outboard of the antenna. To achieve continuous pointing of the MW antenna to the ground station (rectenna), independent rotation about the X-axis is required between the solar arrays and antenna. To accomplish this, the mast is divided into three segments — two solar array sections and an antenna section. Rotary joints at the points where the segments join permit this independent rotation. These joints transmit mast bending and axial loads, but no torsion. All twisting forces acting between the two solar arrays are transmitted by the non-conductive structure acting as torque cells. In the baseline configuration, the electrical power-conductive structure was considered capable of also carrying structural loads. Electrical requirements for the power-conducting bus/structure call for a structure of large diameter and thin-wall tubing for heat-dissipation. Because of the resulting high diameter-to-wall thickness ratio, this type of configuration is structurally unacceptable. Thermal requirements (uniform heating of shape) also eliminate this concept. One possible attempt at solving the problem from a structural point of view is shown in Figure 10. This concept starts with a basic building element. By joining together many of these building elements bigger struts are created. This concept could be considered for all compression struts. The basic building element has to be light-weight, very easily manufactured, and have structural integrity against column failure and local instability. In this baseline configuration, 6061 aluminum alloy was selected for the bus/structure because of its good structural properties and electrical conductivity. All the remaining structure in the plane of the four transverse DC power buses was considered to be also of 6061 aluminum to minimize the thermal warping that would result if dissimilar materials were used in this area. In the selection of the material to use for the structure connecting the solar arrays, the MW transparency of the material had to be considered, because this structure lies in the path of the microwave beam. A mica-glass ceramic was therefore used in this area for the baseline configuration. The remaining SSPS structure was the area that permitted the greatest variation in material selection. This allowed the investigation of various material properties without actually selecting any particular material. Since good structural stiffness was one of the principal requirements, an initial modulus of elasticity with a value equal to that of mica-glass ceramic was selected for the basic solar array structure. This value was used because it approximated the properties found in several common materials now used in the aerospace industry. Structural Mathematical Model.— The structure defined in the baseline configuration was idealized into a structural mathematical model. The method used was based upon the finite-element method of structural analysis which assumes that every structure may be idealized into an assemblage of individual structural components or elements. This idealized structure is then analyzed and the results used to predict the behavior of the actual structure. Verification of the idealization was obtained by utilizing two separate methods. The first method used the Grumman Automated Structural Analysis (ASTRAL) — Comprehensive Matrix Package (COMAP) program (defined in Reference 27). The second method used NASA's NASTRAN program (defined in Reference 28). Both programs employed identical idealization of structure. The ground rules and assumptions used in both models are listed below. To reduce the computer time required to run both problems, the structure was considered symmetrical about the
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