Electric propulsion can be categorized into three groups:

1. Electrothermal Propulsion Systems, which heat a gas with resistive elements, an electrical arc, or through electromagnetic radiation (e.g., microwaves) that is subsequently expanded through a supersonic nozzle to produce thrust;
2. Electromagnetic Propulsion Systems, which use electromagnetic body forces to accelerate a highly ionized plasma; and
3. Electrostatic Propulsion Systems, which use electric fields to accelerate ions.

In addition to possessing suitable exhaust velocity, an EP system must also be able to convert onboard spacecraft power into directed kinetic power of the exhaust stream efficiently (i.e., possess high thrust efficiency) and must generate suitable thrust to ensure reasonably short trip times. The EP system must be as light as possible and obviously must not negatively impact the life or operations of the space vehicle.

Electrothermal propulsion
Electrothermal propulsion systems rely on electric power that is supplied by the spacecraft to heat propellant to very high temperatures. Once heated, the propellant is allowed to expand through a nozzle, where it accelerates to provide thrust. The equation that shows this process for an ideal nozzle is

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Figure ET1: Schematic of electrothermal propulsion system operating principles.

The trick in creating efficient electrothermal propulsion systems is to put a lot of energy into a small mass of propellant without melting the device, and then to extract as much of this energy as possible to produce thrust. Resistojets use an electric heater to put heat into propellant. Resistojets were first used in 1965 with nitrogen on the US Department of Defense Vela satellites. Resistojets have been used for stationkeeping communication satellites for some 20 years. The resistojets used on commercial spacecraft typically have specific impulses of 300 s or more (compared to CP specific impulses of 200 s for the same propellant--hydrazine), peak heater wire temperatures in excess of 2,000° C for thrust efficiencies above 50%, input power levels of 465-885 W and total impulses above 500,000 Ns (figure ET2).

Image and diagram courtesy of Aerojet

Figure ET2: Picture and schematic of the 500 W Aerojet MR-501B Electrothermal Hydrazine Thruster (EHT) resistojet used for communication satellite stationkeeping. Over 200 Aerojet EHTs have flown since 1983. The MR-501B generates up to 360 mN of thrust and specific impulses of 303 s.

Resistojets were seriously considered (in fact, baselined at one point) for drag make-up of the International Space Station. The ISS multipropellant resistojet would use waste water and gases from the crew as propellant. The ISS multipropellant resistojet can operate with CO₂ , Ar, N₂ , air, CH₄ , He, and H₂ as propellant and generate specific impulses that reach nearly 400 s with hydrogen.

The other electrothermal device that is often used is the arcjet. The arcjet operates exactly as a resistojet except it uses an electric arc (sort of like a continuous lightning bolt) to heat the gas to temperatures above 3,000° C. While one of the oldest EP technologies to be studied (since the late 1950s), arcjets entered a 10-year golden age in 1983 with a NASA-industry program to develop hydrazine-propellant arcjets for commercial satellites.

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Figure ET3: Schematic of arcjet operating principles. Propellant is injected near the rear with a swirl. The swirl is used to stabilize the arc through the narrow part of the device (called the constrictor) to prevent it from melting the walls. The swirl also ensures good mixing of the gas and therefore uniform heating. The hot gas emerges from the constrictor and expands (and accelerates) through the nozzle.

Figure ET3 shows the operating principles of an arcjet. The arc in the arcjet is a beam of electrons that is emitted from the tip of the cathode and collected at the positively biased nozzle (anode). Between the cathode and anode is a narrow passageway called the constrictor. As the electrons leave the cathode, the electric fields that exist between the cathode and anode accelerate them. Gas is injected near the base of the cathode with an azimuthal swirl. One can picture the gas flow by imagining that a section of gas flows around and along the cathode in a helix, the center of which is the middle of the cathode/thruster. This flow surrounds both the cathode and then the electric arc in the constrictor. The swirl prevents the arc from kinking and touching the walls, thereby keeping the constrictor from melting. The swirl also helps circulate the gas through the arc, which can have an effective temperature in excess of 15,000° C. Arcjets have been developed for a multitude of applications ranging from stationkeeping of moderate-sized spacecraft (500 W, hydrazine) to a piloted mission to Mars (100 kW, hydrogen). Specific impulses range from approximately 500-600 s on hydrazine to ~2,000 s on hydrogen.

The US Department of Defense recently tested a 26 kW ammonia arcjet in space that produced a specific impulse in excess of 800 s. The major issues that plague arcjets include excessive heating (largely solved with advanced materials) and so-called frozen flow losses, which result from an inability of the nozzle to convert energy deposited in the gas by the arc into directed thrust power. Accordingly, arcjets typically have thrust efficiencies of only 35%; that is, only a third of the input power is converted to useful thrust. However, the simple ease with which arcjets can be integrated with spacecraft that used hydrazine rockets ( I_sp ~220 s) or resistojets ( I_sp ~300) for stationkeeping continues to provide a market for this robust engine.

In December of 1994, a Lockheed Martin Astro Space Series 7000 Telstar 401 satellite owned by AT&T became the world's first commercial satellite to use an arcjet for stationkeeping. By switching from resistojets to arcjets, the Telstar 401 was able to double its propellant efficiency and thus carry a larger, more capable communications payload instead of the additional propellant normally required for its 12-year mission. The hydrazine-propellant arcjets used on this and follow-on commercial satellites have input power levels between 1.8 and 2.2 kW at thrust levels of around 230 mN, specific impulses between 510 and 650 s, thrust efficiencies of 35-40%, and total impulses up to 1.5 million Ns (figure ET4).

Images courtesy of Aerojet

Figure ET4: A shipset of Aerojet MR-510 arcjets for a communications satellite and one of the engines operating during a 1999 test firing at the Aerojet Electric Propulsion test facility in Redmond, Washington. Each arcjet consumes over 2 kW of power for an Isp above 600 s on hydrazine and thrust levels approaching 260 mN. To date (fall 2002), 16 Lockheed Martin A2100tm spacecraft have been launched with MR-510 shipsets.