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upstart from IRC writes:

A deep dive into the Apollo Guidance Computer, and the hack that saved Apollo 14:

In the eight months since the harrowing flight of Apollo 13, engineers made several changes to the spacecraft to reduce the chance of another explosion happening. To help ensure that the crew could make it home if another emergency occurred, an additional oxygen tank and battery were added. The unplanned pause also allowed time for some software updates to be added to the lunar module computer; a particularly welcome addition was the ability of the computer to recognize changes in the height of the surface during the approach to the landing site. With this new capability, the computer would not be confused by the undulating terrain as the vehicle headed toward landing.

[...] A very serious problem cropped up almost immediately after TLI, as Kitty Hawk attempted to dock with the mission's lunar module, Antares. Fingernail-sized latches on the docking probe used to connect the command module to the lunar module failed to catch, and the two spacecraft were unable to dock. Only after repeated attempts was Kitty Hawk able to capture and securely attach Antares. Afterwards, the S-IVB was sent on its way to a lonely but spectacular death and the combined Apollo 14 spacecraft continued the voyage to Fra Mauro.

[...] However, less than four hours before the scheduled landing, controllers noticed that according to the indications on their consoles in Mission Control, the LM's Abort pushbutton appeared to have been pressed. When asked via radio, Shepard confirmed that no one on board Antares had pressed the Abort button—which meant there was a short-circuit or other electrical issue somewhere inside the LM's complicated guts.

This was potentially a mission-ending problem: if the button was pressed and the engine was firing, the LM would immediately begin its abort procedure as soon as the lunar descent started, making a landing impossible.

Under hard time pressure, the ground had to quickly figure out what was wrong and devise a workaround. What they came up with was the most brilliant computer hack of the entire Apollo program, and possibly in the entire history of electronic computing.

To explain exactly what the hack was, how it functioned, and the issues facing the developers during its creation, we need to dig deep into how the Apollo Guidance Computer worked. Hold onto your hats, Ars readers—we're going in.

Original Submission