The COTVq, applicable to SPS construction in GEO, need not handle payloads in as large segments or with as low acceleration limits, but it does need to supply its own energy. Possibilities are solar energy collected by solar cells or thermal systems, nuclear reactors, and chemical propulsion systems. Present analyses indicate that an 0o/Ho chemical COTVc CL U may be the least expensive option. A comparison of various stages and reusability versus expendable choices in terms of propellant-to-payload ratio versus stage mass fraction is shown on figure VI-6. It is seen that the two-stage reusable system requires more propellant than does a single-stage expendable system but it has the advantage of reusing the hardware. Two primary 02/H2 chemical propulsion COTVg configurations have been investigated. The first is a two-stage reusable system with both stages returned to LEO. The second is a 2-1/2 stage vehicle, which is based on the assumption that an empty drop tank may be left with the SPS as parasitic weight without penalty, or perhaps even utilized in the SPS construction at GEO. The configuration and characteristics of the two and one-half stage system, as shown on figure VI-7, are represented as the "nominal" COTVG for the program model work presented in section VII. Partial recovery concepts may be devised. One example suggested by Rockwell International may merit further consideration. In this concept, the engine (modified SSME) and avionics are returned from GEO to Earth ba11istica1ly and recovered at sea. The propellant-to-payload weight ratio as a function of stage mass fraction for a NERVA solid core nuclear propulsion OTV was investigated, and its advantages in propellant weight brought to LEO over the 02/H2 system appeared to be small and may not be preserved if longer reactor life than 10 hours is specified. The reduced propellant requirement, if any, must be balanced against the greater vehicle and space operations cost of nuclear stages and the performance penalty and operations costs associated with disposal of the reactor at the end of its useful life. In the Future Space Transportation System Analysis Study (NAS 9-14323), it was concluded that the solid-core nuclear OTV was a higher cost option than 02/H2 stages. E. Personnel Orbital Transfer Vehicle The POTV will be utilized to transport all personnel from LEO to GEO and return to LEO and to transport high-priority cargo to GEO. The short trip time (less than 1 day) and small payload requirement of the POTV preclude commonality with the high-specific-impulse, low-thrust cargo OTV systems being considered. Therefore, except in the case of the independent high-thrust, chemical-propulsion cargo OTV, the POTV is considered as a special-purpose device optimized for personnel transfer between LEO and GEO.
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