Apollo 11 LM ascent engine circuit breaker

[nicp]

I think it is fairly well known that the handle of a circuit breaker got broken off in the Apollo 11 LM by (I think) Aldrin's backpack while the LM was on the surface of the Moon. I don't recall if it was before or after the EVA.

If memory serves, it was the ascent engine circuit breaker handle that was damaged, so this was a big deal.
The story goes that when required the breaker was closed by Buzz using a pen.

Does anyone know what mitigation was put in place to prevent such a potential disaster in later missions.
Also, I recall seeing a documentary where someone - a flight director? Kraft? - said they figured out a number of ways of getting the ascent engine to run if the breaker could not be convinced to close. This suggests interesting redundancy in the wiring of the LM. Does anyone have detail of this?

Thanks,
Nic

EDIT: Clarity.

[bkellysky]

I'm sure someone will be along soon with more details and references.
In the meantime, I researched this for my talk "Failure is an Option" that included many of the potentially mission-ending issues on the Apollo manned missions.
After Apollo 11, NASA added more guards on the instrument panel. Weight was an issue for the LM, so additions were studied thoroughly.
I don't have the sources at hand, but http://www.collectspace.com/ubb/Forum29/HTML/001034.html at collect space has a good discussion about Buzz and the pen, including a story from Kelly at Grumman (no relation) saying they rerouted the signal around the switch.
bob kelly

[gparker]

From the Apollo 11 radio transcript:

(after the EVA)
"112:56:28 Aldrin: Houston, Tranquility. Do you have a way of showing the configuration of the engine arm circuit breaker? Over. (Pause) The reason I'm asking is because the end of it appears to be broken off. I think we can push it back in again. I'm not sure we could pull it out if we pushed it in, though. Over.

112:56:56 McCandless: Roger. We copy. Stand by please. (Long Pause)

112:57:23 McCandless: Tranquility Base, this is Houston. Our telemetry shows the engine arm circuit breaker in the Open position at the present time. We want you to leave it open until it is nominally scheduled to be pushed in; which is later on. Over."

(and before ascent, after the crew pushed it in)
"123:20:43 Evans: Tranquility, Houston. For your information, the circuitry looks real fine on that ascent-engine-arm circuit breaker.

123:20:54 Aldrin: Roger. I don't think I could get it out now if I wanted to.

123:21:00 Evans: Roger. We copy."

From the Apollo 11 Technical Crew Debriefing:
"ARMSTRONG: As was reported, we broke one circuit breaker with the PLSS and we depressed two others, one on each side, sometime during the operation with the PLSS on the back. So that's an area that we still need to improve on to be able to have confidence that the integrity of the LM itself won't be jeopardized by the operation with the PLSS on the back."

From the Apollo 11 Mission Report, Anomaly Summary:
"Broken circuit breaker knob. The crew reported after the completion of extravehicu­lar activity that the knob on the engine arm circuit breaker was broken and that two other circuit breakers were closed. The engine arm circuit breaker was successfully closed when required for ascent, but loss of the knob would not allow manual opening of the breaker.

The most probable cause of the damage was impact with the oxygen purge system (aft edge) during preparation for extravehicular activities; such an impact was demonstrated in simulations in a lunar module. Circuit breaker guards will be installed on Apollo 12 and subsequent vehicles to prevent the oxygen purge system from impacting with the circuit breakers.

This anomaly is closed."

all available from the Apollo 11 Lunar Surface Journal
https://www.hq.nasa.gov/alsj/a11/a11.html