2. Flex cable - does not fit the joint design concept. 3. Rotary transformer - requires conversion from DC to AC and back to DC, these conversion losses add to the overall thermal problem and reduce the transformer efficiency; the transformer itself will produce high thermal loads (1% efficiency loss equals 100 megawatts of heat which must be rejected). 4. Rolling contacts - fatigue may be a problem; requires development. In addition to the devices listed above slip rings were considered and selected as a best choice for the rotary ball joint because they are state-of-the-art, highly reliable and maintainable. The large surface area which is inherently available on the 7.6 meter diameter ball allows a large number of brushes to be used which results in a low current density (.78 amps/sq cm). It appears that the low current density and extremely low brush speed (approx. 1.5 cm per min.) will allow the slip ring and brush system to operate at a high efficiency. The joint drive system includes the electric motor-gearbox (with adequate redundancy), a constant velocity universal joint and drive shaft as indicated on the drawing (Figure IV-C-8-7) to provide continuous rotation about the primary axis of the joint. Motion about an orthogonal axis (up to ±10°) is obtained by 6 linear actuators arranged in a triangular pattern and mounted between the ball and a "table" that is bearing mounted to the outer race at the juncture of the drive shaft and outer race. The six actuators, by continuously changing their lengths, can position the ball in any attitude within the ±10° limit. If the ground receiving rectenna were located at 50°N. latitude on earth, for example, the antenna should be tilted up about 7.5°. In this case the 6 actuators would rotate the outer race of the ball joint 7.5° and, as the main drive shaft rotated the outer race at a rate of 360° per day, the 6 actuators would be driven to cause the outer race to "cone" through a total angle of 15° (2 x 7.5°) each day. Figure IV-C-8-8 shows the soft suspension system connecting the antenna to the ball joint outer race. A cylindrical configuration outer structural shell is shown (though it may be of various configurations adaptable to the antenna structural arrangement) which connects to the antenna structure. This shell connects to the outer race of the ball joint through 6 suspension struts symmetrically arranged in a triangular pattern. These struts act as soft spring/dampers to allow small linear and angular excursions of the antenna with respect to the ball joint, while minimizing torque inputs to the antenna from the ball joint. The electric current is conducted from the ball joint outer race through literally thousands of small, stranded copper wires to busses at each end of the structural shell. These wires each are located along a radius vector from the center of the ball, and are installed so they are "slack". Small angular
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