Included on the schematic are the switching and regulating switch gear. The latter function being accomplished by selective shorting of the last segments of the 45.5 kV blanket to assure voltage outputs compatible with summing bus voltage characteristics. F. Rotary Joint Mechanical - The rotary joint attached to the hexagonal center carry- through structure is illustrated in Figure R-10. The joint consists of a double set of inner stationary and outer rotating rings. The rings are modified 50-m tri-beam girders fabricated on orbit by beam machines. Rotary joint dimensions are noted in the Figure. Electrical - The rotary joint is utilized to transfer energy through slip rings and brushes from the satellite fixed member to the satellite rotating member upon which the microwave antenna is located. The rotary joint assembly design characteristics are given in Table R-2. G. Attitude Control System (ACS) Figure R-l1 shows an ACS employing high-performance electric thrusters, and use of the Y-POP, X-IOP orientation and inertia balancing to minimize attitude control propellants. This approach employs eight RCS quads. The total RCS propellant requirements (see table) are low, due primarily to the high specific impulse (13,000 s) which is believed to be feasible with the argon ion bombardment thrusters. The ACS attitude reference determination system features charge-coupled device (CCD) star and sun sensors as well as electrostatic or laser gyros and dedicated microprocessors. Five attitude reference determination units are at various locations on the spacecraft to sense thermal and dynamic body bending and to desensitize the system to these disturbances. The control algorithms will feature statistical estimators for determining principal aixs orientation, body-bending state observers or estimators, and a quasi-linear RCS thrust command policy to provide precise control and minimize structural bending excitation. The ACS hardware mass is very small relative to the 30-year propellant requirement.
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