VI. SPACE TRANSPORTATION SYSTEMS A. Systems Requirements and Analysis The SPS transportation system is required to transport building material, subassemblies, equipment, supplies, and personnel to geosynchronous orbit (GEO) at a rate sufficient to establish as many as seven stations per year (assuming a moderate rate of buildup, or scenario B), having masses ranging from 47 x 106 to 124 x 106 kg/station. The largest and most massive payload element is now expected to be the rotary joint between the transmitting antenna and the solar energy collection structure (SECS). This rotary joint measures up to 12 by 10 by 10 m and weighs up to 450 metric tons. Performance and economic considerations dictate that the Earth to low-Earth orbit (LEO) transportation be accomplished by heavy lift launch vehicles (HLLV) designed for the appropriate flight rates and the loads associated with launch, atmospheric flight, reentry, and landing, whereas the LEO-to-GEO transportation vehicles (orbital transfer vehicles (OTV's)) be designed for nonatmospheric loads and high specific impulse (possibly low thrust) propulsion. A single transportation vehicle design suitable for both flight regimes would be a difficult feat with present technology and would be, at best, a compromise design that would not be cost effective compared with separate vehicles. The alternatives open to the power satellite designers that have the largest impacts on the transportation system are (1) construction of the station in GEO, (2) construction of the station in LEO and transporting it to GEO in modules for final assembly, and (3) construction of the station completely in LEO and transporting it to GEO as a single unit. The first alternative is considered to be required by the column/cable structure, whereas the first, second, and third are permitted by the truss configura- ti on. These alternatives affect the OTV design because assembly in LEO, either partial or complete, offers the possibility of using payload-supplied power for LEO-to-GEO propulsion. This is expected to reduce transportation costs because it permits effective use of high-specific-impulse, low-thrust electrical propulsion systems that require relatively small quantities of propellant to be lifted from the Earth's surface. If power is not available from the payload, as in the case of the first alternative, electrical propulsion is still possible, but requires a heavy dedicated power source, the expense of which dictates round-trip flight. Under such conditions, chemical and nuclear propulsion systems become competitive. Two cargo OTV concepts are considered. One is for the SPS configurations that involve primary construction at GEO and therefore requires independent OTV propulsion systems. The second is for SPS configurations that involve construction at LEO, either total or in modules, which can provide energy for propulsion for orbital transfer.
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