continuous torque is also necessary to overcome joint friction. It appears that these torques can be reacted by the structure without serious overall dynamic effects. However, there may be some local dynamic response that can be identified only by analysis of detailed design. If such problems exist, they can be circumvented by isolating the antenna drive from the SPS structure, either by "shock-mounting" the antenna or by reacting the drive torques against reaction jets or momentum storage devices on the antenna. To be conservative, the latter approach has been used for sizing purposes. Reaction jets would tend to create an atmosphere and would consume large quantities of propellant. Control moment gyros (CMG's) or other momentum storage devices are attractive for cyclic torques, although rotor masses of several tons are required. Friction is counteracted within the joint and does not affect the supporting structure. Pointing command inputs can be derived from phase differences in the pilot beam as received at a set of four selected subarrays located 90° apart. A second set of subarrays is used to resolve phase ambiguities. Initial acquisition is based on the known position and attitude of the SPS, but this method is not accurate enough for antenna pointing. The principal development required is the momentum storage device. A CMG for this application would be more than 200 times heavier (^25 000 kg) than the CMG's used on Skylab if designed to the same stresses. 6. Power Distribution A lateral flow distribution system was used to minimize weight (fig. IV-13). Each quadrant of the antenna is independent, with several switchgear per quadrant. Four switchgear per quadrant keep the current at a reasonable level (^11 000 A) and were assumed for estimating purposes. The switchgear serve not only as distribution points, but can be used to bring power up to maximum in a controlled sequence during startup. Power is routed to distribution points (switchgear) serving from one to four subarrays, depending on the power density Figure IV-13.- Typical antenna distribution system.
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