operates at a turbine inlet temperature of 1500°F (~1090 K) as compared to the 2115°F (1430 K) planned for the solar-thermal SPS (Ref. A17). This turbine inlet temperature parameter influences most other system parameters. Weight is directly impacted due to the addition/reduction of materials, and cycle mass flow decreases with increased temperatures, which impacts the cycle temperature ratio. This, in turn, effects the compressor pressure ratio and so forth through a complex chain of parameters. The impact of this temperature is especially pertinent in the area of material selection. Ref. A17 indicates that although tungsten/rhenium and tantalum alloys might be attractive for heat exchanger tubing and manifolds, their characteristics are less well defined than those for the columbium and cobalt alloys which have been baselined. These too, apparently, require some improvements for SPS use. Ref. A4 reports that current systems operating at turbine inlet temperatures of 1500-1800°F (1095-1265°K) have achieved efficiencies in the 35-40% range. It further states that through technology advances which will permit higher temperatures, efficiencies of 50% should be achievable in terrestrial systems. For space systems, Ref. A4 points out that the tradeoffs between peak cycle temperature and thermal collector performance, as well as between heat rejection temperature and waste heat radiator weight, will probably lead to optimum efficiencies of less than 40%. For SPS concepts utilizing nuclear power sources, energy conversion would be accomplished in the nuclear-thermionic approach via a molten-salt breeder reactor (MSBR) , whi^h would produce heat to operate thermionic diodes. In the nuclear-Brayton approach, an MSBR would also be utilized but the conversion would be accomplished by a Brayton turbo/generator arrangement. Ref. A17 states that the MSBR is a unique breeder concept in that a single liquid fuel mixture contains both fissile and fertile fuels, and therefore processing of solid fuel elements is not required. The MSBR concept has essentially been dismissed by ERDA in a recent decision and therefore power generation approaches based on it are probably not valid. Ref. A4 describes the current status of space nuclear systems. These utilize the radioisotope plutonium-238 as fuel, and include the SNAP27
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