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  Backup System

As -indicated previously, the Space Shuttle mission was not amenable to degraded backup system mechanizations, and, initially, no backup to the four-computer, four-string, FO/ FS avionics configuration was included. Eventually, however, considerations of potential generic software errors which could affect all four computers, and concern over the complexity of the primary system with its closely coupled, tightly synchronized approach forced a new look at the possibility of a backup. Constraints imposed on this investigation were that a backup system should in no way degrade the reliability or performance of the primary system, and that no significant crew training impact should result from the mechanization. The result was a concept which used the fifth computer, loaded with unique, independently developed and coded software capable of safe vehicle recovery and continuation of ascent or safe return from any mission situation. A redundant, manual switching concept was devised by which control of all required data buses, sensors, effectors, and displays was transferred to the single backup computer.

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