For this solar photovoltaic approach, a large cryogenic tug would be used for the 1 GWe pilot plant, and an advanced ion propulsion system for the 5 GWe operational plant. Schedules given for the development of these systems are: Large Cryogenic Tug: Development from 1982-1987 Advanced Ion System: Development from 1985-1992 A "maneuver tug" would also be developed for maneuvering and handling materials and equipment at the LEO assembly site. (2) Solar-Thermal SPS For the solar-thermal SPS approach, assembly would be in LEO, with transport to GEO accomplished by "orbit transfer module sets." These would be self-powered (photovoltaic) or SPS-powered electric thrusters which would attach to completed SPS power generation modules (Refs. E9, A12). Reference A12 describes these thruster modules as MPD (magnetoplasma-dynamic) electric thrusters utilizing argon propellant. An LO^/LH^ rocket system is under consideration for propulsion during solar eclipse and for the return from GEO to LEO. Reference E7 introduces the term "electric rocket tugs," which would be used for orbit transfer and are apparently the same as the electric thrusters discussed in Refs. E9 and A12. Reference E7 also points out the need for an orbit transfer vehicle for maintenance and re-supply. In terms of development schedules, Ref. E9 plans for the first orbit transfer module sets to become available in 1989, with a build-up to four systems by 2000. The cargo orbit transfer vehicle would be available in 1991. Reference E7 calls for development of the "electric rocket tug" from 1978 to 1988, which is consistent with the 1989 target date given in Ref. E9. (3) Overall Status and Requirements Both the SPS approaches, as discussed above, would require advanced ion propulsion by 1989 to 1992, and both, to the extent possible, would take advantage of available SPS power during the transfer.
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