while hydrogen is to be used for the second stage because of its superior performance. Personnel Launch Vehicle (PLV) - The PLV transports crews and crew supplies to LEO. The vehicles considered for this function were all derivatives of the STS. MSFC and Boeing have investigated various concepts for replacing the STS solid rocket boosters (SRB's) and have, in particular, studied a twin LOX/LH^ strap-on booster configuration. JSC also investigated several alternatives including series and parallel burns, winged and ballistic configurations and the use of LH^, RP-1, and CH^ (methane) as fuels. Rockwell has proposed a concept for modifying the Orbiter to carry 68 passengers and other concepts have ranged between 50 and 100. Improved booster performance would permit even higher passenger payloads. The booster configuration actually to be utilized for the SPS program will depend on whether it is designed strictly for the SPS traffic requirements or if a suitably modified STS will be available. The reference system is based on a design for SPS requirements and compatibility with the SPS HLLV technology. It utilizes a methane fueled winged booster, series burn and a resized, smaller external tank. (See figure 19.) Cargo Orbit Transfer Vehicle (COTV)- The COTV transports cargo from LEO to GEO. Several approaches to meeting this requirement were considered: A conventional chemically fueled high thrust COTV with a short round trip time and a high degree of reusability, a nuclear COTV featuring a propulsion system capable of high thrust and high specific impulse, and an electrical COTV requiring an extended transit time, but deriving the electrical power to run its ion engines from a solar cell array. The electrical approach was subdivided into two options, the self powered option in which part of the SPS solar array is used to power the COTV and the independent option in which the array remains dedicated to the COTV function. The chemical option is a space-based common stage system with both stages having the same LOX/LH^ capacity and utilizing conventional
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