ground-launched SSPS demands. Present costs of solar panel arrays for space applications are based on manual assembly techniques and are therefore much too high for application7 to an SSPS; they are typically $175,000 per kilowatt. A more reasonable starting point is the 1971 figure of $5,000 per kilowatt for single-crystal wafers 5 cm in diameter. The necessary target figure7 for a competitive SSPS launched from the earth is about $220 per kilowatt, about half the present cost of a large, coal-fired central power station. As an alternative to a photovoltaic array for SSPS power, Woodcock and Gregory^ have considered the use of closed- cycle helium turbines driving conventional electric generators®. In that alternative the specific mass must be reduced from presently attainable values (10 kg/kw) to about 5 kg/kw. To achieve that reduction, Woodcock and Gregory have assumed a development program in which the turbine inlet temperature could be increased to a value considerably higher than that used in current practice. For an earth-launched SSPS the cost of lifting components to geosynchronous orbit from the earth would be critically important. For a photovoltaic SSPS of 0.88 kg/kw, 7 the necessary lift cost figure would be $220/kgsy. For a turbogenerator SSPS of 5 kg/kw and effiency e of 70 per cent, the performance demands on the lift vehicle would be even more severe: $575/kgsy.
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