for the antenna mount to permit antenna rotation of 360° per day about a plane perpendicular to the orbit. These structural offsets eliminate the need for large tilt and roll motions of the antenna. There are many options possible for the gimbal configuration. An extensive trade study and design effort is required to optimize the system from the standpoints of transportation, assembly, complexity, and pointing performance. 7. 3. 9 POWER DISTRIBUTION AND CONTROL SUBSYSTEM Electrical power is supplied by 24 main buses. Circuit breakers are located at the power sources for protection of the power subsystems, and additional circuit breakers are located on the antenna section to provide protection and bus control for the amplitrons. Power is distributed on the antenna system by thin-walled aluminum conductors. Power conditioners are located at the power source, and load voltage is sensed remotely. 7. 3. 10 ROTARY JOINT SUBSYSTEMS Present concepts for SPS require the power transmitting antenna to rotate about one or more axes with respect to the main structure. Rotation is necessary to allow the solar collectors to be Sun oriented while the transmitting antenna remains pointed to a fixed ground rectenna. (See subsection 7.1. 6. 3 for a discussion of the required antenna operational attitudes.) Rotary joints must provide structural stability and mechanisms to provide the proper antenna orientation as well as a means to transfer power. Preliminary analysis has indicated that the use of slip rings and brushes for transferring power across a 360c continuously rotating joint will be more efficient and have less mass than other concepts. Flexible cables are prime candidates for use across joints that have limited freedom. The highest rate of rotation necessary is 360° per day. With a design lifetime of 30 years, brush wear should not be a problem if slip ring diameters are held in the order of tens of meters. Several existing combinations of brush and slip ring materials have shown low enough wear rates to be considered for this application. Unless active cooling is incorporated, indications are that the brush areas that limit brush current densities to reasonable levels will not be the determining factor in sizing the rotary joint, rather structural and thermal considerations will determine the joint dimensions.
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