rectangular subarrays (9.31 meters by 10.75 meters, 100 meters2) are attached in a determinate manner to the triaxial grid points of the secondary truss. Two initial adjustments would be required to achieve boresight alignment of each subarray during the construction phase. Again, the three-point determinant support system could not transmit any flatness distortion. The subarrays would also be an open structure which could utilize the rigidity of the waste heat thermal radiator associated with each microwave generator. Figure IV-C-7-2 illustrates a Klystron system set orthogonal to the beam to minimize the heat rejection path. Although the Klystron, wave guide, and thermal radiator would be a relatively rigid unit, the aluminum wave guides would have a significantly different thermal expansion coefficient than the pyrolytic graphite radiators or the graphite polyimide subarray structure. To accommodate the contraction associated with an eclipse, each wave guide unit would have to be structurally isolated from the surrounding units within the subarray. The aluminum wave guides could be dimpled in a random pattern to create an effective stiffness with a minimum distortion and loss. A high-Young's modulus low thermal expansion graphite material would be used for the MPTS structure. Electrical conduction grade aluminum is required for electrical transmission. Different material requirements preclude efficient integration of the two systems. The structural configuration shown in figure IV-C-7-3 was originally conceived for a large solar collector; however, it might be modified to serve as a structure for the transmission module. This configuration is composed of a single outer compression ring with a triaxial grid pattern of cable stretched over each "drum" surface and then tied together to form mirror symmetry paraboloids. For the solar collector, each triangular unit of one paraboloid surface would contain a flat reflector. Although this configuration would afford sufficient dynamic stability as a solar collector, the tension level required to provide sufficient stiffness for an antenna system is prohibitive. The transmission module is a complex design task which requires relatively detailed study to achieve an efficient design. Of particular importance is the coupling of the structural requirements and the thermal environment. Figure IV-C-7-4 illustrates two detailed design approaches toward minimizing thermal distortion. The use of thermally opaque structural elements is an approach toward minimizing the thermal distortion which can arise in thin walled closed members. This allows the use of structurally efficient closed members but reduces the potentially significant thermal gradients 100°K across the diameter for a thin walled isolated aluminum cylinder normal to the solar flux). The use of structural/ material configuration to advantage is also illustrated by the thermally stable configuration shown in figure IV-C-7-4. Here the distance from point A to B is independent of the temperature level of this structural unit. Detailed design features such as these will be required for simple, low cost approaches to achieving the structural requirements. a. References 1. Microwave Power Transmission System Studies, Fourth Engineering Review, Contract NAS 3-17835, Raytheon Company for Lewis Research Center, December 12, 1974. 2. Brodie, S. B., et al.: Orbital Assembly and Maintenance Study, NAS 9-14319, August 1975.
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