power supply which was used in the Apollo lunar program and the SNAP19 which was used on Nimbus satellites and Pioneer's 10 and 11. Conversion efficiencies were on the order of 4 to 6%, and power degradation rates were from 2.4 to 5.5% per year. Improved space nuclear power systems are feasible but do not have much relevance to extremely high powered SPS systems. Ref. A17 states that the nuclear-thermionic SPS concept is not feasible with projected 1985 technology. The reason given for this is that the temperatures involved would limit the efficiency of the thermionic diodes to 23%, thus 77% of the total energy would have to be dissipated as waste heat and this would require more pumping power than the electric power produced. An entirely different reactor than the MSBR would be needed to make effective use of thermionic conversion. b. Implementation Requirements Exhibit 21 lists some of the energy conversion requirements discussed in the preceding subsection, as well as several others which are more specifically implementation-oriented. From the exhibit, it can be seen that cooling requirements would be met through approaches ranging from passive cooling fins to heat exchangers utilizing liquid metal cooling loops. The radiators required for ultimate heat dissipation into space in all but the solar-photovoltaic approach present the potential for micrometeorite impact damage, as do the solar reflectors. Some radiator approaches would utilize armor, and others "meteroid bumpers." For the solar-thermal concepts, a metallized plastic film such as aluminized Kapton would be used for the solar reflectors. Concentration ratios of more than a 1000 would be required. These reflectors would, in turn, focus onto the cavity absorbers. Insulation and low emissivity external coatings for the absorbers would be required to limit heat loss through the walls. The rotary joint is required to transmit power from the conversion devices to the microwave system. Its maximum pointing error would be limited to about 1 arc minute, and it would have to be movable in 2 axes.
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