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If I understand it, the alarm during the first Lunar descent was due to the computer being overloaded by too much RADAR data. That sounds like something that they would have thought to simulate, but it seems that everyone was surprised by it. Why didn't they simulate that particular type of failure (computer overflow in general, and RADAR-caused overflow in particular?)

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    $\begingroup$ You are assuming they didn't have a plan for it, that's not a good assumption. $\endgroup$ – GdD Dec 23 '20 at 11:50
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    $\begingroup$ Anyone who's tried to bomb-proof their software only to find a problem 3 months later knows the answer here :-( $\endgroup$ – Carl Witthoft Dec 23 '20 at 12:12
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    $\begingroup$ istr they did pretty much that exact failure in a sim. Will look for reference. $\endgroup$ – Organic Marble Dec 23 '20 at 13:32
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    $\begingroup$ Stanislav Lem's Pilot Pirx story Ananke is about this case. It may provide fictitious reasons. $\endgroup$ – Hans-Peter Stricker Dec 24 '20 at 10:39
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    $\begingroup$ @CarlWitthoft And had to write software to meet some high ranking executive's arbitrary deadline. "If Kennedy wants to get to the Moon before the decade is out, he can come down here and debug the cursed thing himself!" -- Probably some Apollo software engineer. $\endgroup$ – Schwern Dec 25 '20 at 6:05
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They did simulate the debugging alarms, such as the 1201 and 1202. From Apollo: The Race to the Moon*:

On July 5, just eleven days before the launch, [...] the scenario included one of the computer alarms that [Jay] Honeycutt (one of the simulation supervisors) had discovered. When the alarm went off, the controllers didn't know what to do with it.

The Guidance controllers subsequently put together a list of the different alarms and how they should be handled. When the 1202 and 1201 alarms occurred on Apollo 11, Steve Bales (Guidance controller) and Jack Garman (in the Guidance back room) knew how to handle the alarms; as long as they weren't continuously firing, the descent was still ok to proceed.

As for why the general issue of executive overflow (the guidance computer being overloaded) wasn't more thoroughly tested: as much as the Apollo program simulated and tested, there was still a limit to the number of different scenarios they could test in the time they had. Again from Apollo:

[Identifying and learning how to handle the computer alarms] was a pain in the ass, many of [the Guidance controllers] thought, because there were so many failure modes on a descent that were much more likely to happen.

* Chapter 24, end of section 3

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    $\begingroup$ Simulating real radar data in a realistic way must have been a real challenge, if it was even possible. $\endgroup$ – GdD Dec 23 '20 at 16:18
  • $\begingroup$ Can you give a chapter or page # in the book? (Mine falls open to Chapter 21 part 2, about simulation training in general) $\endgroup$ – Organic Marble Dec 23 '20 at 21:00
  • $\begingroup$ @Organic Marble edited in. $\endgroup$ – DylanSp Dec 23 '20 at 21:07
  • $\begingroup$ @DylanSp many thanks. $\endgroup$ – Organic Marble Dec 23 '20 at 22:04
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As DylanSp's answer notes, the 1201/1202 alarms were simulated, but the details of the computer overload that caused them on the Apollo 11 flight were complex, and were not specifically simulated prior to the mission.

According to Mindell's Digital Apollo:

The trouble was that the rendezvous radar and the rest of the guidance system had different electrical power supplies. They both ran on alternating current of the same frequency, but had different phases (i.e. their alternating sine waves were out of sync). When the change in the [rendezvous radar mode] switch procedure was tested in the lab, technicians connected both to the same power supply, which caused them to run in phase, even though they would be out of phase in the spacecraft...

On Apollo 11, the power supplies on the LM fell into a particularly unfortunate phase angle. Hence the computer and radar were not in sync, causing the angle counters on the rendezvous radar to constantly increment or decrement in response to random electrical noise, sending nearly the maximum rate of data to the computer. The computer struggled to increment or decrement its counters for tracking the radar angles, which used up about 15 percent of its processing time.

More details can be found in Don Eyles' paper titled Tales From The Lunar Module Guidance Computer.

NASA went to great lengths to realistically test and simulate as much as they were able, but a few issues like this did slip through the cracks.

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