5. A chemical propulsion system will be required at least for attitude control to assure Sun orientation during the occultation. 6. If the numerous electric thruster start transients cannot be allowed, a chemical propulsion system will be needed to climb to a continuously sunlit orbit. 7. In addition to providing attitude control, propulsion (electric and/or chemical) will be needed for atmospheric drag makeup and solar pressure corrections. 8. Since the physical size of the SPS limits maneuverability, the transfer propulsion system (assuming a continuously sunlit orbit) will have to be integrated into the SPS structure such that continuous tangential thrusting can be allowed without plume impingement or on-off operation of electric thrusters. Operating electric thrusters in an on-off manner as required by occultation of the SPS is undesirable because of thruster thermal cycling during on-off propulsion operations. Until further system definition indicates a more desirable optimization, a chemical propulsion system is assumed to boost the SPS to a sunlit altitude and inclination such that continuous electric propulsion can be expected. The exact orbital parameters are yet to be derived (considering low cost), consequently, the chemical propulsion system required to perform the transfer has not been sized. However, it will be desirable to begin the trip in advance of solstice to provide a transfer window and to maximize the available unocculted trip time. Should the geosynchronous transfer burn not be initiated during the launch window, the SPS may have to remain in LEO for another year until the Earth's shadow is again in the proper position with respect to the SPS orbit. The electric propulsion system required to perform the LEO to GEO transfer must be selected on the basis of low transfer cost (dollars/kilogram SPS). The various orbit parameters and mission requirements must be established. Then, for a given SPS (size, mass, and acceptable acceleration level), the characteristics (number of thrusters, masses, etc.) for different types of electric propulsion systems to satisfy the mission requirements are computed. The cost of the electric propulsion systems are computed based upon a credible cost model. The transfer cost based upon the inherent range of performance (specific impulse) for each type of electric propulsion system and the transfer cost versus varying propulsion times are calculated. Other parameters to be considered include launch cost and the economic factor of radiation degradation. The type, size, and required performance of the electric propulsion system to satisfy the mission requirements at the lowest cost can then be selected.
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