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Interesting: he says that the famous 1202 error may sometimes crash the spacecraft and sometimes land successful in some other cases. And also that it depends o

Interesting: he says that the famous 1202 error may sometimes crash the spacecraft and sometimes land successful in some other cases. And also that it depends on many factors. So, the sane thing to do at that moment, AIUI, was to abort the landing. The Apollo 11 landing was not a tragedy be sheer luck. IIRC Armstrong asked ground control about 1202 and they tell him to ignore it. Ground controllers, probably unknowingly, bet Armstrong and Aldrin lives.

wrs 3y ago
Not unknowingly at all. They had worked out the criteria for all the program alarms ahead of time. You can see their “cheat sheet” here: https://www.space.com/apollo-11-guidance-officer-remembers-moon-landing-drama.html https://www.space.com/apollo-11-guidance-officer-remembers-m...
- devilbunny 3y ago
  The video strongly implies that the conditions for 1201/1202 alarms might, or might not, lead to a crash. The take in the video is that they got lucky both in sim and reality.
  - garaetjjte 3y ago
    I don't know if CuriousMarc discovered something new, but AFAIK: Cause of alarms is described there: https://www.doneyles.com/LM/Tales.html https://www.doneyles.com/LM/Tales.html Alarms indicated system restart, but because landing program was designed to automatically restart with data from last checkpoint, checklists deemed it was safe to continue. The trouble though was what was causing the restarts: due to unexpected CPU time stealing from hardware, new SERVICER jobs were scheduled before previous one finished! And previous job wasn't removed from list immediately (that only happened when everything restarted with alarm), which mean that with load fluctuations previously interrupted stale jobs could be continued. In effect there were multiple active instances of routine that was never designed to be reentrant, potentially stomping over global variables and submitting stale commands. The reason that effects of this were relatively benign was that the last thing SERVICER did was updating display, and that's where unintended behavior appeared. In case hardware was stealing a bit more time and jobs would be interrupted near throttle command submission, effects would have been worse. See also https://www.ibiblio.org/apollo/Documents/LUMINARY-memo-138.pdf https://www.ibiblio.org/apollo/Documents/LUMINARY-memo-138.p... >It is now known that the stacking up of Servicer jobs which precedes a 1201 or 1202 alarm in the event of TLOSS can cause completely wrong throttle and attitude commands, and in one special case, random branching. This happens because the Servicer job can clobber its own erasables. Luckily this did not happen to Apollo 11, which encountered TLOSS estimated at up to 15%. EDIT: I watched the talk and I'm not sure whether he implies there's additional CDU problem, or just discussion about exact amount of CDU-caused TLOSS, so I hope he will explain that on the channel.
- elteto 3y ago
  Correct. And had done extensive testing of these particular alarms. Straight from NASA: https://history.nasa.gov/alsj/a11/a11.1201-pa.html https://history.nasa.gov/alsj/a11/a11.1201-pa.html
jazzyjackson 3y ago
Here's another lecture I really enjoyed walking through the operation of the guidance computer and the events that lead to the 1202 error, which the speaker sums up as an "out of memory" error resulting in an automatic reboot. He says the cause was a hardware bug taxing the machine with empty status updates, combined with Buzz typing "verb 16 noun 68" to have the display auto-update some stats - he was crashing the machine but didn't put together why it was happening because it never crashed the machine in simulation. https://youtu.be/B1J2RMorJXM https://youtu.be/B1J2RMorJXM
duckhelmet 3y ago
> Interesting: he says that the famous 1202 error may sometimes crash the spacecraft and sometimes land successful in some other cases .. The Apollo 11 landing was not a tragedy be sheer luck. I recall reading somewhere that the 1202 error was caused by someone switching on the rendezvous radar, in the event of a quick take-off. This caused the computer to continuously seek for a radar lock on the orbiting craft that was not going to occur. Nothing like this happened in the simulator.

wrs 3y ago

Not unknowingly at all. They had worked out the criteria for all the program alarms ahead of time. You can see their “cheat sheet” here: https://www.space.com/apollo-11-guidance-officer-remembers-moon-landing-drama.html https://www.space.com/apollo-11-guidance-officer-remembers-m...

devilbunny 3y ago
The video strongly implies that the conditions for 1201/1202 alarms might, or might not, lead to a crash. The take in the video is that they got lucky both in sim and reality.
- garaetjjte 3y ago
  I don't know if CuriousMarc discovered something new, but AFAIK: Cause of alarms is described there: https://www.doneyles.com/LM/Tales.html https://www.doneyles.com/LM/Tales.html Alarms indicated system restart, but because landing program was designed to automatically restart with data from last checkpoint, checklists deemed it was safe to continue. The trouble though was what was causing the restarts: due to unexpected CPU time stealing from hardware, new SERVICER jobs were scheduled before previous one finished! And previous job wasn't removed from list immediately (that only happened when everything restarted with alarm), which mean that with load fluctuations previously interrupted stale jobs could be continued. In effect there were multiple active instances of routine that was never designed to be reentrant, potentially stomping over global variables and submitting stale commands. The reason that effects of this were relatively benign was that the last thing SERVICER did was updating display, and that's where unintended behavior appeared. In case hardware was stealing a bit more time and jobs would be interrupted near throttle command submission, effects would have been worse. See also https://www.ibiblio.org/apollo/Documents/LUMINARY-memo-138.pdf https://www.ibiblio.org/apollo/Documents/LUMINARY-memo-138.p... >It is now known that the stacking up of Servicer jobs which precedes a 1201 or 1202 alarm in the event of TLOSS can cause completely wrong throttle and attitude commands, and in one special case, random branching. This happens because the Servicer job can clobber its own erasables. Luckily this did not happen to Apollo 11, which encountered TLOSS estimated at up to 15%. EDIT: I watched the talk and I'm not sure whether he implies there's additional CDU problem, or just discussion about exact amount of CDU-caused TLOSS, so I hope he will explain that on the channel.
elteto 3y ago
Correct. And had done extensive testing of these particular alarms. Straight from NASA: https://history.nasa.gov/alsj/a11/a11.1201-pa.html https://history.nasa.gov/alsj/a11/a11.1201-pa.html

jazzyjackson 3y ago

Here's another lecture I really enjoyed walking through the operation of the guidance computer and the events that lead to the 1202 error, which the speaker sums up as an "out of memory" error resulting in an automatic reboot. He says the cause was a hardware bug taxing the machine with empty status updates, combined with Buzz typing "verb 16 noun 68" to have the display auto-update some stats - he was crashing the machine but didn't put together why it was happening because it never crashed the machine in simulation. https://youtu.be/B1J2RMorJXM https://youtu.be/B1J2RMorJXM

duckhelmet 3y ago

> Interesting: he says that the famous 1202 error may sometimes crash the spacecraft and sometimes land successful in some other cases .. The Apollo 11 landing was not a tragedy be sheer luck. I recall reading somewhere that the 1202 error was caused by someone switching on the rendezvous radar, in the event of a quick take-off. This caused the computer to continuously seek for a radar lock on the orbiting craft that was not going to occur. Nothing like this happened in the simulator.