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Spacecraft electric propulsion

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For vehicles other than spacecraft that are propelled by electric means, see Electric vehicle.
6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory.

An electrically powered spacecraft propulsion system uses electrical energy 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, but electrodynamic tethers work by interacting with a planet's magnetic field.[1]

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 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] As of 2013, over 200 spacecraft operated around the world use electric propulsion for stationkeeping, orbit raising, or primary propulsion.[5] 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][6] Also, an electro-rocket with an external power source (transmissible through laser on the solar panels) has a theoretical possibility for interstellar flight.[7][8] Electric propulsion is not suitable for launches from the Earth's surface, because the thrust is too weak.

History

The idea of electric propulsion for spacecraft dates back to 1911, introduced in a publication by Konstantin Tsiolkovsky.[9] Earlier, Robert Goddard had noted such a possibility in his personal notebook.[10]

The first in-space demonstration of electric propulsion was an ion engine carried on board the SERT-1 (Space Electric Rocket Test) spacecraft, launched on 20 July 1964.

Electrically powered propulsion with a nuclear reactor considered for interstellar Project Daedalus and was rejected because of small acceleration and big time travel.[11]

Types

Ion/plasma drives

Main article: Ion thruster

This type of rocket-like reaction engine uses 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 in three families based on the type of force used to accelerate the ions of the plasma:

Electrostatic

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.

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 the linear motion of 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.[12] Electrothermal systems by Aerojet (MR-510) are currently used on Lockheed Martin A2100 satellites using hydrazine as a propellant.

Electromagnetic

Main article: Plasma propulsion engine

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

Non-ion drives

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.[13] 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.

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 manueuver near a planet.

See also

References

  1. ^ Michel Van Pelt Space Tethers and Space Elevators Springer, 2009 ISBN 0387765565, page 24
  2. ^ a b c Choueiri, Edgar Y. (2009) New dawn of electric rocket Scientific American 300, 58–65 doi:10.1038/scientificamerican0209-58
  3. ^ a b "Electric versus Chemical Propulsion". Electric Spacecraft Propulsion. ESA. Retrieved 17 February 2007.
  4. ^ http://fluid.ippt.gov.pl/sbarral/hall.html
  5. ^ W. Andrew Hoskins et al. "30 Years of Electric Propulsion Flight Experience at Aerojet Rocketdyne", Paper IEPC-2013-439, 33rd International Electric Propulsion Conference, Washington DC, October 2013. http://www.iepc2013.org/get?id=439
  6. ^ Choueiri, Edgar Y. (2009). New dawn of electric rocket
  7. ^ Laser-Powered Interstellar Probe G Landis - APS Bulletin, 1991
  8. ^ Geoffrey A. Landis. Laser-powered Interstellar Probe on the Geoffrey A. Landis: Science. papers available on the web
  9. ^ Palaszewski, Bryan. "Electric Propulsion for Future Space Missions (PowerPoint)". Electric Propulsion for Future Space Missions. NASA Glenn Research Center. Retrieved 31 December 2011.
  10. ^ 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. doi:10.2514/1.9245.
  11. ^ PROJECT DAEDALUS: THE PROPULSION SYSTEM Part 1; Theoretical considerations and calculations. 2. REVIEW OF ADVANCED PROPULSION SYSTEMS
  12. ^ Template:Ru icon "Native Electric Propulsion Engines Today". Novosti Kosmonavtiki. 1999. Archived from the original on 6 June 2011.
  13. ^ 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
  • Aerospace America, AIAA publication, December 2005, Propulsion and Energy section, pp. 54–55, written by Mitchell Walker.

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