transportation basically falls into the same two categories; however, there are a number of issues yet to be resolved that exaggerate the number of options for propulsion systems that are candidates for orbital transportation systems. The basic issue, from an orbital transportation viewpoint, is SPS assembly location (LEO versus GEO). Propulsive options that exist for SPS transportation when assembly is in LEO are low thrust electric propulsion devices that can capitalize on the electric power generation capability of an assembled SPS or SPS module. Electric propulsion is also desirable for the transfer of SPSTs or assemblies from LEO to GEO, since the lightweight structure for SPS requires very low acceleration (< 10 “3 Earth gravity). When electric propulsion is employed as the cargo orbital transfer system, personnel transportation between LEO and GEO is accomplished via a high thrust system with trip times to GEO of hours rather than the months dictated by low thrust options. If geosynchronous orbit assembly is assumed, the nature of the orbital transfer system changes because of the lack of a ''free" power source provided by the payload. GEO assembly does not eliminate electric propulsion as a viable option; however, the need for an independent power source and the trip times associated with low thrust propulsion make high thrust propulsion a viable candidate. Further, when high thrust propulsion is employed for cargo orbital transportation, the need for persomiel and critical logistics transportation, as well as cargo, can be met with the same system. The SPS assembly orbit location selection is dependent on a large number of factors other than the transportation issue and will only be resolved with significant attention over the next several years. Since this is the case, this section of the report will discuss transportation systems that meet both requirements. 12. 2. 2 HEAVY LIFT LAUNCH VEHICLE SYSTEM The HLLV concepts described here are sized to deliver payloads to a 500 km circular orbit at 28.5° inclination. Two candidate HLLV's are described. Configuration details are given for a two-stage ballistic vehicle, followed by a description of a ballistic single stage-to-orbit (SSTO) vehicle. Cost comparisons for each vehicle are also provided. The two-stage configuration, shown in Figure 12-5, has two fully recoverable ballistic stages. The upper stage uses seven space shuttle main engines (SSME). The lower stage has nine new LOX/RP-1 high chamber pressure engines. A 8 896 442 N thrust engine was selected to be compatible with current test stand capability.
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