.[11]

Electrically-powered propulsion with a nuclear reactor was considered by Dr. Tony Martin for interstellar Project Daedalus in 1973, but the novel approach was rejected because of very low thrust, the heavy equipment needed to convert nuclear energy into electricity, and as a result a small acceleration, which would take a century to achieve the desired speed.[12]

The demonstration of electric propulsion was an ion engine carried on board the SERT-1 (Space Electric Rocket Test) spacecraft,[13][14] launched on 20 July 1964 and it operated for 31 minutes.[13] A follow-up mission launched on 3 February 1970, SERT-2, carried two ion thrusters, one operated for more than five months and the other for almost three months.[13][15][16]

By the early 2010s, many satellite manufacturers were offering electric propulsion options on their satellites—mostly for on-orbit attitude control—while some commercial communication satellite operators were beginning to use them for geosynchronous orbit insertion in place of traditional chemical rocket engines.[17]

Types

Ion and plasma drives

These types of rocket-like reaction engines use electric energy to obtain thrust from propellant carried with the vehicle. Unlike rocket engines, these kinds of engines do not necessarily have rocket nozzles, and thus many types are not considered true rockets.

Electric propulsion thrusters for spacecraft may be grouped into three families based on the type of force used to accelerate the ions of the plasma:

Electrostatic
Main article: Ion thruster

If the acceleration is caused mainly by the Coulomb force (i.e. application of a static electric field in the direction of the acceleration) the device is considered electrostatic.

Ion drives are essentially particle accelerators that shoot streams of particles out the rocket's exhaust jet. Particle accelerators currently in use are not for propulsion; their main uses are in research and industry for the purposes of producing effects for measurements of scientific interest (as for elementary physics studies like at CERN's Large Hadron Collider), or producing effects on a target such as in nuclear spallation or ion implantation.

Thus the kinds of particle accelerators needed to serve as ion drives are very different from conventional accelerators in their construction and operating parameters. There are two main parameters that a particle accelerator must balance against one another: beam energy and beam current (particle density in the beam). Generally, conventional accelerators are optimized either for very high particle energy and low current or low energy and high current. The primary obstacle to the satisfaction of these parameters is that all the particles comprising the beam are electrically charged (else they could not be accelerated), so they all repel and jostle one another inside the beam volume, and thus resist collimation. The higher the beam energy (acceleration inside the accelerator) and the higher the particle density, the greater the resistance the particles offer to further acceleration and collimation.

To be effective as a means of propulsion, the accelerator used as an ion drive should have the highest possible beam energy and beam current simultaneously. No such accelerator has yet been constructed that could produce more than a few tens or hundreds of newtons of thrust.

Electrothermal

The electrothermal category groups the devices where electromagnetic fields are used to generate a plasma to increase the temperature of the bulk propellant. The thermal energy imparted to the propellant gas is then converted into kinetic energy by a nozzle of either solid material or magnetic fields. Low molecular weight gases (e.g. hydrogen, helium, ammonia) are preferred propellants for this kind of system.

An electrothermal engine uses a nozzle to convert the heat of a gas into linear motion in its molecules, so it is a true rocket even though the energy producing the heat comes from an external source.

Performance of electrothermal systems in terms of specific impulse (Isp) is somewhat modest (500 to ~1000 seconds), but exceeds that of cold gas thrusters, monopropellant rockets, and even most bipropellant rockets. In the USSR, electrothermal engines were used since 1971; the Soviet "Meteor-3", "Meteor-Priroda", "Resurs-O" satellite series and the Russian "Elektro" satellite are equipped with them.[18] Electrothermal systems by Aerojet (MR-510) are currently used on Lockheed Martin A2100 satellites using hydrazine as a propellant.

  • Arcjet
  • Microwave arcjet
  • Resistojet
  • Variable specific impulse magnetoplasma rocket (VASIMR)

Electromagnetic
Main article: Plasma propulsion engine

If ions are accelerated either by the Lorentz force or by the effect of electromagnetic fields where the electric field is not in the direction of the acceleration, the device is considered electromagnetic.

Non-ion drives

Photonic

Photonic drive does not expel matter for reaction thrust, only photons. See Laser propulsion, Photonic Laser Thruster, Photon rocket.

Electrodynamic tether
Main article: Electrodynamic tether

Electrodynamic tethers are long conducting wires, such as one deployed from a tether satellite, which can operate on electromagnetic principles as generators, by converting their kinetic energy to electric energy, or as motors, converting electric energy to kinetic energy.[19] Electric potential is generated across a conductive tether by its motion through the Earth's magnetic field. The choice of the metal conductor to be used in an electrodynamic tether is determined by a variety of factors. Primary factors usually include high electrical conductivity, and low density. Secondary factors, depending on the application, include cost, strength, and melting point.

