9 . 5 the lunar surface or the trailing Lagrangian point LS, the assumed colony location. A vehicle must be built to transfer the payloads left by the shuttle/DOL launch system to the desired destination. Using Hohmann minimum energy transfers and patched conic analysis as an approximation to the highly complex three-body problem in Earth-Moon transfers, the total velocity change from low earth orbit to either LS or lunar orbit is remarkably similar: 3.918 km/sec to LS and 3.899 km/sec to a circular lunar orbit at an altitude of 185 kilometers. Therefore, a single optimized vehicle is applicable to either transfer. The first decision made in connection with the design of the orbital transfer vehicle (OTV) is that conventional space shuttle main engines are used for propulsion. These engines give a specific impulse of 455 seconds,near the theoretical maximum for their hydrogen-oxygen reaction. They are throttleable, going from 50 to 109 percent of their rated maximum thrust of 2.09 million Newtons (9.3). Since the OTV is only for interorbit transfers,there is no necessity to design it for a thrust to weight ratio greater than one,so one SSME is assumed to be the primary propulsion system. Judicious design of OTV reaction control system engines firing in the +X (aft) direction should allow for the use of these smaller engines as backups, in the event of a failure of the primary SSME. The choice of a chemical engine over the logical competitor, a NERVA class nuclear engine, is primarily based on economic reasons. As in the choice of a space shuttle derivative Earth-launch system, it is felt that RDT&E costs for the OTV should be minimized. It is necessary for an OTV to be operational by year 7 from program go-ahead (see Section IX.3). This requirement could be uncertain with an untried nuclear engine. The OTV performs a routine mission of orbital transfer and rendezvous,and is therefore designed as an unmanned system. Unmanned orbital rendezvous is routine for the Soviets, and is baselined for the American space tug to be developed in the early 1980's. However, while the system is unmanned, it should still be a man-rated system,in order to serve also for personnel transport during working crew rotations. For this reason,the SSME is doubly attractive, due
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