VI. SPACE TRANSPORTATION SYSTEM A. SYSTEM REQUIREMENTS (E. M. Crum, Future Programs Office) 1. Background and Related Work The magnitude of the SPS program will require a dedicated, optimized transportation system to transfer men and cargo from Earth to the SPS position in geosynchronous space and, in the case of men and a small portion of the cargo, return them to Earth. The system will include growth versions of the Shuttle to transport men, Heavy Lift Launch Vehicles (HLLV) for cargo to low Earth orbit (LEO), and orbital transfer vehicles (OTV) to transfer men and cargo to geosynchronous orbit (GEO). Several investigations and contracted studies, although independent of the SPS, have contributed to the understanding and data base available for the HLLV elements of the transportation system. These studies include the forecast analyses done for the space transportation system section of the agency "Outlook for Space" study (NASA SP-386, January 1976) in which transportation costs of $20 per pound ($44/kg) payload to low Earth orbit were predicted and the Systems Concepts for STS Derived Heavy Lift Launch Vehicles study (NAS 9-14710) that investigated alternative design concepts and made similar cost predictions. The Shuttle Growth Study (Booster and External Tank Options (NAS 8-32015) which was contracted to Rockwell International in June 1976 will provide indications of what performance increases and cost reductions may be expected for the transportation of crews and high priority cargos to LEO. The orbital transfer vehicle has not been studied to the same extent but propulsion studies beginning with those for the shuttle tug have given some insight into the possible contributions of various propellants, nuclear engines and staging schemes while the MSFC Solar Electric Propulsion Studies provide a starting point for the consideration of low thrust, high specific impulse systems. 2. Requirements The requirements placed on elements of the transportation system are in general known well enough to define and evaluate conceptual designs. The primary uncertainty at this time is whether or not the payload will provide power to the OTV on the trip from LEO to GEO. This hinges, in part, on whether the SPS is to be assembled, or partly assembled at a LEO staging point and then transported to GEO with the capability of providing power to the OTV enroute, or whether it is to be assembled, and, perhaps, manufactured at GEO with no capability to provide power during the LEO to GEO transfer. The truss type SPS is amenable to either assembly option while the column/cable configuration is considered for GEO assembly only. The decision concerning LEO versus GEO assembly will be determined by overall system costs and will not be made in the near future.
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