IBM System/4 Pi

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The IBM AP-101S Space Shuttle General Purpose Computer is a member of the System/4 Pi family

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,[1] which had a 16-bit word length and 16,384 words of memory with a custom input/output assembly.[2]

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,[3] 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.


  1. ^ Jenkins, Dennis (April 5, 2001). "Advanced Vehicle Automation and Computers Aboard the Shuttle". NASA History Homepage. NASA. Retrieved 27 October 2013. 
  2. ^ "Skylab Space Station". eoPortal. ESA. Retrieved 27 October 2013. 
  3. ^ Technical Description of IBM System/4 Pi Computers. Owego, NY: Federal Systems Division of IBM. 1967. Retrieved 27 October 2013. 

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