Solar array power is controlled by vacuum circuit breakers near the buses. Voltage is controlled by turning groups of strings on or off, depending on load requirements. Two sections of the array provide the required voltage at the sliprings using the sheet conductor voltage drop to achieve the required voltage at the sliprings. Power transfer across the rotary joint is accomplished by a slipring/ brush assembly. Mechanical rotation and drive is provided by a mechanical turntable 350M in diameter. The antenna is suspended in the yoke by a soft mechanical joint to isolate the antenna from turntable vibrations. F. Rotary Joint As previously stated, power transfer from the solar array section to the microwave antenna is accomplished via a rotary joint (figure B-8) using a slip ring/brush assembly. Mechanical rotation and drive is provided by a mechanical turntable 350 m in diameter. The antenna is suspended in the yoke by a soft mechanical joint to isolate the antenna from turntable vibrations. The antenna is mechanically aimed by CMG's installed on its structure. A position feedback with a low frequency bandpass allows the mechanical turntable to drive the yoke to follow the antenna and also provide sufficient torque through the soft joint to keep the CMG's desaturated. Figure B-9 illustrates the electrical components of the rotary joint. Coin silver (90% silver and 10% copper) was selected for the slip-ring material and a silver-molybdenum disulfide brush with 3% graphite was selected. With a o design using a brush current density of 20 amps/cm only about 40 kW of power is dissipated in the rotary joint. The projected brush/sl ip-ri ng wear is very o small (.0289 to .0617 cm/year). G. Attitude Control System The attitude control system (ACS) includes all operational elements and software required to maintain orbit station keeping and attitude control of the SPS in the operational orbit or to establish attitude control from an initially uncontrolled condition. The ACS is an electric propulsion system with four installations, one at each corner of the SPS solar array system. A typical
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