Controversial

A number of propulsion methods have been proposed, where it is unclear that they can work according to the currently-understood laws of physics, including:[20]

Steady vs. unsteady

Electric propulsion systems can also be characterized as either steady (continuous firing for a prescribed duration) or unsteady (pulsed firings accumulating to a desired impulse). However, these classifications are not unique to electric propulsion systems and can be applied to all types of propulsion engines.

Dynamic properties
Further information: Reaction engine § Energy use

Electrically-powered rocket engines provide lower thrust compared to chemical rockets by several orders of magnitude because of the limited electrical power possible to provide in a spacecraft.[3] A chemical rocket imparts energy to the combustion products directly, whereas an electrical system requires several steps. However, the high velocity and lower reaction mass expended for the same thrust allows electric rockets to run for a long time. This differs from the typical chemical-powered spacecraft, where the engines run only in short intervals of time, while the spacecraft mostly follows an inertial trajectory. When near a planet, low-thrust propulsion may not offset the gravitational attraction of the planet. An electric rocket engine cannot provide enough thrust to lift the vehicle from a planet's surface, but a low thrust applied for a long interval can allow a spacecraft to maneuver near a planet.

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See also

References
  1. Mazouffre, Stéphane (1 June 2016). "Electric propulsion for satellites and spacecraft: established technologies and novel approaches". Plasma Sources Science and Technology. 25 (3): 033002. doi:10.1088/0963-0252/25/3/033002. ISSN 0963-0252.
  2. Choueiri, Edgar Y. (2009) New dawn of electric rocket Scientific American 300, 58–65 doi:10.1038/scientificamerican0209-58
  3. "Electric versus Chemical Propulsion". Electric Spacecraft Propulsion. ESA. Retrieved 17 February 2007.
  4. Electric Propulsion Research at Institute of Fundamental Technological Research
  5. Beyond Frontiers Broadgate Publications (September 2016) pp20
  6. Lev, Dan; Myers, Roger M.; Lemmer, Kristina M.; Kolbeck, Jonathan; Koizumi, Hiroyuki; Polzin, Kurt (June 2019). "The technological and commercial expansion of electric propulsion". Acta Astronautica. 159: 213–227. doi:10.1016/j.actaastro.2019.03.058.
  7. Choueiri, Edgar Y. (2009). New dawn of electric rocket
  8. Laser-Powered Interstellar Probe G Landis - APS Bulletin, 1991
  9. Geoffrey A. Landis. Laser-powered Interstellar Probe Archived 22 July 2012 at the Wayback Machine on the Geoffrey A. Landis: Science. papers available on the web
  10. Palaszewski, Bryan. "Electric Propulsion for Future Space Missions (PowerPoint)". Electric Propulsion for Future Space Missions. NASA Glenn Research Center. Retrieved 31 December 2011.
  11. Choueiri, Edgar Y. (2004). "A Critical History of Electric Propulsion: The First 50 Years (1906–1956)". Journal of Propulsion and Power. 20 (2): 193–203. CiteSeerX 10.1.1.573.8519. doi:10.2514/1.9245.
  12. PROJECT DAEDALUS: THE PROPULSION SYSTEM Part 1; Theoretical considerations and calculations. 2. REVIEW OF ADVANCED PROPULSION SYSTEMS Archived 28 June 2013 at the Wayback Machine
  13. NASA Glenn Contributions to Deep Space 1
  14. Ronald J. Cybulski, Daniel M. Shellhammer, Robert R. LoveII, Edward J. Domino, and Joseph T. Kotnik, RESULTS FROM SERT I ION ROCKET FLIGHT TEST, NASA Technical Note D2718 (1965).
  15. NASA Glenn, "SPACE ELECTRIC ROCKET TEST II (SERT II)" Archived 27 September 2011 at the Wayback Machine (Accessed 1 July 2010)
  16. SERT Archived 25 October 2010 at the Wayback Machine page at Astronautix (Accessed 1 July 2010)
  17. de Selding, Peter B. (20 June 2013). "Electric-propulsion Satellites Are All the Rage". SpaceNews. Retrieved 6 February 2015.
  18. "Native Electric Propulsion Engines Today" (in Russian). Novosti Kosmonavtiki. 1999. Archived from the original on 6 June 2011.
  19. NASA, Tethers In Space Handbook, edited by M.L. Cosmo and E.C. Lorenzini, Third Edition December 1997 (accessed 20 October 2010); see also version at NASA MSFC; available on scribd
  20. "Why Shawyer's 'electromagnetic relativity drive' is a fraud" (PDF). Archived from the original (PDF) on 25 August 2014.
  • Aerospace America, AIAA publication, December 2005, Propulsion and Energy section, pp. 54–55, written by Mitchell Walker.

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