Challenger: A True Story of Heroism and Disaster on the Edge of Space

by Adam Higginbotham

The shuttle hardware was deliberately designed to incorporate as many redundant systems as possible, and some components were duplicated several times to provide backups in case of failure. For example, the shuttle was equipped with a total of five IBM avionics computers, so that if one malfunctioned, the crew could continue their mission as if nothing had happened; if two or three failed, the orbiter could keep flying without obvious difficulty, but the crew might have to return to Earth earlier than planned. These components, with multiple redundancies, NASA designated Criticality 3, or 2R; those with a single backup were designated Criticality 1R. But those parts on the Critical Items List that were reviewed with the most care before launch were the Criticality 1 components, with no redundancy at all; if any one of these failed in flight, the result would spell disaster: “Loss of vehicle, mission and crew.”