wear. Total contact area for all the brushes, which are made of silver molybdenum disulfide with 3% graphite, is about 2 x 10 cm , so the current density in the brushes is about 20 amperes per square centimeter. The brushes slide along the slip rings at speeds of about 1 to 2 meters per hour. Attitude control system (ACS). The attitude control system provides thrusters, sensors, and computers to perform station-keeping and orientation control for the power satellite. For the gallium option satellite, a total of 35 operating argon ion thrusters are needed (100 for the silicon option power satellite, since it is more massive); to provide the necessary redundancy (assuming once-a-year maintenance), the Reference Design provides 64 thrusters (160 for the silicon option) , 16 at each corner of the photovoltaic array. Each thruster is independently gimballed and controlled. Chemical thrusters are also installed to provide control during eclipse by the Earth. Th attitude control system uses 34 MW of power (average), mainly for the ion thrusters, in the gallium option, somewhat more for the silicon option. Propellant requirements for the ion thrusters are on the order of 40 to 100 tons of argon per year, along with 1.5-5 tons of liquid oxygen and liquid hydrogen for the chemical thrusters. Propellants are stored cryogenically, with electric refrigeration and (possibly) reliquefaction. Living/working quarters. The Reference Design system does not provide any living quarters or working facilities aboard the power satellites. Maintenance is assumed to be done by annual visits of maintenance crews visiting from the GEO base. Addition of living and working quarters for both inspection teams and a small permanent maintenance team (5 to 20 people) may be cost-effective. Further, if the owners of the power satellite are concerned about saboteurs infiltrating inspection teams, they may wish to have their own security personnel aboard the power satellites to monitor the inspectors. No additional DDT&E costs would be incurred, since the quarters could be identical (except for total size) to those provided at the GEO base, including a solar flare shelter. These living quarters would most likely be placed near the middle of the photovoltaic array, either directly behind it or at one side, to provide a constant thermal balance (except during eclipses), minimal travel times and distances to all parts of the power satellite, and a safe
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