IBM System/4 Pi



The IBM System/4 Pi is a family of radiation hardened avionics computers used, in various versions, on the B-52 Stratofortress bomber, the F-15 Eagle fighter, E-3 Sentry, NASA's Skylab, MOL, and the Space Shuttle, as well as other aircraft. It descends from the System/360 mainframe family of computers.

The Skylab space station employed the model TC-1, which had a 16-bit word length and 16,384 words of memory with a custom input/output assembly.

The top-of-the-line 4 Pi is the AP-101, used in the B-52. The U.S. Navy used a similar variant, the AN/ASQ-155, in the carrier based A-6E/A-6E TRAM medium attack aircraft. The Shuttle is controlled by five AP-101 computers, four of which are arranged in a redundant configuration, with the fifth as backup.

The name of the system is derived from the fact that the angular measure of a complete sphere (solid angle) is 4π steradians, while the angular measure of a complete circle is 360 degrees; hence System/4 Pi and System/360. This implies that System/4 Pi is a version of the IBM System/360 for the three-dimensional world of avionics.