multi-mode engines which operate on LOX/LH2 during 2nd stage burn only. The configuration very nearly approximates a minimum GLOW vehicle for the prescribed payload. Staging conditions are such that a "heat-sink" booster requiring minimum thermal protection may be utilized. Horizontal Takeoff SSTO HLLV - The horizontal takeoff, single-stage-to- orbit concept represents a more advanced technology option for SPS. This concept was considered because of the operational problems related to multiple daily launches of very large vertical takeoff concepts and the overall operational flexibility. Without a commitment toward accelerated advanced technology programs, it is not apparent that this configuration can meet current SPS technology readiness requirements. The winged vehicle, Figure R-23, is a delta flying wing consisting of a multi-cell pressure vessel. The wing contour is a supercritical air-foil with leading edge modified to improve supersonic and hypersonic performance. The cargo bay floor is designed similar to the C5-A military transport to permit the use of Airlog cargo loading and retention systems. Cargo is deployed in orbit by swinging the forebody to 90 or more degrees about a vertical axis and transferring cargo from the bay on telescoping rails. Ten high-bypass, supersonic-turbofan/air-turbo exchanger/ramjet engines with a combined thrust of 1.4 x 106 lb are mounted under the wing. The inlets are projected by retractable ramps that close the inlets and fair the bottom surface into a continuous surface suitable for reentry. Three uprated SSME-type rocket engines (total thrust = 3.2 x 106 lb) provide the required thrust above the sensible atmosphere. The vehicle is capable of placing a 91,000 kg payload in a 550 km equatorial orbit. Cargo Orbital Transfer Vehicle - The payload required for construction is transported to GEO using a dedicated electric OTV. This concept is illustrated in Figure R-24. The OTV is sized to carry 4 x 10^6 kg (8.8 x 10^ lb) of payload for a LEO-GEO trip time of 133 days. Approximately ten OTV flights are required to transport the mass required for the construction of each SPS. GaAlAs solar cells also are used in this concept to provide power for propulsion. As for
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