and translational motions between the ball joint and antenna would meet a very minimal resistance from these wires. In conjunction with the copper conductors would be auxiliary tension cables or struts, again positioned radially to react translational movements but not rotation. These tension cables essentially "moor" the antenna structure to the outer race and provide a soft connection. The total system then functions in a manner outlined as follows: 1. The CMG's control antenna pointing attitude in response to pointing error signals. 2. An attitude difference between the antenna and ball joint outer race is detected from position transducers within the 6 suspension struts. 3. The attitude difference signal is used as a basis to change the ball joint drive rate and/or the 6 angular position actuator rates, to cause the ball joint outer race to, in effect, track the antenna. The ball joint drive system, then, functions only to overcome the friction in the joint and drive the joint to keep up with the antenna motion within a half-degree or so. The structure in the ball joint area is very stiff, and the friction is basically constant. Therefore, the difficulty in designing this servo-system is minimal. The nearly constant friction torque is considered to be a feature of the ball joint, which allows 3 axis freedom within a single bearing system, without any starting and stopping motion — with resulting static and dynamic friction coefficients — as would result in a discrete axis (gimbal) arrangement. The control system can allow the CMG's to be desaturated by feeding an angular impulse into the SPS through the joint drive and suspension system at a low rate over a long period of time, without significant complications from low spring rate SPS structure. The overall size and weight of the ball joint is estimated to be well within the volume and payload weight capabilities of the Heavy Lift Launch Vehicle. Therefore, it would be completely manufactured on earth and at least two joints launched in the one launch vehicle. A likely technical problem is heat rejection from the relatively compact configuration. Preliminary analysis indicates that the use of very low current densities (amps/square cm.) may reduce the electrical heating in the joint to a low enough rate so that radiation cooling is adequate. Heat pipes may be used to conduct heat from the outer race outward to larger area radiating surfaces. Cooling of the inner conducting tube of the 2 concentric tubes would be aided by not making the outer tube solid (i.e. orovide holes in the outer tube to expose some of the inner tube surface).
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