turbine inlet (1984 technology), which is near the upper temperature limit for the superalloys. The Oberhausen Brayton unit in Europe has been in operation for 11 years with a turbine inlet temperature of 131 OF and air as the working fluid. With an inert gas working fluid it is felt that the Oberhausen technology could be extrapolated to build an SPS turbine with a 1600F inlet temperature. Increases in materials technology would be required for a 1900F turbine, particularly since the absorber would have to operate at 2300F, but it is not unreasonable to assume, based on past technology increases, that technology advances in superalloys and refractories will occur within the next ten years, which will make the 1900F turbine inlet reasonable for the SPS. Small gains in absorber efficiency, coupled with advanced aerodynamic turbine and compressor design techniques, will contribute to an increase in overall system efficiency. More exotic schemes such as cooling of turbine blades, ceramic turbine parts, and advanced recuperator materials are also possibilities for the future. The Boeing-study-assumed turbo-compressor-generator package size of approximately 300 MW per unit is consistent with NASA projections for maximum payload weight in the HLLV (1,000,000 lbs) and is based on a specific weight for the machine of 3.3 Ib/kW, The heat rejection concept originally proposed by Boeing was a gas radiator system in which the working fluid was piped directly to the radiator panels where heat pipes were employed to distribute heat to the fins. The gas radiator was favored because even though the pressurized gas tubes were somewhat larger and heavier than liquid tubes, the heavy interface heat exchanger involved with the use of the liquid loop was avoided. However, as preliminary design studies progressed, it became evident that since the recuperator had to essentially be in a pressure vessel, a liquid loop would not be as heavy as originally thought since the interface heat exchanger could be included in that same pressure vessel. When this was realized, the liquid radiator system began to look more attractive from standpoints other than weight, such as working fluid pressure drop and meteoroid protection. To meet SPS life requirements, a liquid metal, as opposed to a conventional organic fluid, was chosen. Further, conventional electrically driven fluid pumps were baselined to flow the liquid through the radiator system. It is felt that the liquid radiator system is a good choice. Abundant technology on fin-tube space radiator systems exists, and scale-up to the order of the SPS involves only problems of engineering design and not new technology. Adequate surface coatings for the 30-year life requirement may be a problem. Excellent radiator surface coatings are presently available for space radiator systems, but 30-year endurability has not been proven. In the event a chosen coating has life limitations; e.g., due to damage by ultraviolet radiation, a coating maintenance program would be considered, and this is not an insurmountable problem. Statistical data on meteoroid damage is available, but more data is needed at geosynchronous altitude. This, too, will be available in time, and meteoroid protection to accommodate these environmental conditions is an engineering design, and not a technology problem.
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