the Rankine cycle system concept with either organic working fluids for lower-temperature operations (approximately 700°F), or liquid metals (mercury, NaK, or potassium) for higher temperatures, was indeed attractive. Unlike ground-based power systems, space systems must dissipate waste heat by radiation, which makes size and weight a function of the fourth power of the radiating temperature. Since the radiator is the heaviest component of the system, power plant weight can be minimized by designing a minimum-area radiator system. This can only be done by raising the radiating temperature. Thus to capture as large a portion of Carnot (ideal) cycle efficiency as possible, a high turbine inlet temperature is desirable. Technology advances brought about by intensive development efforts continued over the years to improve Rankine cycle systems. However, as longer missions were planned, the advantages of the Rankine cycle were diminished by problems such as decomposition of the working fluid in organic Rankine cycle systems, and internal corrosion in the liquid metal systems. These problems are sharply accelerated by elevated temperatures and long-duration operational requirements. As developmental problems in extending Rankine cycle technology persisted, the Brayton cycle began to look more attractive, Early applications of open-cycle Brayton systems came in the aircraft and utility industries. Extensive research and development programs on compressors and turbines for aircraft propulsion have been conducted for over 35 years. Early gas turbine work in the utility industry concentrated on airbreathing gas turbines for peakload power plants, and later for base-load and hybrid (Brayton peakload, Rankine base-load) systems. Work on closed Brayton cycle space power systems above one kilowatt for extended mission durations (months or years) began in the late 1950's and has progressed to its present high degree of sophistication in component and system technology. The closed Brayton cycle has also found application in the marine industry, rails, vehicular transportation, and propulsion. Government contracts (with one company^ (1963-1974) totaled more than twenty million dollars. The Federal Republic of Germany presently has five closed Brayton plants in operation4, the largest of which is at Gelsenkirchen with a continuous output of 17.25 megawatts and a 30 percent overall thermal efficiency. This plant has operated for a total of 48,000 hours as of May 31, 1975. An earlier plant built at Ravensburg (2.3 megawatts, 25% efficiency) had 110,000 hours of operation on May 31, 1975. Thus it is evident that there is ample conversion system technology to draw upon for the design of the SPS. However, while the conversion device is the heart of the system, two other essential components are the reflector-absorber subsystem and the heat rejection subsystem. Figure IV-B-l-c-1 schematically illustrates these subsystems for the SPS application.
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