(175°C). However, there are questions of the extent of outgassing and its potential effect on the open tube converters as well as questions of stability over 30 years in the space environment. An all aluminum solution must be carried forward until these questions are resolved, and a concept of sectoring the subarray into independent 5-meter segments was devised to alleviate the aluminum deflection problem. Figure 24 illustrates the tradeoff between capital cost of power lost by increasing subarray size versus capital savings for reduced electronic investment. Subarray dimensions of 18M x 18M were selected because 18M is within the acceptable range and corresponds to the maximum dimensions that can be carried in the Space Shuttle Orbiter, which could be expected to play a key role in early development flights. The detailed implementation of a subarray with amplitrons is shown in Figure 25. Phase reference electronics are shown, as are the mounting of circuits for power control (crowbars). Screwjacks at the corners provide for mechanical attitude adjustment of the subarray to compensate for installation errors and for deflections that may arise over years of operation. The fully packed tubes with thermal radiators touching represent the highest power density that can be implemented, and so this is a centrally located subarray. Power taper toward the edge of the antenna is achieved by spacing the tubes farther apart. Maximum radiated power for this unit is 7 megawatts, an impressive figure by any earth based standards. 4. MECHANICAL SYSTEMS Fine pointing by electronic phase control directs the power beam to an accuracy of about 0.04 arc sec (about 10M at Earth), but as noted earlier there is reduced efficiency if mechanical pointing is not reasonably accurate. An error of 1 arc minute corresponding to a power loss under 1% was selected for the design goal and is accomplished with control in elevation and azimuth as shown in Figure 26. The azimuth rotary joint is located at the mast interface with a solar oriented power source for which relative rotation is 360° per day. Additional antenna motion in azimuth and elevation is required to compensate for spacecraft (power source) limit cycling which would nominally be on the order of 1 degree [Glaser, 19741. The principal disturbance torque on the antenna by far is the frictional torque in azimuth due to contact pressure between the brushes, carrying electric power, and the rotary joint ring. It is estimated at about 10$ Nm (8 x 10$ Ft-Lb). Details of the rotary joints are given in Figure 27. Power is carried across the azimuth interface by silver alloy brushes and slip rings, and across the elevation drive by flexible cable where motion is limited to ± 8 degrees. Orientation drive is nominally by DC torque motor
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