The chemical system selected to boost the SPS from LEO to a continuous sunlit orbit is O2/H2 using a high pressure, pump fed, tug type engine. This selection is preliminary and is based on the high specific impulse of O2/H2 (approximately 470 s) minimizing the launch cost because of less propellant needed in LEO. Integrating high thrust chemical engines into the structurally fragile SPS presents a problem. This problem is still under investigation. Also, the large initial mass of the SPS in conjunction with the AV that the chemical system is to satisfy requires a long burn time (possibly up to 24 h). The types of electric thrusters considered in this study are the MPD, the arc jet, the ion, and the resistojet. The arc jet and the MPD thruster are considered as variations of the same plasma device and will both be categorized under the broad heading of MPD thruster. The resistojet thruster is eliminated because of its inherent low specific impulse characteristic. The ion thruster is well known, whereas the MPD thruster is not so well understood. Table 7-7 summarized the physical, electrical, and performance characteristics of an ion thruster and two MPD thruster concepts that are potential candidates for use on the SPS. The ion thruster and one of the MPD thrusters were proposed by The Boeing Company. The other MPD thruster is a Jet Propulsion Laboratory (JPL) proposal. Several versions of the ion thruster concept have been operated in space, and a 30 cm beam diameter design will be used on the SEPS. The 100 cm ion thruster presented in Table 7-7 was postulated and its characteristics were obtained by extrapolation from 30 cm thruster data. These data have high confidence. In contrast, the MPD data are speculative; only some general designtrend historical data exist on which to base its characteristics. This becomes apparent by contrasting the Boeing and JPL proposals. However, many MPD concepts have been designed and ground tested with appreciable success. Although ion thrusters are essentially state-of-the-art and have the potential to deliver an attractively high specific impulse, MPD thrusters appear to offer many advantages when applied to the SPS. The most significant difference is the number of power supplies. A 30 cm ion thruster with a single cathode may require as many as 14 separate power supplies for operation. Each hypothetical 100 cm thruster with 10 cathodes may require as many as 31 power supplies, if the cathodes must be independently supplied. Ion thrusters cannot be ganged on power supplies because of transient interactions. Thus, each ion thruster must have its own power processing unit which, as shown in Table 7-7, can be quite massive. In contrast, MPD thrusters may be ganged in series or parallel onto a common power supply providing a vastly simpler propulsion system. Since both arcs and magnets tend naturally to operate at low voltage and high current and since there are few active components in an MPD thruster, the power supply required for operation can be very simple. MPD thrusters
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