If it wasn’t for Dick Koos it seems certain that the Apollo 11 moon landing would have been aborted just 10 minutes from completing the assignment given by President Kennedy eight years earlier.

Dick Koos was a simulation supervisor, or in NASA’s short hand, a “SimSup”, pronounced “Sim Soup”. His job was to produce problems for the entire team, so they could iron out any problems before they presented themselves 384,000 km from Earth. It was July 5 1969, just 11 days before the launch of Apollo 11. In the lunar landing simulator was the Apollo 12 backup crew of Dave Scott and Jim Irwin. Koos loaded case number 26, which had not been tried before. Soon enough the crew were trying to land their simulated lunar module (LM, pronounced “lem”) when the computer started complaining. The program controlling the LM’s descent to the surface had run out of memory, and rebooted. Its last words were “Program Error – 1201”.

The simulations are as much for training the team at Mission Control as they are for the astronauts. Possibly even more so. Mission Control was a masterpiece of organisation and management. Sitting in front of the big screens was a room full of Flight Controllers with names like FIDO (Flight Dynamics Officer), BOOSTER (the rocket engine controller) and the man in charge of all the Flight controllers, the Flight Director, known simply as FLIGHT. Each Flight Controller has a back room full of support staff to help with details that the Controller may not have on hand. And in these back rooms they have the contact information for the people who designed, built and installed each system in the spacecraft. Nothing was left to chance.

An error in the LM computer falls to GUIDO, the Guidance Officer. Today the GUIDO was the 26 year old Steve Bales. Like everyone else, he had no idea what the 1201 error meant. It had not happened in any simulation so far. He asked the back room. The man there in charge of computing was Jack Garman, a year younger than Bales. He consulted the manual from MIT, who had built and programmed the LM computer. The manual said the computer was overloaded and the alarm meant it would “bailout”. He and Bales decided that with an unknown error on the computer in charge of the landing, the only safe response was to abort the landing. They made this recommendation to FLIGHT and that was the end of the simulation.

Dick Koos was not happy. Everything else was working. If this really happened just above the Moon’s surface, is aborting the right response? The Flight Director for this simulation was the famous Gene Kranz. Fate would put Kranz in charge for almost every important Apollo event. You know him – the crew cut ex-marine with the white vest. (Flight control teams were identified by a colour: Kranz’s team was white so his wife made him a new white vest for each mission.) Kranz instructed Bales and Garman to go through every possible computer error and decide which really did require an abort and which could be ignored, and the mission continued.

The LM computer keeps track of what it is doing in case of such an overload. It assigns each calculation a priority, and constantly keeps a record of where it is so that it can restart from the current situation. When the computer reboots it reads in the tasks it was doing, in order of their assigned priority. Thus it is able to continue the higher priority calculations, meaning that the important jobs keep going. Therefore a 1201 does not necessarily mean an abort is needed. It means that the least important jobs are lost, but if they are not critical to the mission, then the mission can continue. Garman re-evaluated which errors really required an abort and which did not. He prepared a hand-written sheet which was stapled into the manual at the last minute.

Just 15 days after the simulation, almost unbelievably, as Apollo 11’s LM Eagle approached the Moon, the crew received a 1202 error. Now, the crew in Eagle had not seen this error before. It was the Apollo 12 crew who were in the simulator when Koos gave them the 1201 error to deal with. But fate had arranged for Bales to be the GUIDO for Apollo 11 and had also placed Garman in the back room. They consulted their hand-written notes and realised that a 1202 error was similar to a 1201. The 1201 error meant that the computer was out of memory for calculations. In the 1202 error the computer could not start a new program. It’s a very similar problem, also classed as a “bailout” error. But after the recent re-evaluation, it is no longer considered as cause for an abort. So they announced “We are GO on that alarm, FLIGHT.” Over the next few minutes they received repeated 1202 errors and some 1201s. They told Armstrong and Aldrin to continue. The rest is, as they say, history.

But the first time the team was exposed to a 1201 or 1202 error, they aborted. If Koos had not tried it out on the Apollo 12 crew, it is almost certain that Apollo 11 would have aborted also.

