no advantages could be ascertained by placing nuclear reactors in geosynchronous orbit. Failure mode corrections would appear to be more difficult in space than on earth, and potential poisoning at geosynchronous orbit altitude could present a long- range problem to the future use of this very important earth orbit. The Brayton cycle solar thermal concept was rejected because it requires a more advanced material technology than the Rankine cycle for the turbomachinery (ceramic vs refractory metal) as a result of higher turbine inlet temperature. The flat (nonconcentrated) array was preferred for the silicon solar cell concept because of construction simplicity and comparable costs at a concentration ratio (CR) of 2.0. Gallium arsenide concepts have lower costs when concentrators are employed. Costs at a concentration ratio of 2 and 5 were comparable, but the CR=2 was preferred because flat 1/2-mil Kapton concentrators can be used. At a CR/5, the optics problem is more severe and construction becomes more difficult. Of the three concepts shown in Fig. 3, the GaAs concept was preferred because of lower cost. The silicon solar array SPS concept was higher in cost even though the unit cost of the array blanket for silicon is estimated to be only 2/3 the cost of a GaAs array blanket. This cost disadvantage for GaAs arrays is more than offset by a reduction in the array size because of the GaAs array reflectors (solar blanket area is reduced to half the silicon blanket area) and a much lower array mass (which reduces transportation mass). The lower array mass is attributed to lower solar blanket mass per unit area for GaAs cells and the use of very light weight reflectors. The end result is a cost savings of about 0.68B for the average cost of a satellite when a GaAs solar array is used. This compares to a satellite total cost of 5.3B for the GaAs concept. The GaAs solar cell also has the ability to anneal damage from natural space radiation at the normal operating temperature of 125°C, which occurs at CR=2. Silicon solar cells require an annealing temperature of 500°C to remove radiation damage. SUMMARY OF RECENT ROCKWELL SPS STUDIES During the past year, Rockwell continued the system studies under contract to NASA/MSFC that started in March, 1977. During this period, Rockwell activities focused on the following: Definition of the GaAs NASA/DOE Reference System Definition of alternatives to the Reference System End-to-end analysis and definition of SPS construction (ground and space elements) Conceptual definition of a 2-stage, parallel-burn Heavy Lift-Launch Vehicle (HLLV) and the Electric Orbit Transfer Vehicle (EOTV) and Personnel Orbit Transfer Vehicle (POTV) Experiment/verification program definition Definition of the GaAs NASA/DOE reference system The reference system defined by NASA and DOE contains the two satellites shown in Fig. 4. The major difference between the two satellites is the energy conversion approach; one uses silicon solar cells in a planar, nonconcentrated array and the other uses GaAs solar cells in a planar array with CR=2. The microwave antenna on both satellites is located at one end of the satellite. Power at the utility interface on the ground is 5 GW. Construction of the satellite is accomplished at geosynchronous orbit.
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