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.