maximized instead of converter current. The optimum collector temperature is found to be about half the emitter temperature. Two heat transfer mechanisms were accounted for in these calculations: electron cooling and radiation cooling. The effect of electron cooling is much greater than that of radiation cooling. Heat conduction was not accounted for in this analysis but should be considered. Radiator. The radiator is one of the most massive parts of the system. Required radiator area is determined by the amount of waste heat, the heat rejection temperature and the ambient temperature. Ambient temperature was taken as 4 K. External heat inputs considered were solar radiation (1399 W/m2) and Earth's emittance and albeido of 243 W/m2. The emissivity of the radiator was taken as 0.9 and its solar absorptance as 0.21. The following equations were used to determine the required radiator area: where Q is the external heat input power density in watts per square meter; Pt the internal heat generation in watts; Jeff the effective radiator area in square meters; er the Stefan Boltzmann constant (5.67 E — 8 W/m2/K4); e the emissivity; a the absorptivity; F a shape factor for solar radiation; G the solar radiation on a plane normal to the sun (1399 W/m2); and T the radiator temperature in K. The payload was assumed to shield the radiator from the emitted and reflected radiant load from Earth, so the external heat input is solar alone. The radiator configurations for both systems are shown in Fig. 10. The radiator temperature was nearly equal to the condenser temperature in the Rankine system and to the collector temperature in the thermionic system. Fig. 11 (a, b) compares the required radiator areas for the two systems as a function of principal design variables. The radiator is covered by beryllium armor to protect it from micrometeoroid damage. The beryllium armor thickness was selected to protect the radiator heat pipes from punctures. These sodium heat pipes are used to keep the radiator isothermal. Micrometeoroids were assumed to have an average density of 0.5 g/cm3 and an average velocity of 20 km/s.
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