3.2 ANTENNA STRUCTURAL CONCEPT 3.2.1 General Arrangement The MPTS antenna is 1 km (3280 ft) in diameter by 40 meters (131.2 ft) deep. The antenna is assembled in two rectangular grid structural layers, Fig. 3.2-1. The primary structure is built up in 108 x 108 x 35 meter bays using triangular girder compression members 18 meters long and 3 meters wide. The secondary structure 18x18x5 meter bays (Section B-B, Fig. 3.2-1) are used as support points for the waveguide subarrays. Dimensions of the secondary structure will vary with selection of the optimum subarray size. (The 18 x 18 meter size is typical). A mechanical screw jack system (Detail C, Fig. 3.2-1) is used as the interface with the subarrays and provides the flexibility of mechanically aligning the waveguides in orientation and position. This feature desensitizes the configuration requirements on assembly tolerances and thermal deflection accuracies. The antenna-to-spacecraft interface (Detail D, Fig. 3.2-2), uses a 360" rotary joint (azimuth) about the spacecraft (SSPS) central mast and a limited motion (±8°) rotary joint for North-South steering (elevation). The azimuth rotary joint uses two slip-rings and brush assemblies for power transfer (Section F-F, Fig. 3.2-2). One routes plus current, the other negative. The azimuth drive assembly utilizes a geared rail support structure (Section E-E, Fig. 3. 2-2) and motor driven 4-wheel truck roller assembly. The elevation drive utilizes flexible cable for power transfer and a geared rail drive system similar to the mechanism used for azimuth control. 3.2.2 Rotary Joint A recommended approach for concept definition consisting of rollers and tracks had tentatively been made. Power is transferred across the azimuth interface by silver alloy brushes and slip rings, and across the elevation drive by flexible cable. The orientation drive is by DC torque motor with spur gear drive. Design of the antenna mechanical interface requires selection of the gearing, bearings, motor, power transfer device and lubrication. Reference 8, containing design details and analysis for a space station solar array rotating joint, has been used as a source of pertinent design data. Applicable data from both Ref 7 and 8 has been repeated in this report for convenience. 3.2. 2.1 Gears The choice of gears to meet the 1 arc-min pointing accuracy requirement and 30-year life is a major issue in control system design. Depending upon the extent of the gear ratio,
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