The Longest Tech Support Call: Apollo 14 Computer Hack

Deep-voiced and aptly named [Scott Manley] posted a video about the computer hack that saved Apollo 14. Unlike some articles about the incident, [Scott] gets into the technical details in an entertaining way. If you don’t remember, Apollo 14 had an issue where the abort command button would occasionally signal when it shouldn’t.

The common story is that a NASA engineer found a way to reprogram the Apollo guidance computer. However, [Scott] points out that the rope memory in the computer wasn’t reprogrammable and there was no remote way to send commands to the computer anyway.

The initial patch had the astronauts use the DSKY to clear a bit that would prevent an abort from occurring. However, there was a chance that some other code would set that bit again as part of normal operations. If the bit was set and the switch malfunctioned, you could get a spurious abort. Engineers sent them another procedure to fool the computer into thinking it was already executing an abort which solved that problem.

However, this required a lot of effort during operations to make everything work. [Scott] shows you the nouns and verbs used and explains them in detail. It is rare to find such a technical treatment of this story and the NASA engineers were certainly doing a hack in the truest sense of the word.

The DSKY is no stranger to the pages of Hackaday. We’ve even talked about the odd rope memory before. While Apollo 13 got the big budget movie, the Apollo 14 hack is a great story, too. And without it, we’d have never had a golf shot on the moon.

Filed under: classic hacks

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