Electrically powered spacecraft propulsion

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For vehicles other than spacecraft that are propelled by electric means, see Electric vehicle.

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 throughout the solar system 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] However, Electric propulsion is not a method suitable for launches from the Earth's surface, as on average the thrust for such systems is too weak.

History[edit]

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]

Electrically powered propulsion with a nuclear reactor was considered Dr. Tony Martin for interstellar Project Daedalus in 1973, but the novel approach was rejected because of very low thrust, much weight needed to convert nuclear energy into electrical equipment and as a result a small acceleration, which would take a century to achieve the desired speed.[11]

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

By the early 2010s, many satellite manufacturers were offering electric propulsion options on their satellites—mostly for on-orbit attitude control—while some commercial commsat operators were beginning to use them for geosynchronous orbit insertion in place of traditional chemical rocket engines.[12]

One such satellite system is the Boeing 702SP which can be launched as a pair on a lighter-weight dual-commsat stack—two satellites conjoined on a single launch—and was specifically designed to take advantage of the lower-cost SpaceX Falcon 9 launch vehicle.[13][14] The first two commsats of this design are planned for a paired launch in early 2015.

Types[edit]

Ion and plasma drives[edit]

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[edit]

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[edit]

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

Electromagnetic[edit]

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[edit]

Photonic[edit]

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

Electrodynamic tether[edit]

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.[16] 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.

Unconventional[edit]

The principle of action of these theoretical devices is not well explained by the currently-understood laws of physics.[17]

Steady vs. unsteady[edit]

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[edit]

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.

See also[edit]

References[edit]

  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. ^ de Selding, Peter B. (20 June 2013). "Electric-propulsion Satellites Are All the Rage". SpaceNews. Retrieved 6 February 2015. 
  13. ^ Svitak, Amy (2014-03-10). "SpaceX Says Falcon 9 To Compete For EELV This Year". Aviation Week. Retrieved 2015-02-06. But the Falcon 9 is not just changing the way launch-vehicle providers do business; its reach has gone further, prompting satellite makers and commercial fleet operators to retool business plans in response to the low-cost rocket. In March 2012, Boeing announced the start of a new line of all-electric telecommunications spacecraft, the 702SP, which are designed to launch in pairs on a Falcon 9 v1.1. Anchor customers Asia Broadcast Satellite (ABS) of Hong Kong and Mexico's SatMex plan to loft the first two of four such spacecraft on a Falcon 9.... Using electric rather than chemical propulsion will mean the satellites take months, rather than weeks, to reach their final orbital destination. But because all-electric spacecraft are about 40% lighter than their conventional counterparts, the cost to launch them is considerably less than that for a chemically propelled satellite. 
  14. ^ "Boeing Stacks Two Satellites to Launch as a Pair" (Press Release). Boeing. 2014-11-12. Retrieved 2015-02-06. 
  15. ^ "Native Electric Propulsion Engines Today" (in Russian) (7). Novosti Kosmonavtiki. 1999. Archived from the original on 6 June 2011. 
  16. ^ 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
  17. ^ "Why Shawyer’s ‘electromagnetic relativity drive’ is a fraud". 
  • Aerospace America, AIAA publication, December 2005, Propulsion and Energy section, pp. 54–55, written by Mitchell Walker.

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