4. Bus Material — Aluminum reduces conductor mass by 57 percent as opposed to copper for a given temperature and has been selected for the structural material. 5. Bus Configuration — As discussed previously, thin conducting sheets or tubes with favorable thermal view factors are preferred for power conduction. However, consideration must be given to reducing power loss caused by interaction with the space plasma. Power loss is proportional to exposed conducting surface area and can be significant under certain plasma conditions. 7.1. 4. 7 REQUIRED TECHNOLOGY ADVANCEMENTS The primary concern in the power distribution subsystem was the large structures required for power transmission. The major technology requirement lies with the assembly of large structures in space; however, the conduction of power across structural joints must be given attention. The most immediate technology concern relates to the interaction of the space plasma with high voltage solar arrays. Studies are underway to further analyze the phenomena, but additional effort is required to obtain experimental verification of power loss calculations and proposed solutions to reduce sparking. A considerable effort is in progress to study the other voltage problem — spacecraft charging. This activity should be continued. Three types of in-line commercial circuit breakers have potential application for the SPS: (1) crossed field (Penning) discharge devices with interrupting capacity of 100 kV, 2 kA, (2) vacuum interrupters with capacities to 20 kV, 15 kA, and (3) modified ac circuit breakers as preferred by the Europeans. Studies should be undertaken to find the most suitable breakers. The baseline SPS utilized a partial shunt switching regulator to control bus voltage, but there are several competitive system options that would require dc-to-dc or dc-to-ac and ac-to-dc power conditioners. The list of candidate active circuit components that should be evaluated includes hydrogen thyratrons with capacities to 50 kV, 10 kA and solid state thryistors with capacities to 2 kV, 1 kA. 7.1. 5 FLIGHT MECHANICS 7.1. 5.1 STATION KEEPING Station keeping for the SPS is concerned primarily with two major phases: the assembly in low Earth orbit and the operational phase in geosynchronous equatorial orbit.
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