shuttle-flight-computers

Why did the space shuttle have 5 identical flight computers? Dave Mohr

The presence of the 5th computer was not to provide an odd number of computers to avoid the ‘dilemma’ situation. In fact, the 5th GPC was programmed with software written by a completely different group of people from the group that programmed the 4 ‘primary’ computers. Early in development, it was feared that some sort of ‘generic’ bug in the primary software could cause a ‘collapse’ of the 4 primary computer ‘set’. If that happened, the outcome would be catastrophic. So, a 5th computer with a ‘Backup Flight System’ (called ‘BFS’) was created. BFS ‘listened’ to the primary 4 computers all the time, and would take over if the primary computer ‘set’ collapsed, or if called into action by a push-button that sits on top of the control stick. BFS was never used in the Space Shuttle program.