The structural weight for the solar cell array concept employed in this study is less significant to the baseline selection process than other systems (solar cell blankets, reflectors, power distribution system, and attitude control system). This may not be the case for other concepts (particularly those which might require large concentrations of the solar flux) such as the Brayton cycle "Power Sat" approach proposed by Boeing (reference 2) or selective spectral reflection concepts which may enhance the performance of solar cells. The significance of the structural configuration is far greater with respect to factors such as attitude control, assembly, electrical conduction lengths and simplicity of design than it is from the standpoint of weight. The station weight is driven primarily by the large number of small "light" components as opposed to the small number of "heavy" components. The structural requirements, low loads, and minimum gauge considerations lead to the idea of few large structural members. The large members minimize the significance of joints and can provide the three-dimensionality required for dynamic stability. Consideration of the low loads, low thermal expansion, and stiffness requirements lead to the selection of graphite as a reference material for the SPS structure. The column buckling limits shown as a dashed curve in figure IV-B-3-4 were used to size compression members as will be discussed below. Cable/Column Configuration: The cable/column configuration shown in figure IV-B-3-2 enables a concentration of all the array compression loading into only six main columns. All other significant structural members of the array are cables as illustrated. Twelve main tension cable systems join the columns to form an overall rigid structure. Less than 3000 N (600#) tension is required in these main cables to provide dynamic stability of the array (0.25 N-m membrane tension). The solar cells and reflectors are basically suspended by cables (or tapes to facilitate attachment to cell and reflector substrates) in 200 m by 200 m units. The tension in the cables supporting the solar cell/reflector unit is 22 N (5#). These units are suspended in 1 km2 modules which are guyed to the columns as illustrated in figure IV-B-3-2. Electrical conduction outside the cell blankets is handled by the reflector sheets (for low temperature and therefore low resistance). Electrical transmission to the antenna is in conductor sheets suspended along the main cable system at the outer edge of the array. This configuration was sized for delivery of 10 GW on the ground. Two antennas are provided for 5 GW transmission to two separate locations. This also affords symmetry to the configuration for attitude control. The control system could utilize thrusters at the exterior tips of each column or a combination of thrusters and current loop control via the exterior main cables. The latter system would eliminate the need for thrusters in the vicinity of the antenna. The six columns are segmented into 3.6 km lengths and guyed with intermediate main cables for static elastic stability of the configuration (stays may be required for torsional stability). The columns are tiered as illustrated in figure IV-B-3-5 and contain bracing cables (not shown). Although these bracing cables are less than 10 percent of the column weight, strict control of the tension in these cables is required for the static elastic stability of the trusses and columns. The cylindrical tube elements must be opaque (open) to thermal radiation to prevent thermal distortion. The compression element, bracing requirements, and guying requirements for the graphite composite columns were determined from a length to radius of gyration (L/p) of 100 as indicated in
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