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Ion thruster

Form of electric spacecraft propulsion

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This image of a xenon ion engine, photographed through a port of the vacuum chamber where it was being tested at NASA's Jet Propulsion Laboratory, shows the faint blue glow of charged atoms being emit
NASA · Public domain

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Lab · Class 9–12Propulsion Systems

Electrostatic thruster ions are accelerated by the Coulomb force along the electric field direction. Temporarily stored electrons are reinjected by a neutralizer in the cloud of ions after it has passed through the electrostatic grid, so the gas becomes neutral again and can freely disperse in space without any further electrical interaction with the thruster.

By contrast, electromagnetic thruster ions are accelerated by the Lorentz force to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.

Ion thrusters in operation typically consume 1–7 kW of power, have exhaust velocities around 20–50 km/s (12–30 mi/s, Isp 2000–5000 s), and possess thrusts of 25–250 mN and a propulsive efficiency of 65–80%; experimental ion thrusters have achieved 100 kW (130 hp), 5 N (1.1 lbf).

The 1998 Deep Space 1 spacecraft changed velocity by 4.3 km/s (2.7 mi/s) with its ion thruster, and consumed 73.4 kg (162 lb) of xenon. The 2007 Dawn spacecraft achieved velocity change of 11.5 km/s (7.1 mi/s), though with less efficiency, having consumed 425 kg (937 lb) of xenon.

Applications include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters), use as a main propulsion engine for low-mass robotic space vehicles (such as Deep Space 1 and Dawn), and serving as propulsion thrusters for crewed spacecraft and space stations (e.g. Tiangong).

Ion thrust engines are generally practical only in the vacuum of space as the engine's minuscule thrust cannot overcome any significant air resistance without radical design changes, as may be found in the 'Atmosphere Breathing Electric Propulsion' concept. The Massachusetts Institute of Technology (MIT) has created designs that are able to fly for short distances and at low speeds at ground level, using ultra-light materials and low drag aerofoils. An ion engine cannot usually generate sufficient thrust to achieve initial liftoff from any celestial body with significant surface gravity. For these reasons, spacecraft must rely on other methods such as conventional chemical rockets or non-rocket launch technologies to reach their initial orbit.

Origins

Robert H. Goddard first suggested the use of electric propulsion in 1906 and later expanded the idea in a document for the Smithsonian titled Report Concerning Further Development, having presented two early patents in 1920 (US1363037, Method Of And Means For Producing Electrified Jets Of Gas) and 1931 (US1809115, Apparatus For Producing Ions). From 1910 to 1928 he carried out intermittent work using electrical devices in experiments aimed at determining the feasibility of ion propulsion for space travel. The Ion Collector together with the Ion Propulsion Test Tube are preserved in the National Air and Space Museum. In 1925–1926 he had two of his students working with him on this subject, producing independent results. The technique was recommended for near-vacuum conditions at high altitude, but thrust was demonstrated with ionized air streams at atmospheric pressure. The idea appeared again in Hermann Oberth's Wege zur Raumschiffahrt (1929; Ways to Spaceflight), where he explained his thoughts on the mass savings of electric propulsion, predicted its use in spacecraft propulsion and attitude control, and advocated electrostatic acceleration of charged gases.

The concept of an ion thruster spacecraft was first popularized in a science fiction story by American Jack Williamson in 1947. It subsequently became a staple of genre science fiction stories.

A working ion thruster was built by Harold R. Kaufman in 1959 at the NASA Glenn Research Center facilities. It was similar to a gridded electrostatic ion thruster and used mercury for propellant. Suborbital tests were conducted during the 1960s and in 1964, and the engine was sent into a suborbital flight aboard the Space Electric Rocket Test-1 (SERT-1). It successfully operated for the planned 31 minutes before falling to Earth. This test was followed by an orbital test, SERT-2, in 1970.

On the 12 October 1964 Voskhod 1 carried out tests with ion thrusters that had been attached to the exterior of the spacecraft.

An alternate form of electric propulsion, the Hall-effect thruster, was studied independently in the United States and the Soviet Union in the 1950s and 1960s. Hall-effect thrusters operated on Soviet satellites from 1972 until the late 1990s, mainly used for satellite stabilization in north–south and in east–west directions. Some 100–200 engines completed missions on Soviet and Russian satellites. Soviet thruster design was introduced to the West in 1992 after a team of electric propulsion specialists, under the support of the Ballistic Missile Defense Organization, visited Soviet laboratories.

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