would be available. The Shuttle will be capable of up to 500 reuses without major refurbishment; turnaround time is two weeks (Ref. E3). In the area of orbital transfer vehicles, the Atlas-Centaur is currently available. It is an expendable vehicle, and its payload capacity is 1 ton (900 kg) (Ref. A9). In the 1980's, a tug may become available for use with the Shuttle system. The tug engine would be a small-scale version of the Shuttle main engine, and its development is expected to somewhat parallel Shuttle development. A 20 mission life for the tug has been postulated. Its payload capacity would be 3,000 pounds (1,400 kg) and it is designed for ground-based propellant loading, tug/payload assembly, and refurbishment (Ref. E3). Ion engines, which are under consideration for orbital transfer, are now under investigation at the Lewis Research Center (LeRC). To date, less than 3,500 hours of operation have been accumulated in space, and less than 8,000 hours in ground tests. Thruster diameter, an important parameter, is currently on the order of 12 to 60 inches (30 to 150 cm) (Ref. A2). b. SPS Transportation System Requirements To meet the transportation system requirements for SPS development and deployment, four basic types of vehicles will be needed which can be categorized as follows: • A heavy-lift launch vehicle (HLLV) • An orbital transfer stage • A personnel and equipment transport • Interim tvpes of vehicles for use during the development of the above three While all vehicles discussed in the documents examined are accommodated by the above four categories, efforts during the various studies have spawned a multitude of system types, names and acronyms. Exhibit 17 lists these, together with an indication of intended use and source reference. With respect to the first category, i.e., the HLLV, there is fairly general agreement that a payload weight capacity of about a half million pounds (225,000 kg) will be needed. This capacity is required because of the massive amount of hardware and material that must be placed into
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