Once the thruster type and propellant were selected, investigations were conducted to determine optimum trip time, and the desirability of utilizing the SPS array itself to provide power for the ion engines. In the self powered approach, studies were completed of assembling the complete SPS in LEO and self-powering to GEO as well as constructing the SPS in eight modules in LEO and self-powering each to GEO. For the second option, thruster and power processing systems are located at four corners of the satellite module and connected to a gimbal system to enable required pointing (Figure A-22). A joint cost optimization of Isp and trip time resulted in a selection of a 180 day transfer at 7000 seconds electrical Isp. The effective Isp, after accounting for losses for attitude control thrusting and the use of chemical propulsion during transits of the Earth's shadow is about 3000 seconds. This high specific impulse option therefore requires about 0.25 kg of propellant per kg of payload to GEO compared to about 2.1 kg of propellant per kg of payload to GEO for the chemical COTV option. Approximately one-quarter of the SPS solar blankets on each module are deployed for the transfer from LEO to GEO; the remainder are deployed from their shipping boxes at GEO. The deployed arrays will be degraded by Van Allen belt radiation absorbed during the transfer but will be annealed at GEO to regain most of the lost efficiency during the final checkout and preparation process. The ion thrusters and propellant tanks would remain in GEO as an integral part of the SPS, thus incurring no propellant penalty for returning them to LEO. A second electric solar array option, called the independent electric OTV, involves the construction of a fleet of reusable electric powered round-trip vehicles and their associated solar array in LEO. These vehicles are used to transport SPS material which is fabricated in GEO. Potentially, this option offers the performance advantages of the LEO con- struction/self-power concept and the monolithic satellite construction operations associated with the GEO construction/chemical COTV.
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