(3) Summary For both solar-photovoltaic and solar-thermal approaches, a fully reusable HLLV with a half-million pound (225,000 kg) payload capacity is being considered. One contractor gives the date that this should be operational as 1988, the other 1992. In either case, several vehicles would be needed to meet the logistic constraints for the required number of flights. For the most part, the power station contractors have not yet defined the HLLV parameters in any great detail. In addition to the work done by the power station contractors, NASA has performed some in-house studies of its own. Reference E5, the NASA forecast for 1980 to 2000, discusses both the HLLV and the VTOVL (vertical take-off, vertical landing) vehicles. This forecast predicts that chemical propulsion will continue to be used throughout this period, although electrical propulsion options may become available for low thrust orbit-to-orbit propulsion toward the latter part of the period. This study recommends future NASA emphasis on low cost, earth-to-orbit transportation. Reference A4, a NASA assessment of the power station concept, identifies the HLLV as a critical item, but states that present studies represent a good start, and that shuttle technology development must precede a concerted effort in the HLLV area. This document states that the rocket engines, propellant tanks, and avionics developed in the shuttle program can be assembled into a partly reusable heavy lift vehicle for primary transport of large structural elements and material. Based on these factors, it concludes that while the HLLV is an extremely critical item in the long run, it is less so now. Reference A2, a contractor report, similarly points out that operating experience in shuttle transportation of prototype elements to LEO is essential to the orderly evolution of a very high volume transportation system. b. New Transfer Stage There are several possible mission profiles for the ultimate placement of an SPS in geosynchronous orbit (GEO). With the exception of direct transport to GEO and assembly at that altitude, all profiles
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