You may think that this was enough drama for our first manned landing on another astronomical body. But wait, there is more. When the LM separated from the command and service modules for the descent to the Moon’s surface, there was a small amount of air left in the tunnel between the two spacecraft. This pressure gave a slight extra kick to the LM. So it separated from the command module just a little faster than intended. This extra speed was small, but as the LM descended closer to the Moon it accelerated under the force of the Moon’s gravity and the LM’s excess speed increased.

Part of the landing procedure was to visually check landmarks on the Moon as the astronauts flew over them, to ensure they were on course. They also checked the computer’s estimates of the speed by calculating it from the time it took for the landmarks to pass. There is no such thing as too many checks. Not when you are so far from home. These checks revealed that they were further down range than intended and also travelling faster than planned.

This is not necessarily a problem. But the Moon is not a smooth sphere. The Apollo 11 landing site had been carefully chosen to be relatively flat and clear of rubble. Now they were going to fly over the site previously chosen, and examined by Apollo 10 just months before. Neil Armstrong entered program 66 into the LM computer. This allowed him to take manual control of the attitude of the spacecraft while the computer kept the vertical speed at a value that Armstrong chose with a switch in the LM. He immediately tipped the LM forward to get some extra horizontal speed so he could fly over a large boulder field which was exactly where the new trajectory would have them land. Avoiding the boulders was essential, of course. But it was taking precious fuel.

The LM had a finite amount of fuel. If they were not able to land by a certain time, they were to abort the landing. In this case they would return to the command module which was awaiting them in orbit. That return took fuel as well, of course, because the LM had to climb against the Moon’s gravity. So this amount of fuel had to be held in reserve, for an abort.

Hence there was a “bingo” fuel call, which meant “land in 20 second or abort”. This was not negotiable. There would not be enough fuel to return to the command module if they did not either land or abort within 20 seconds of the bingo call.

As Armstrong flew them over the boulders, and looked for a safe place to land, Aldrin was reading out critical flight data. Specifically, their height above the moon, in feet, as well as their descent rate and forward speed, both in feet per second. The safe limits for landing were 10 feet per second down and only 4 feet per second forward. Exceeding the latter limit resulted in a significant risk that the LM would tip over. This would not be a good result…

They were zipping across the moon at 50 feet per second and descending at 20 feet per second. At a height of only 300 feet Aldrin calmly announces “You’re, uh, pegged on, uh, horizontal velocity.” This is Buzz’s way of saying that the forward motion “speedometer” was showing its maximum value, and they were actually travelling even faster than that. To land they need to be below 4 feet per second. Armstrong immediately tips the LM backward to slow their speed, while still trying to avoid the boulders.

Mission control in Houston is counting down the seconds to the bingo call. The situation is unbelievably tense. Don’t forget, they had the 1201 and 1202 computer errors going off as well, at the same time. The CAPCOM, or Capsule Communicator, is the only person allowed to communicate with the LM from Houston. This person is always an astronaut from the backup crew, so that he knows the mission as well as those in the spacecraft. For the Apollo 11 landing the CAPCOM was Charlie Duke, who would fly to the Moon on Apollo 16. Charlie was, in his words, “giving a running commentary” when Deke Slayton, the Director of Flight Crew Operations, punched him in the side and said “Shut up Charlie, let them land!” At that time the flight director, Gene Kranz, wisely said to all controllers “From now on the only callouts will be for fuel.” Hence the final stage of the landing was handed over to the two people most trained to do it, two humans who were holding their nerve despite all the tension and chaos surrounding them.

Listening to the recordings you can hear Houston calling out the time to the bingo call……”60 seconds”……”30 seconds”……and then finally the LM touched down with 18 seconds of fuel left. It was 4 miles (6.4 km) further down range than the original plan.

The Apollo 11 landing was a masterpiece of management, planning and, in the final analysis, gutsy flying from a team with nerves of steel. It is estimated that some 400,000 people were involved in making the Apollo program a success. Every one should be proud of their role in that achievement.

John C Lattanzio
Monash Centre for Astrophysics
https://research.monash.edu/en/persons/john-lattanzio