The dress rehearsal that saved Apollo 11

In this post we will look at one of the many odd coincidences that saved the success of the Apollo project: The dress rehearsal that saved the Apollo 11 moon landing.

The dress rehearsal saved the Apollo 11 landing

A few weeks before liftoff Mission Control held a dress rehearsal of the landing. The leading Flight Director Gene Kranz was in charge of the team, and he was fully confident the test would prove successful and would be an excellent confidence booster for the crew before things got serious. But Apollo 11’s Simulation Supervisor (Call-sign "SIMSUP") Dick Koos had other plans. In the midst of the simulated landing when everything was going perfectly and according to the book, he induced computer alarms 1201/1202 into the Lunar Module’s computer.

The computer system was part of navigation and thus belonged under GUIDO Steve Bales. But no one on the GUIDO team, neither Bales nor anyone on his team of specialists, had any idea what the alarm codes from the Apollo computer meant. All they could understand was that the system was overloaded, and since one of the mission rules was that a landing must not be carried out without a functioning navigation system he chose to abort the mission and carry out a successful emergency abort instead. Gene Kranz was sure he and his team had made the right decision, and passed the test. He was wrong.

Supervisor Dick Koos introduced computer error 1201/1202 at the dress rehearsal - The Mission Control team failed and the formidable premiere lay ahead!

Code 1201 and 1202 meant the computer was so overloaded that it couldn’t perform all its tasks. The team had been right so far. But how did the computer actually act when it was overloaded? There were three categories of computer tasks ranked according to their importance. A Priority 1 task was the most important, for example performing a navigation task or an engine control task. A Priority 3 task was the least important, for example updating the clock on the astronauts’ instrument panel. Alarms 1201/1202 simply meant that the computer was dropping low priority tasks whereas all critical tasks continued to be carried out, in other words, the navigation system was still basically functional.

Gene Kranz and his team were notified by SimSup that aborting the mission had been a critical error. This was hardly a morale booster, and Kranz gave Bales some homework to do: All computer alarms and their significance must henceforward be listed systematically so they could be deciphered immediately if necessary. The dress rehearsal was a failure and the formidable premiere lay ahead.

On the morning of Sunday 20 July 1969, Gene Kranz and his team got to work before dawn. The great moment was approaching. He called his anxious, young team together for the final pep-talk: "We're getting ready to make history. You know what we're about to do. From the day that we were born we were destined to be in this room this day. I've trained, I've absolute confidence in everyone in this room, but I want you to know something: No matter what happens to us this day, I will stand behind every decision that you make. However it goes, when we walk out of this room, we walk out of this room as a team." [1]

Gene Kranz was actively using his faith to inspire his men with confidence in Providence and the faithfulness of God. He believed, and he induced the faith, that they were all destined and enabled by God to carry out the task before them. This gave them the peace and confidence to perform. GUIDO Steve Bales, who was only 26 at the time, says about his Flight Director’s talk. "I can’t even say that today without getting choked up about it. That was the best, best thing he could have said to me." [1]

During the next half hour Bales had to bear his share of the difficult decisions on his young shoulders.

The landing of Apollo11 was a 30 minutes breathtaking drama. Communication problems, computer problems and fuel-shortage threatened to blow the mission. The Eagle landed with 17 seconds of fuel left.

Seven minutes prior to landing at approx. 30,000 feet above the lunar surface an alarm lit up on the astronauts’ instrument panel. The alarm was set off by the Lunar Module’s navigation system and accompanied by an automatic shutdown and re-start of the computer. The error was type 1202! Everyone in the control room was dumbfounded. Of all the thousands of possible errors here was 1202, which they had only seen once before, at the dress rehearsal when they had aborted the mission and failed the test.

7 minutes before landing - 1202 Alarm. "The significance of this is not lost on any of us"!

Gene Kranz recalls: "Duke [CAPCOM] muses aloud on the Flight Director loop, “It’s the same one we had in training.” [The precise words according to the Audio-recordings [4, go to time=24:50] were "Yeah, the same thing we had". It is not verified that it is the voice of Duke on the loop] He audibly expresses our collective feeling, almost wonderment. These were the same exact alarms that brought us to the wrong conclusion, an abort command, in the final training run when SimSup won the last round. This time we won’t be stampeded. The significance of this is not lost on any of us."[2]

The fact that the alarm code, which had only been tested once, actually occurred the following week during the actual landing, and that what the team had learned from that test was what actually saved the mission, is one of the small, odd events that occurred throughout the Apollo project, and which many believe to be miraculous.

Was it mere chance that had inspired SimSup Dick Koos, to get this exactly right for the dress rehearsal?

The error arose, by the way, because Buzz Aldrin used his intuition and deviated from the mission plan. The astronauts’ intuition was normally a powerful and important tool based on their talent and extensive experience. It was an unwritten but approved practice that out in the real world the pilots’ intuition trumped the mission plan. Astronauts could often sense things that planning hadn’t accounted for.  But this time it was almost disastrous. Aldrin chose to keep the tracking radar on that took the bearings of the Command Module. This radar was designed to stay on for a while after the separation of the two vessels in case something went wrong and an efficient emergency abort maneuver was required and the crew had to get home quickly. After a while it was supposed to be turned off manually, but Aldrin chose to let it stay on. On the spur of the moment he decided not to let go of the lifeline. This overloaded the computer and it began sending 1202 alarms, restarting, and dropping low priority tasks, as explained above.

For safety reasons Buzz Aldrin left a radar on, that should have been shut down - causing the computer to be overloaded.

This time, however, Gene Kranz and Steve Bales were not taken off guard. Armstrong and Aldrin knew nothing about the Code 1202, because the dress rehearsal had been carried out by the back-up crew. They demanded to know what the alarm meant. Kranz had CAPCOM inform them that they could just ignore it. “We are GO on that alarm” [4]. Without the dress rehearsal’s “small miracle” in all likelihood they would have aborted Apollo 11’s landing maneuver, and men would not have walked on the Moon in 1969!

"Yet it was not accomplished by human endeavor alone. It was strengthened with the spirit of God within those who accomplished it." (Simulation Supervisor Dick Koos, who orchestrated the dress rehearsal that saved the landing)

[1] Failure is Not an Option. Dir. Rushmore DeNooyer and Kirk Wolfinger. Documentary. 2003.
[2] Gene Kranz, Failure is not an Option, ISBN:0743200799
[3] Carol Mersch, The Apostles of Apollo, p. 141. ISBN:9781940222097
[4] https://www.hq.nasa.gov/alsj/a11/A11_landing_FD_loop.mp3

See also this discussion thread: http://www.collectspace.com/ubb/Forum29/HTML/001672.html