IBM System/4 Pi

The IBM System/4 Pi is a family of avionics computers used, in various versions, on the F-15 Eagle fighter, E-3 Sentry AWACS, Harpoon Missile, NASA's Skylab, MOL, and the Space Shuttle, as well as other aircraft.

[3]) Previously, custom computers had been designed for each aerospace application, which was extremely costly.

There would be an automatic switchover (taking on the order of one second) to the backup in the event of a critical failure of the prime.

[19] The software management effort was led by Harlan Mills and Fred Brooks.

The Skylab flight software development process incorporated many lessons learned during the IBM System/360 Operating System project, as described in Brooks' 1975 book The Mythical Man-Month.

It remained in service on the Space Shuttle because it worked, was flight-certified, and developing a new system would have been too expensive.

The upgrade to the AP-101S in the early 1990s replaced the core with semiconductor memory and reduced the size from two to one chassis.

The early AP-101 variants used IBM'S Multipurpose Midline Processor (MMP) architecture.

As such, it represents the first manned spacecraft computer system with hardware intentionally behind the state of the art.

Four operated in sync, for redundancy, while the fifth was a backup running software written independently.