F. Stebbins, R. C. Ried/ Structures and Mechanics Division IV-C-7. Structure the microwave transmission antenna presents one of the most difficult tolerance problems in the SPS system. Although the precise targeting of the microwave beam is achieved through electronic phasing of the 7854 subarrays per antenna, each of the subarrays must maintain a relative mechanical pointing accuracy of 3.2 arc minutes to achieve the desired efficiency. As with the array, the antenna structure must have sufficient stiffness to maintain dynamic stability; in the case of the antenna, however, the control system is the design driver. Actually, the attitude control system selected for the microwave power transmission system is critical to attaining dynamic stability and, therefore, boresight alignment of the individual subarrays. The large size and mass and the planar character of this antenna system prohibit a natural frequency greater than the desired attitude correction frequency. The microwave power transmission system design requires a close integration (control system, structure, microwave radiator system and electronic phasing) to keep the dynamic motion of the antenna to a desirable form and level. In contrast to the array, the antenna undergoes a daily revolution with respect to the sun and, therefore, potential thermal strain becomes a significant design consideration. The most critical structural design requirements are: (1) minimization of the dynamic motion of the subarrays and maintenance of a maximum-time-average boresight alignment, (2) initial alignment of the subarrays with respect to the antenna boresight, and (3) accommodation of relative thermal strain occurring during an occultation. Two previous transmission module designs by Raytheon Company/Grumman Aerospace Corporation (reference 1) and Martin Marietta Corporation (reference 2) used different structural configurations to support 18-meter by 18-meter subarrays. The Grumman design consists of a two "bed" approach. The first "bed" is a primary structure and furnishes overall stiffness. The second "bed" serves as secondary structure to join the subarrays to the prime structure. The Martin Marietta Corporation design is a single bed concept of cubical structure attuned to the subarrays. Although the latter approach offers a possible advantage in the repetitive nature of the assembly process, it is massive. Figure IV-C-7-1 illustrates a potential design for the power transmission module prime structure. Four concentric polygons of 6, 12, 18 and 24 sides are held under hoop compression by radially joining cable tensile members. This places node points at radial distances of 125, 250, 375, and 500 meters, respectively, for the 1 kilometer diameter antenna. As illustrated, each circumferential compression (hoop) member is 130 meters long and consists of a rectangular frame 65 meters by 130 meters with cable cross bracing. The outer hoop members could have a decreased depth to minimize the effects of thermal strain associated with the daily solar cycle and yet retain sufficient stiffness. The radial symmetry of the prime structure affords a lower distortion due to the radial symmetry of the power generation, operating temperatures and radially varying loads. Ninety-six planar truss secondary structural units are attached to the prime structure in a determinate manner at three points, each as illustrated in figure IV-C-7-1. Three points form a plane which will not introduce warping forces from contiguous structural units due to thermal distortion (as would occur during an eclipse). The planar secondary truss is made up of tetrahedrons which form tri axial patterns on the outer surfaces. As illustrated in figure IV-C-7-1,
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