and 3 meters deep. The secondary structure is used as support points for the waveguide subarrays and is built up as 18 x 18 x 5 meter bays. The total antenna structure/mechanical system weight is using aluminum and using graphite/epoxy (or polyimide). The antenna-to-spacecraft interface uses a 360° rotary joint for antenna motion perpendicular to the orbit plane (azimuth joint) and a limited motion rotary joint, ± 10 deg, for North-South pointing (elevation joint), Fig. 2-13. Two slip ring assemblies (one for plus power and one for power return) are used for power transfer across the azimuth rotary joint and flex cable is used across the elevation joint. Both the azimuth and elevation joint drive assemblies utilize a geared rail about the diameter of the support structure and four DC brushless motor driven roller assemblies. The structure to waveguide interface uses three gimballed screw jack assemblies (Fig. 2-14) to provide a mechanical tuning system for alignment of the waveguides after construction. Up to 40. 5 cm of linear motion can be used to correct thermally induced antenna tip deflections and can also be used to correct a maximum expected 4 arc-min subarray misalignment. Figure 2-15 is a typical conceptual design of a mechanical locking mechanism for structural joints. The girder interconnect fitting is similar to a docking drogue which utilizes a spring-loaded ball lock for fastening with the tri-beam end fitting. 2.2.3 Configuration Analysis 2.2.3.1 Thermal Analysis A refined thermal analysis of the antenna conceptual design concentrated efforts on the following: • Selection of the tri-beam element longeron cross section to minimize maximum temperature and thermal gradients • Identifying the limit waste heat at the center of the antenna as a function of structural vertical member material • Defining range of thermal gradients between primary and secondary structural caps as a function of sun position relative to the antenna Figure 2-16 presents the maximum temperatures and thermal gradient across three candidate structural cross sections: tubular, rectangular hat, and triangular hat. The tube is the worst from a thermal standpoint. The use of aluminum tubing near the center
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