of achieving a totally sunlit orbit would be cost effective. Other economic factors regarding allowable trip time will need to be factored into these studies. 7.1. 5. 2. 3 ELECTRIC PROPULSION PERFORMANCE EVALUATION AND REQUIREMENTS Two designs or concepts of electric ion thrusters received significant attention for application to the self-powered SPS orbital transfer capability. These are a 100 cm electric ion thruster and a 10 cm MPD thruster as defined in the midterm review of NASA Contract NAS8-31444, ''Payload Utilization of SEPS (PLUS)by The Boeing Company on March 2, 1976. These thrusters are only concepts and will require a substantial technology advancement effort to be available in the SPS projected operational time frame. For reasons that will become apparent during this discussion, the MPD electric thruster concept appears to have significant performance advantages over other electric thruster concepts and is therefore a suggested baseline for SPS application. Figure 7-21 shows relative performance expectation and system masses for both the 100 cm ion thruster and the MPD thruster propulsion systems. From Figure 7-21 it is seen that a short trip time (less than 100 days) produces an exponential rise in the propulsion system mass for the 100 cm ion Figure 7-21. Relative SPS total mass versus LEO to GEO orbital transfer time.
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