In Their Own Words: Don Eyles on Apollo 12 Pinpoint Landing
Don Eyles, Apollo Software Engineer explains the challenges engineers and astronauts faced in getting the lunar module to make a precision moon landing. Eyles worked on the software and computer engineering team for the lunar module at the MIT Instrumentation Lab (now Draper).
"Every mission had to accomplish more than the one before.
Apollo 11 landed four miles from its intended site. So what? To land at all was a triumph, beating Kennedy's goal by almost half a year. But to explore more scientifically interesting sites the LM would have to land more accurately. So Pete Conrad, the Apollo 12 commander, was handed a new goal — to make a pinpoint landing, within easy walking distance of an unmanned Surveyor spacecraft that had landed in the Ovean of Storms two years previously.
Less than a week after Apollo 11 splashed down, MIT engineers George Cherry, Allan Klumpp, Don Eyles, and Nick Pippenger traveled to Houston for a meeting chaired by Bill Tindall, to consider the problem. Tindall summarized the discussion in one of his famous Tindalgrams, titled 'How to land next to a Surveyor — a short novel for do-it-yourselfers.'
One problem was the thrust created by the LM's water boiler (used for cooling), which was not accounted for by navigation. That alone could cause a big miss. On the other hand, if tracking information from Earth-based radars could be incorporated into the LM's navigation, much of the error could be corrected. As the LM came around the Moon for the last time before the landing it was in the ideal position for ground radars to measure its position and velocity. That is where the MIT Instrumentation Lab could help, if it could figure out a way to incorporate the radar data, at the last possible minute, into the LM navigation. The result was a new noun, Noun 69, added to the vocabulary of verbs and nouns by which the crew and computer communicated. It would have been hard to correct the navigation itself, so the effect of the new noun was to move the landing site. Same difference!
Another response to the problem was in the training of the astronauts, and for that a magnificent tool was available, the LM Mission Simulators (LMS) located in Houston and at Cape Canaveral. These sims had terrain displays for the LM's windows that were accurate enough to train for landing at a particular spot. Shortly after Bill Tindall's meeting, the Apollo 12 commander, Pete Conrad, invited MIT lunar-landing specialists Klumpp and Eyles to Cape Canaveral to tell him about the new noun and to show them, on the LMS, what it was like to operate the LM. Inside the simulator the cockpit was accurate in every detail. Out the LM's large front windows was an optically accurate simulation of the lunar surface, derived from a large relief map that was constructed for each landing site. Training focused on the crucial moment at the start of the visibility phase when the spacecraft pitched forward and the lunar surface became visible. Would the astronauts recognize the pattern of craters that marked their target? It was a moment of peak excitement in every mission. If the astronaut could quickly orient himself, the MIT-designed redesignation system could show him where the LM was headed, and gave him a way to shift that point if necessary.
Following his meeting, Tindall had written, 'If we land within walking distance, it is my feeling we have to give most of the credit to Lady Luck.' Three months later he expressed carefully qualified optimism. 'As long as the systems work as well as they have in the past, we have a pretty good chance of landing near the Surveyor.' In fact, by applying the radar correction, using Noun 69, early in the landing, and using the redesignation system when they got closer, not only Apollo 12, but every landing from then on, touched down at its precise target."
Apollo 12 went down in the history books as a successful lunar mission despite the fact that the Saturn V rocket was struck not once, but twice, by lightning during the launch of the mission.