E. Space Transportation Systems Considerations The transportation system is a key element in the overall SPS program. Systems selection is dominated by the need to achieve the lowest cost possible in terms of dollars per mass to orbit. The mission of the transportation system is to carry personnel and material between earth and the required locations in space. Performance and economic considerations dictate that transportation between earth and low earth orbit (LEO) be accomplished by vehicles designed for the appropriate flight rates, payloads and the loads associated with launch, atmospheric flight, reentry and landing, whereas transportation between LEO and geosynchronous orbit (GEO) be accomplished by orbital transfer vehicles (OTV's) designed for non- atmospheric loads and high specific impulse (possibly low thrust) propulsion. A single vehicle design suitable for both regimes would be difficult with present technology and would require a compromise design that would not be cost competitive with separate, functionally optimized vehicles. The LEO staging location is at approximately 500 km altitude and at the inclination of the launch site. The SPS transportation system consists of four basic vehicle types (plus their associated support facilities) which respond to the requirements of their operational regimes and the differing needs of human and material cargo. The Heavy Lift Launch Vehicle (HLLV) and the Personnel Launch Vehicle (PLV) handle material and personnel traffic between earth and LEO while the Cargo Orbital Transfer Vehicle (COTV) and the Personnel Orbital Transfer Vehicle (POTV) satisfy the requirements between LEO and GEO. Several studies have investigated the vehicle design characteristics that respond most cost effectively to the payload and launch rate requirements of a wide spectrum of potential missions. These studies have provided a data base of design features supporting the SPS study. The Heavy Lift Launch Vehicles Study (NAS 9-14710), contracted to Boeing in July 1975, investigated the effect of payload and annual mass to LEO requirements on vehicle configurations for the
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