reduction was based on higher operating temperatures and thus lower cell efficiency. Any change in this number produces large weight and size variation in the overall station as demonstrated by the weight increase from 17.8 to 31.35 X 106 KG. The Boeing transmitting antenna is about the same size as Raytheon's, but uses twice the number of amplitrons. Heat rejection capabilities of the tubes and surrounding structure are not sufficient to allow this, so that either a larger antenna or two antennas would be needed. Other JSC comments have been incorporated into the study in the evaluation column. Performance degradation as a result of long term exposure to the radiation, material, and space vacuum environment may be a significant problem for both the photovoltaic and Brayton cycle systems. Performance degradation of silicon solar cells of the type proposed for the reference design SPS is discussed in Section IV-B-1-6 of this report. The ADL/Raytheon/GAEC concept allows six percent reduction in power output during the first 5 years of operation. No discussion of continued reduction in output after five years was presented. Optimistic estimates would indicate a continued degradation of about one percent per year. In the case of the Brayton cycle system, environmentally induced degradation occurs at both the solar concentrator and the waste heat radiator. Based on accelerated life tests of selected reflective materials subjected to proton flux, Boeing reports an approximate four percent per year reduction in concentrator reflectivity. The proposed means of repairing this condition is to utilize multilayer reflective material so that a degraded surface may be peeled off, thus exposing a new surface.. Boeing estimates a 30 percent loss in radiator areas as a result of meteoroid impact. This is accommodated by oversizing the radiator initially. Loss of radiator area results in higher compressor inlet temperatures which, in turn, reduces overall cycle efficiency. Approximate analysis indicates that a 30 percent radiator area reduction would cause 27 percent per reduction in cycle efficiency or about 0.9 percent per year reduction in output power. This assumes that all other factors remain constant. Restoration of damaged radiator area may be accomplished by on- orbit repair or replacement of damaged sections. In summary, environmentally induced performance degradation will occur in photovoltaic and closed Brayton cycle systems. Unless on-orbit repair can be accomplished, hardware replacement will be necessitated resulting in significant increased mass-to-orbit to maintain design power output. Table IV-APP-2 provides a summary listing of advantages and disadvantages of the photovoltaic and Brayton cycle systems. This qualitative comparison is based primarily upon information found in the existing system design.
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