NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

NASA SP-504: Space Shuttle Avionics System

Section 3  Power Distribution

The size and complexity of the Space Shuttle vehicle forced a number of changes in the approach to power distribution and control followed on previous space programs. As an example, a single-point ground standard, if imposed in the dc distribution system, would have resulted in a 2268-kilogram (5000 pound) weight penalty and, therefore, had to be relaxed. A multipoint system with structure return was used; however, isolation was maintained between primary power returns and LRU chassis and these returns were brought out to controlled points on the fore and aft payload bay bulkheads. Another example is the extensive use of remote power controllers and computer-controlled load switching, employed both to reduce weight and to reduce the crew's workload.

Other issues faced in the power area included the redundancy level of the system and the requirement for battery backup to the fuel cells. A three-bus distribution system was selected on the basis of both weight and reliability considerations. The battery issue was also resolved on the basis of weight and reliability. Even the minimum loads required for safe return would have required a prohibitively heavy battery complex. The system which evolved was two-fault tolerant, with extensive cross-tie capability and multiple feeds to critical LRU's to enhance failure tolerance and to protect against transients.

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