upwards from 50 to 90 days, (Ref. A4, A9) * but no problems associated with this were cited except for the system's transit through the radiation belts and the impact on fleet size. Trajectory shaping could minimize this transit duration although it would lengthen the overall transit time. Solar cell power could not be used during the radiation belt transit since the cell arrays would have to be stowed to prevent radiation degradation. Also, if the propulsion system derives its power from the SPS, provisions for independent propulsion for the return trip would have to be included (Ref. E6) . Chemical orbital transfer vehicles have also been considered to some extent although gravitational forces on the structure could be a severe problem. The transfer times would be quite short (1 day) and payload capacities of 250 tons (225,000 kg) have been estimated (Ref. A9). Such a vehicle would apparently be an outgrowth of shuttle/tug development, and could conceivably be available by the 1990's. Nuclear electric transfer vehicles are also a possibility. Ref. A9 indicates that they would require 90 days for the transfer, and would have a payload capacity of 100 tons (90,000 kg). Ref. E3 describes an Alpha nuclear rocket engine design and the Gamma engine which would be its successor. Original schedules called for an Alpha engine to be available in 1982, and a Gamma engine in 1984, but these schedules are no longer applicable since the nuclear rocket program has been cancelled. "Self-propelled" SPS modules are also a possibility. This would involve long transit times to GEO (75 days), but payload equivalents of 3500 tons (3.2 x 10^ kg) could be accommodated. For any type of orbital transfer equipment that is itself quite large, orbital assembly would probably be required. The tradeoff is several orbital transfer stages operating in tug-like fashion (Ref. E6). Since ion propulsion is most attractive for both the photovoltaic and thermal SPS approaches, as indicated above, both would require advanced ion propulsion by 1989 to 1992. A key concern in the development of such a system is the large thruster diameter that will be required. *Earlier documents such as Ref. Fl and A2 estimate transfer times of 6 to 12 months.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==