J. C. Jones & J. D. Bradley Spacecraft Design Division IV-C-8 ROTARY JOINT A rotary joint or system of joints is required to point the antenna toward the rectenna on earth. Major functional requirements of the rotary joint may be summarized as follows: 1. Provide structural connection between antenna and SPS while allowing proper angular motion. 2. Transfer electrical energy across rotating interface. 3. Contain a mechanical drive system to rotate the joint. Two orthogonal axes are required for pointing the antenna. A third axis, on or parallel to the line of sight from the antenna to the rectenna is necessary in some configurations to maintain polarity of the antenna with respect to the rectenna on earth. However, this axis is not required for our reference configurations and orbits identified to date, assuming polarity differences of one or two degrees are tolerable. An additional axis is necessary for some arrangements to allow for positioning the outer axes so that the angular motion about those axes is, basically, uniform. In this study several logical arrangements were identified, a general evaluation of each arrangement was made, and a configuration concept was selected. The selected joint configuration was evolved from consideration of numerous, overall "system" factors. The following paragraphs will attempt to explain those considerations. Figure IV-C-8-1 illustrates two antenna mount configurations, both applicable to two antenna, solar oriented SPS configurations. It is a basic principle that the antenna must rotate approximately one revolution per day - relative to the SPS - about an axis (real or imaginary) parallel to the earth's polar axis. The arrangement shown as configuration 1 requires that the angular velocity about both axes vary cyclically twice each day. The antenna is also moved through large excursions relative to the solar array (almost 1/3 km normal to orbit plane, and .8 km parallel to orbit plane). The dynamics involved are judged to be intolerable. Configuration two improves the situation somewhat by adding an axis "C" which allows axis "A" to be positioned parallel to the earth's polar axis. Now, motion about axis A is, basically, uniform at 1 revolution per day. However, the mass center of the antenna is constantly being moved relative to the SPS, causing radical changes in the SPS inertia distribution. Also the attitude control of the SPS becomes involved with antenna pointing, requiring a change in attitude of 23.5° in a cyclic manner during each year. Figure IV-C-8-2 shows three configurations which partially or completely eliminate the problems of the prior two configurations.
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