The Computer Hack That Saved Apollo 14

Video from the channel Scott Manley about Apollo 14, linked on its Above Ten Thousand entry. Duration: 11:22. Language: English. Video type: Documentary.

Scott Manley explains how Apollo 14's Moon landing was saved when, on February 5, 1971, with Alan Shepard and Edgar Mitchell in lunar orbit aboard Antares, it was discovered that the abort switch was intermittently sending the abort signal due to a short circuit. If it triggered during descent, the computer would cancel the landing and return the module to orbit.

Mission Control turned to Don Eyles, an expert on the Apollo guidance computer. The video clarifies that the computer was not reprogrammed: the program resided in read-only memory woven at the factory, and only the two kilowords of erasable memory could be touched. The solution involved manipulating status bits that the astronauts typed into the DSKY. Simply disabling the LETABORT flag was risky, because the engine-ignition routine would turn it back on. Eyles's idea was to make the computer believe it was already running abort program 71, so the monitoring routine would not trigger it.

The sequence: before ignition, set the major mode to 71; 26 seconds after ignition, manually throttle the engine to maximum, because with the ZOOMFLAG flag not set the engine wouldn't climb to 100%; afterward, set that flag, zero out LETABORT, and return the major mode from 71 to 63, the descent program. As a backup there was the lunar module's secondary abort computer. Apollo 14 landed successfully.