2.2 TASK 2 - CONCEPT DEFINITION 2.2.1 Mission Analysis The mission analysis effort objective was to define flight scenarios for subsequent assessment of transportation system performance requirements. Figure 2-8 is a top level functional flow of the SSPS assembly sequence. Two flight plans for assembly and transport to geosynchronous orbit were developed: • Low altitude assembly and transport to geosynchronous using solar electric propulsion (SEP) • Assembly just above Van Allen belts and transport to geosynchronous using SEP. A baseline SSPS, Fig. 2-9 was assumed for mission analysis and subsequent estimates of traffic models and fleet sizes. Performance capabilities of the transportation system are summarized in Fig. 2-10. Shuttle performance of 65, 000 lb (29,400 Kg) can be expected up to an altitude of 190 n mi. The Cryo Tug, used in the flight plan with assembly at 7000 n mi, has a payload capability of 36, 800 lb (16,700 Kg) in a Tug recoverable mode. SEP size and performance data for the two flight modes are presented in Fig. 2-10. A SEP system efficiency of 0.7 and a specific weight of 15 Ib/kw (6. 8 Kg/kw) was assumed in the stage sizing. The 0.7 efficiency is equaled or exceeded by today’s technology. Overall system specific weight is consistent with projected solar cell weights for the SSPS itself. Specific weight of the power conditioning and subsystems is based on a projected four fold improvement in technology (using today’s technology would result in an overall system specific weight of ). A 190 n mi assembly site would require continuous orbit keeping propulsion to compensate for air drag. Figure 2-11 indicates that uncorrected air drag effects would result in assembly entry after one to 16 months depending upon configuration . The spread in is indicative of the SSPS configuration with solar blankets deployed and retracted. A 16-lb thrust (70 newton) SEP stage would be required for the orbit keeping function. A propellant expenditure of 44 Klb (20, 000 Kg) is projected. 2.2.2 Antenna Structural Definition The MPTS antenna is 1 km in diameter by 40 meters deep. Fig. 2-12. The antenna is assembled in two rectangular grid structural layers. The primary structure is built-up in 108 x 108 x 35 meter bays using triangular girder compression members 18 meters long
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