This system is referred to as the ''closed Brayton cycle" and has been demonstrated for Earth-based use in sizes up to 50 MW. In a power satellite a number of engines (e. g., 50) may be used to promote redundancy. The major moving assembly of the engines, the turbocompressor, is supported on gas bearings for long life. The second law of thermodynamics permits*useful work to be removed only from a heat engine which has a temperature differential across it. Thus, a cooling system is required in the form of a radiator to reject heat to space. This cooling system and a heat exchanger (called a recuperator) are also part of the gas circuit. The radiator utilizes pumps which circulate a liquid metal that has picked up heat from the helium flow. The liquid metal passes through panels composed of tubes and fins that dissipate the heat. Meteoroid puncture of these tubes is a potential problem; however, it is possible to align the panels ''edge on" to the prevalent meteoroid direction so that the puncture rate is acceptable. An alternative radiator panel would be one composed of heat pipes in which each has its own inventory of metallic working fluid. As with the photovoltaic concept, structural and attitude control systems are required, along with the transmitter and its rotary joint. 1. 2. 4 ADDITIONAL POWER GENERATION ALTERNATIVES The thermionic converter is a potential alternative to thermal engines or solar cells. These passive devices use high temperature thermal energy to produce direct current electricity. High solar concentration ratios (over 1000) are required to achieve the necessary temperatures. Because of the high temperatures, the necessary cooling can be accomplished by fins attached to the thermionic diodes. Finally, instead of a solar energy source, it may be possible to use nuclear reactors. These could energize turbomachines to produce electric power for the microwave transmitter. Breeder reactors could be used to extend our uranium resources; ''bomb grade" fuels bred in these reactors would remain in geosynchronous orbit. 1. 2. 5 AUXILIARY SYSTEMS The space transportation of power satellites is generally considered to take place in two stages. Large reusable ''space freighters" would be used to reach low Earth orbit. Another orbit transfer system would be used on the way to geosynchronous orbit.
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