(word processor parameters LM=8, RM=78, TM=2, BM=2)
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August 3, 1990
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PRIMARY PROPULSION
Electric propulsion for primary spacecraft thrust is of interest for both near-Earth and interplanetary missions. Near-Earth applications include the spiral-out maneuvers from low to high orbit and Earth escape.
Once at high orbit the thruster may also be used for stationkeeping. Interplanetary missions include flights out of the ecliptic and flybys past, or rendezous with, asteroids, comets, and planets.
A primary electric propulsion stage could offer large payload advantages as a commercial tug in conjunction with the space shuttle.
The interest in electric propulsion derives mainly from the reduction in propellant requirements relative to chemical propulsion due to operation at increased specific impulse.
One way to compare the capability of an electric propulsion spacecraft with that of a chemical system is to consider the total impulse delivered by two such systems.
A 1500-kilogram electric propulsion spacecraft with 500 kilograms of propellant can deliver slightly more total impulse than a 2914 Delta rocket stage, which has a mass of 5500 kilograms including 4500 kilograms of propellant. Although exact comparisons are subject to details of the propulsion system configuration, the comparison just given is illustrative of the propellant savings, and, hence, overall mission performance increases, achievable with electric propulsion.
To explain more fully the characteristics of a primary electric propulsion system, the major elements are discussed in the following sections.
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