on the interior walls. The cavity is insulated to reduce energy loss through the walls. The high temperature insulation is composed of multiple thin layers of refractory metals separated by zirconia or yttria pellets. Reflection and reradiation losses through the aperture are functions of cavity geometry. Nuclear Reactors Dr. J. Richard Williams of the Georgia Institute of Technology was the consultant on reactors for this study. Breeder reactors were baselined because uranium resources will not last throughout the baselined program duration without them. Candidate reactor types were evaluated on the basis of their availability by the time required, complexity of the fuel reprocess system, and achievable temperature with the required 30 year life (baselined for all satellite systems). Power Distribution Photovoltaic systems all use series chains to produce 20 000 V or more as required by the transmitter. Because the thermionic systems require rotary converters to step up the output which is limited to approximately 25 V, the thermionics and Brayton cycle can use alternating current distribution at high voltage. Photovoltaic systems will thus use slip rings at the rotary joint to the transmitter, while the other systems can use a rotary transformer. Nuclear power systems do not require a rotary joint because they are not Sun-facing. All systems require switchgear at various points in the distribution network. Solar Cells Two solar cell types were selected for emphasis: conventional silicon cells and gallium arsenide (GaAs) heterojunction cells. Performance in the 1990's was estimated by the projection of past improvement trends, taking fundamental limitations into account where required. A photovoltaic optimizer model assessed the impact of solar concentration, cell cooling, and cover glass thickness on total satellite cost. Silicon and GaAs systems were found to achieve a cost benefit from solar concentration and cooling. Solar concentration reduces the total cell ''buy" and allows thicker cover glass with lower mass penalty. Cooling fins behind the cells reduce their temperature and raise cell efficiency. SYSTEMS The previously described subsystems were integrated by optimization models to select design points for the total systems. Several initial findings were:
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