transfers energy to the helium loop which, in turn, drives the Brayton turbomachines. As in the solar system, waste heat is rejected through a gas-to-liguid heat exchanger to a liquid metal cooling loop. The mechanical configuration is not clear for this system, but its ac output will be provided to the microwave generator for transmission to the ground. (4) The Power Relay Satellite This concept consists of an on-orbit reflector which receives microwave beams from earth-based power sources and reflects them back to earth near a power distribution point. The relay function is generally seen to be passive and to consist of a primary 0.6 mi. (1 km) structure that is built up of 354 ft. x 354 ft. x 66 ft. (108 m x 108 m x 20 m) deep bays. A substructure would provide support for the microwave reflector. The ground transmitting antenna would be a phased array with wave guides and convertors similar to the orbital antenna in the power generating satellites but larger. The receiving antenna would be essentially the same. The feasibility of the PRS significantly depends on large-scale, highly efficient transmission of energy by microwaves, as well as very high precision reflectors and attitude control. It is not currently seen to be competitive with ground systems for distances less than about 3500 miles (5600 km). Furthermore, the high power density of the up-beam in the earth's atmosphere may violate reasonable safety standards. 2. Materials Materials are the basic building blocks of all space vehicles and several materials improvements are of particular interest to the SPS. These include weight reduction of structural materials, high temperature materials (particularly inportant for nuclear and solar thermal conversion processes), lightweight and durable reflecting surfaces and large cost reductions in solar cells. Temperatures encountered in the heat exchanger tubing used in the nuclear or solar thermal systems are on the order of 1200°F - 1600°F (925K - 1150K) which combined with weight restrictions, a capability to withstand mechanical stresses, and reasonable
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