INPUT POWER Figure VI-D-1-16 shows that electrical power demand on the SPS is roughly equal at about 35% of total capacity for the variable efficiency MPD-arcjet thruster operating within its optimum range at about 80 KM/sec. Input power is an important consideration since it defines the amount of SPS solar energy collector which must be exposed to Van Allen Belt radiation damage. This factor, in conjunction with orbit transfer time (or time in the radiation belt) determines total SPS generating capacity loss. Assuming that 30% of the power used for propulsion is lost due to radiation damage, the total station power loss is 10.5%. OCCULTATION AND TRANSFER TIME Loss of sunlight due to occultation by the Earth results in the periodic loss of SPS electrical power and attitude control. An early, generally proposed solution to the problem consisted of providing a chemical thrust system for attitude control and propulsion during occultation periods. This results, however, in many periodic start-up/run/shutdown operating cycles of both the SPS and the COTV chemical and electrical propulsion systems. Boeing proposes a different approach that first chemically transfers the SPS to an intermediate, continuous sunlight orbit and then electrically completes the GEO transfer in a continuous operation. This requires a careful combination of departure window, switchover orbit altitude and inclination, and transfer time limitation to stay in sunlight throughout the ascent. Although a switchover orbit inclination of about 55 degrees allows continuous electrical operation from the 270 NM SPS assembly orbit, Boeing proposes and utilizes a 10 day window, 1500 KM (810 NM) altitude, and 34 degree switchover orbit, and 60 day (maximum is 62 days) GEO transfer time as the optimum combination of factors. OPERATING LIFE Although not considered by Boeing in their recent studies, MPD thruster life predictions are perhaps the most speculative of all factors, with estimates ranging from a JSC in-house estimate of 3 months to JPL's recent assumption of 70,000 hours (about 97 months). The former fits comfortably within a single SPS transfer mission with expendable thrusters and the latter suggests the possibility of further engine utilization at GEO for SPS attitude control after the orbit transfer operation. The longer lifetimes appear to be questionable and a nominal value of 7 or 8 months (about 5,000 and 5,800 hours, respectively) within a total range of 2 to 24 months is suggested for current study purposes. ACCELERATION Electrical propulsion acceleration of the "reference" system varies from 10-4 g at start of thrust to 1.25x10“^ g at thrust termination. REPRESENTATIVE SYSTEM An exhaustive and accurate design of the MPD-arcjet COTV appears beyond our grasp at this time due to uncertainty in thruster performance
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