Computer glitch fouls Apollo 11 moon landing

Apollo 11 crew portrait. (Left to right Neil Armstrong, Michael
Collins, and Buzz Aldrin.) Photo by NASA

Neil Armstrong and Buzz Aldrin excitedly descended to the moon. About 6,000 feet above the lunar surface, the Lunar Module‘s Guidance Computer (LGC) that was landing the craft, failed. Although this occurred 40 years ago, the lessons learned from Apollo 11's disconcerting landing remain relevant today.

As mentioned in a previous column, during descent, the LGC issued 1201 and 1202 alarms and repeatedly rebooted. As a result, Neil Armstrong, the Commander of the Lunar Module (LM) Eagle, had to take over for the computer and land manually. He did so skirting over nasty looking rocks and landing with precious little fuel left in the LM's tanks.

The cause of this was nothing more than an innocent communications foul up. It occurred months before Apollo 11 ever left the launch pad. It was an example of the kinds of technical communications issues that continue to plague business computing today.

AVCO's Electronics Division manufactured a device called the Digital Uplink Assembly (DUA.) It was the DUA’s job to decode commands from the Manned Space Flight Network (abbreviated MSFN, and pronounced “misfin”) and hand them off to the LGC. It was like a super modem connecting the Lunar Module to MSFN's set of tracking stations located across the Earth.

It occurred to NASA sometime prior to the launch of Apollo 11 that, as the LM approached the moon's surface, there would be a need to transmit data to the LGC at a rate faster than was previously expected. Therefore, NASA asked AVCO if the DUA’s maximum baud rate could be increased. (Remember that this is back in the days of modems, analog tones used for data and 300 baud.) After some testing, AVCO reported that, yes, the maximum rate could be increased and, indeed, they would do so.

The response pleased the NASA folks greatly. However, it appears that in their joy NASA failed to convey word of this change to the LGC's manufacturer. As a result (oversimplifying a bit) as it approached the lunar surface, the DUA shifted into high gear in processing telemetry data. Since the LGC couldn’t keep up, it did what it was supposed to do in an overload situation. It rebooted.

As reported in The Lunar Module Computer, “As planned, the software rebooted and reinitialized the computer, and then restarted...each time a 1201 or 1202 alarm appeared, the computer rebooted.”

As we know now, in the end, the landing occurred, history was made and the course of mankind was changed. It was one of those rare moments that shows what can be accomplished when people work together. However, this event also shows how important effective communications can be for success when it comes to business computing.

(A side note. Since the DUA had a radio transceiver that was used for data, AVCO had engineered in FM voice capability. It was almost an afterthought. This turned out to be fortuitous and a lifesaver, quite literally, on Apollo 13. When the Command Module experienced damage due to the oxygen tank explosion, the astronauts had to return to Earth by riding in the LM. The voice capability in the DUA allowed the astronauts to save precious power by turning off the LM’s primary radios and still be able to communicate with Mission Control in Houston.)

(Image to the left: The Lunar Module's computer interface, the DSKY. NASA photo by Dennis Taylor.)