1.4 ELECTRICALLY - POWERED COTV - HIGH SPECIFIC IMPULSE (W. F. Perlich, Future Programs Office) 1.4.1 GENERAL Electrically powered propulsion systems of the previous section all derive their thrust by thermal propellant heating in a pressure chamber and normal exhaust via an engine nozzle. Maximum specific impulse values between 440 and 3000 seconds are predicted. This section investigates thruster concepts in which the propellant is ionized and electrostatically or electromagnet!cally accelerated to very high exhaust velocities. With argon as a propellant, maximum specific impulses of 5,000 to 20,000 seconds appear to be attainable. Two specific designs falling within this context are selected as representative and most appropriate for COTV/SPS functions: a relatively straightforward ion design and a combined magnetoplasmadynamic-arcjet (MPD-arcjet) design with external magnetic coil for arc control and stability. Both independent operation and operation dependent upon SPS electrical power is considered. 1.4.2 THRUSTERS 1.4.2.1 PROPELLANT SELECTION Mercury and cesium have been almost exclusively selected as a propellant for early engine development because of easy ionization, high density/efficient storage, and compatibility with electrical thruster design considerations. They are not acceptable for the SPS orbit transfer function, however, because of scarcity, high cost, and for mercury, a serious environmental compatibility question. Therefore, argon has been selected as the propellant for both ion and MPD-arcjet thrusters since it is cheap and abundant, possesses good density/storage and performance qualities, is nonpolluting, and unlike hydrogen and helium, possesses excellent frozen flow efficiencies as shown in Figure VI-D-1-15. Its major disadvantage, cryogenic handling and storage, is not considered an overriding issue. 1.4.2.2 ION CHARACTERISTICS The ion/argon thruster design is based on previous extensive electrical oropulsion R&D activities, including the Lewis Research Center (LeRC) development of their 30 centimeter mercury bombardment ion thruster and propulsion system. The magnitude of the effort required for extrapolation of the LeRC design to a larger size (30 to 150 centimeters) and argon propellant is uncertain but is generally considered to be entirely feasible.
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