changes with respect to the SPS. To eliminate the torque input to the antenna from gravity gradient and dynamic unbalance, the antenna configuration is changed from a disc to a cone—the cone having the same mass moment of inertia about all axes. Figure IV-C-8-6 shows the overall configuration of the antenna installed on either an SPS configuration that is solar oriented or one which is oriented perpendicular to the orbit plane (P.O.P.). The solar oriented configuration requires the dog-leg support and an additional rotary joint to allow the dog-leg support to be properly positioned with respect to earth as the solar array attitude changes to track the sun. A counterweight of approximately 1,000,000 kg is required to obtain equal inertia distribution for the antenna. The CMG's may be located at the counterweight position, reducing the requirement for "dead" weight. The ball joint is located at the antenna mass center. This is very important--so that the antenna has minimum externally applied torques. The major design problem is, now, the ball joint. Figures IV-C-8-7 & 8 show the ball joint design for a dual antenna system (10 GW SPS) and Figure IV-C-8-10 shows a slightly different joint configuration for a single centrally located antenna system (5 GW SPS). The joint is a spherical ball approximately 7.6 meter diameter with an outer race that can rotate continuously about the axis of the ball connecting shaft and can oscillate up to 10° about any orthogonal axis, in the same manner as a conventional ball joint. The sliding contact surface between the ball and outer race provides the structural support (bearing) function and the transfer of electrical energy. The ball and connecting shaft (actually 2 concentric shafts), and the outer race are divided as indicated in the figure by insulation to provide the separate positive and negative electrical conduction paths. Electrical brushes provide the actual siiding/bearing contact between the ball and race. The brushes would be individually spring loaded to maintain proper contact pressure and to allow compliance with the probably-irregular-surface of the ball. It is estimated that about 5000 brushes, each having a 64.5 square centimeter contact area, would be used in both the positive and negative halves of the joint. The concept is to maintain the maximum possible contact area for transfer of electricity, considering the large size of the joint and the probable deviations from a perfectly spherical surface. Listed below are comments about some of the devices that were considered as possible candidates for transferring power across the rotary joint (*see reference at the bottom of the page for a discussion of these devices): 1. Power clutches - requires switching (interruption of current flow) which presents an arcing problem; concept requires development. *Design Data Handbook for Flexible Solar Array Systems, Report No. LMSC-D159618, March 1973.
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