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The button to stop the descent engine of Lunar Module was not "just a button": it evolved and changed multiple times during development, and this is one of the many existing designs:

stop button

What are all those levers and switches surrounding the button? Which, by the way, is not the same of the start button: stop button is on the left of the assembly, oriented to the left:

stop button detail

I know it was a "pushbutton", which remained in locked position after first press, and required a second press to be reset: stop pushbutton operational mode Page 675 of handbook)

So possibly some of those levers are the locking mechanism... or not? The left lever should be the descent rate switch, as shown in another picture:

descent rate

But I cannot find a specific document which describes in detail this assembly. I can only find several photos showing different fashions of same assembly...

fashions

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2 Answers 2

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According to Harrison Schmitt (Apollo 17 Lunar Module Pilot) the strange lever assembly is a safety guard for the Stop button. (emphasis mine)

enter image description here

From Schmitt's website

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    $\begingroup$ a trivial bit of information about the name for the type of switch action described by jumpjack in his question (which remained in locked position after first press, and required a second press to be reset) This type of switch action is called "alternate action" . It can be accomplished by several mechanisms. This one seems to be an over-center latching mechanism. The most widely used alt-ac mech (but likely not recognized as such) is the clicker on the top of pens! $\endgroup$
    – BradV
    May 19, 2022 at 17:58
  • $\begingroup$ @BradV I took Schmitt's comment to mean that the lever is a "switch guard", intended to prevent the button from being inadvertently depressed. Not the part of the action that kept it actuated. Shuttle had switch guards too but they were more like little doors with windows in them, not a lever thing as shown in the question. i.imgur.com/Z72Whk8.png $\endgroup$ May 19, 2022 at 18:22
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    $\begingroup$ I'm familiar with switch guards... have specified COTS (Commercial Off The Shelf... catalog items) guards on flight deck equipment as well as designed a couple of custom guards. A real fun guard was a spring loaded cover with with window not too different from what you show in comment. It was painted bright red and covered the EDM button on a Flight Control Panel. EDM stood for Emergency Descent Mode. When pushed in ordered the FCP and flight control computers to override other inputs and get aircraft to a preset altitude ASAP. $\endgroup$
    – BradV
    May 19, 2022 at 20:29
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    $\begingroup$ I think what I see in the LM photo is an over-center latching mechanism that locks button when button is pushed and requires person to lift a tab on linkage to release the over-center latch and allow button to be pushed again. BUT... this is all my guess from image. No text anywhere says anything like this. $\endgroup$
    – BradV
    May 19, 2022 at 20:34
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Here is a sketch I threw together (reverse engineered from linked engineering dwg that has better resolution. zone J2 is what jumpjack cut/pasted, zone D2 shows better clarity of same mechanism) of how it looks to me. The safety guard mechanism DOES NOT appear to be an over center mechanism but rather pin in notch. My original guess in comments seems to be wrong. The red spring in my sketch drives pin into detent notch, preventing switch from being pushed again. This lockout can be disengaged by pushing down on the tab provided on the green link.

edit: I'd like to add that I don't think my answer conflicts with Organic Marble's

second edit: for those not familiar with alternate action switch mechanisms... here goes.

Lets say (just for easy round numbers) that full button travel is .50 inches. For the alternate action mechanics within the switch body to become enabled the button must pop back out roughly 60% of full travel. If the button is kept from returning over 60% the switch cannot perform its alternate electrical function and the original function remains locked in. The lockout mechanism illustrated keeps the button from returning. A person could pound and pound on the "top button-pusher" but never reset the switch unless the release tab on lockout gets pushed first. IF the lockout is released the button is now free to return to original position (100% return). The original button push function is still enabled but the alternate action mechanism is now free to operate. NOW a second full button push will disengage the first function and engage the second function.

enter image description here

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  • $\begingroup$ you are inventing, more than providing evidence of this ancient design. $\endgroup$
    – jumpjack
    May 21, 2022 at 7:27
  • $\begingroup$ @jumpjack Yes, to a certain extent you are correct. However... if you examine closely you can see the mechanical links in sketch directly correspond to engineering dwg. I know from direct usage in aircraft product design how an alternate action switch mechanism actuates. The only part(s) I invent are the notch in slot and spring. There seems to be evidence of spring in better clarity eng dwg, zone D2. So, OK... what else would the contraption be there for if not a resettable lockout? $\endgroup$
    – BradV
    May 22, 2022 at 3:25

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