A concentration factor of 2 will reduce the efficiency of an 18% silicon solar cell to about 14% at the operating temperature when heat rejecting coatings are used for the solar cell array. Increases in solar cell thickness and lower efficiencies will be reflected in increased capital cost because more material will have to be transported into orbit. The exposure to the space environment is projected to result in logarithmic degradation of silicon solar cells with a 6% loss of the original efficiency after the first five years. Micrometeroid impacts are projected to affect 1% of the solar cells during a 30-year operational lifetime. Recent progress in gallium arsenide solar cells has renewed interest in their use in the SSPS. The advantages of gallium arsenide solar cells are the higher efficiencies that have been reported at high concentration ratios and the lower susceptibility to degradation in the space environment. In addition, gallium arsenide solar cells may be produced at about one-tenth the thickness of silicon solar cells. Thus gallium arsenide deserves attention as an alternative solar cell material. As a result of studies over several years, the design for the SSPS based on the silicon solar cell array configuration shown in Figure 4 has evolved. The two solar collector panels are designed to provide a power output of about 8500 MW which results in an effective power output at the receiving antenna bus bar of about 5000 MW. A 100-meter diameter central mast and stiffened carried-through structure running through the assembly provide structural integrity. A microwave transmitting antenna is located between the two solar collector panels. The solar collector panels are arranged to face the sun continuously while the microwave antenna will rotate once a day with respect to the solar collector in synchronous orbit. The solar collector panels are supported by both nonconducting and conducting structures which carry the power to the microwave generators via the central masts. Rotary joints are provided at the perimeter of the central mast to allow rotation of the microwave transmitting antenna. Dielectric materials are used for the continuous support structure which is transparent to the microwave beam. These joints are the only major continuously active components in an otherwise passive satellite. Analyses of structural stiffness indicate that conventional, analytical techniques and structural design techniques are applicable to the SSPS. The structure of the SSPS will be subjected to thermal stresses and distortions induced by thermal gradients during the eclipses of the SSPS by the Earth's shadow for a short period before and after the equinoxes. The dwell time in the Earth's shadow will reach a maximum of 72 minutes during this period. Because the structure is so large, the thermal exposure cycle could cause it to oscillate; this possibility has to be evaluated in terms of fatigue effects which could shorten
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