began in the early 1960s. Development funding was provided by industry, the National Aeronautics and Space Administration (NASA), and the Department of Defense. In 1962 a 3 kW demonstration system (Fig. 1) was built and tested with an electrical resistance heater to simulate a radioisotope heat source [8], In 1963 the United States Air Force funded a 12-month effort to conduct component development on compressors, lithium fluoride thermal storage, and space radiators. Also in 1963, NASA initiated funding of development work on a 10 kW solar powered system designated Engine A. This was a recuperated engine with a turbocompressor and a separate turbine-driven alternator. Extensive development work was conducted on radial and axial turbines and compressors, solar collectors, recuperators, gas bearings, controls, and other components. Based on the Engine A experience, NASA embarked in 1965 on Engine B, a program to produce a 10 kW (nominal) CBC system for space applications [9]. Fluid Systems was selected by NASA to design and produce this unit. The major difference from Engine A was in the configuration of the turbomachinery. The Brayton Rotating Unit (BRU), Fig. 2, had a single rotating element consisting of the rotor of a brushless alternator mounted between a centrifugal compressor and a radial inflow turbine. Padtype journal and thrust gas bearings, using the working fluid as the lubricant, supported the rotor. This was a highly successful program culminating in 52 000 hours of closed loop system testing in a vacuum chamber. These hours were accumulated on two units, one of which ran for 41000 hours including 38000 hours of essentially unattended operation. At the completion of this test, the performance of the turboalternator compressor was identical to that measured the day that it was installed. During the 1960s and early 1970s many studies of space power systems were conducted that utilized the closed Brayton cycle with input energy provided by nuclear
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