4. Transportation Propulsion systems are expensive to develop and they are usually the major component of a vehicle system and, considering fuel, comprise the majority of the weight at launch. Therefore, propulsion technology improvements can substantially improve performance and reduce cost. Most researchers in the SPS area consider that a successful program will require a new generation of launch vehicles, including a heavy lift launch vehicle (HLLV) which can place on the order of 500,000 lbs (225,000 kg) into low earth orbit. This compares to a shuttle capability of 30 tons (27,000 kg). This problem is currently under study at the Johnson Space Center and at various contractors. It is discussed more fully in Section IV of this report. It is also generally agreed that the SPS would be assembled (at least partially) in low earth orbit and then transferred to a geosynchronous orbit using some new orbital transfer vehicle (OTV). Ion propulsion is an often mentioned option for this stage although cryogenic tugs do offer one alternative. NASA (Ref. A6) has outlined a rather detailed list of tasks for the development of the OTV and Section IV of this report also deals with the development of a new transfer stage. 5. Large Space Structures All investigators agree that considerable advances are needed in the fabrication and assembly of large space structures prior to development of the first operational SPS. Section IV, which contains a rather detailed discussion of this area, lists 14 requirements/considerations that must be addressed. The Johnson Space Center sponsored a study conducted by Martin-Marietta which dealt in detail with the orbital assembly and maintenance of large space structures. Many other studies dealing with the design and construction of these systems have been completed or are in progress. Both Marshall Space Flight Center and the Johnson Space Center have proposed alternate designs to the configuration proposed by A.D. Little, et al., for the photovoltaic system based primarily on structural considerations.
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