# Why were contact sensors put on three of the Lunar Module's four legs? Did they ever bend and stick out sideways?

In the video Brian Eno on Apollo starting at 08:37 there is Apollo-era movie footage of a lunar module taken from the command module as it prepares for descent towards the Moon.

As it rotates from this perspective it is clear that exactly three of the landers four legs have a very long "stick" protruding from the bottom of the pads for sensing contact with the Moon's hard surface.

1. Why three specifically?
2. They look pretty long; have they ever bent on contact and ended up sticking out from under the side of the pad rather than down into regolith?

• Pretty sure this is either explained in an answer to another question or can be found on Wikipedia. I believe there would have been one on each of the four legs except that the one on the "ladder" leg was removed out of concern that upon landing, the probe might be bent up such as to present a puncture hazard to the space suit of an astronaut exiting the spacecraft and descending the ladder. I think there is photography (still and/or video) showing the "remains" of a probe sticking out the side of one or more of the lander legs. Best I recall, it looked like they were laying pretty much flat. – Anthony X Jul 29 '19 at 3:16
• I saw the images on a recent documentary on the Apollo program (either PBS or CNN) - related to the 50th anniversary; not sure about finding an online source. – Anthony X Jul 29 '19 at 3:26
• @AnthonyX I don't think there's a standalone QA for it yet, so you should make that comment an answer. – Russell Borogove Jul 29 '19 at 3:49
• AS11-40-5917 is a photo of the +Y (right-side) footpad from Apollo 11, with probe bent outward. lpi.usra.edu/resources/apollo/frame/?AS11-40-5917 – Russell Borogove Jul 29 '19 at 3:52
• I'm pretty sure that one of the 100s of Apollo things I've read in the last month claimed that Armstrong asked to have it removed; sadly I've no idea which article or paper it was. – Organic Marble Jul 29 '19 at 10:52

It is correct that the probe on the forward footpad was omitted to avoid interfering with the ladder:

The probe located on the forward landing gear was deleted because of a concern that the failed probe could interfere with crewmen descending the LM ladder. [p. 8]

There was more than one probe for redundancy. In particular, there was no electrical latch for the signal, so if one probe had made contact but then failed, the CONTACT light would turn off. But if a second probe made contact, the light would stay on.

The probes are hinged in the middle and are designed to bend a particular way upon contact. There's no report of malfunction in the mission reports or in D-6850. However, D-6850 reports that the contact switches were the most common component of the landing gear to fail during manufacturing or testing. Accidentally tripping them was also a common problem while placing the LM inside the Saturn Lunar Adapter:

The item of greatest concern has been the possibility of misuse of landing-gear hardware inside the SLA where the working area is cramped. Several instances of inadvertent actuation of probe switches have occurred on vehicles being readied for launch. In addition, concern for misuse of landing-gear hardware, such as using struts for handholds or footholds, prompted a special training program for personnel working inside the SLA to preclude any further incidents. [p. 52]

One of the final checks on the launch pad was to make sure these probes had not been accidentally actuated.

Source: Apollo Experience Report: Lunar module landing gear subsystem, NASA Tech Note D-6850.

Some pictures from Apollo 11 of the landing gear – struts, footpads and contact probes.

Three Apollo 12 images:

An Apollo 16 image:

The two probes bend straight up on the left of the left and right footpad.

I found no Apollo 17 images with visible contact probes.

So yes, some contact probes did bend and stick out sideways. Even straight up on the last two images.

All photos are from NASA Apollo Lunar Surface Manual, see.

• These photos are great Uwe, thank you! This is something we usually never get to see. – uhoh Jul 29 '19 at 12:36
• @uhoh I could not resist to add some more of these very beautiful pictures. – Uwe Jul 29 '19 at 12:50
• Where did you get the photos from? – FGreg Jul 30 '19 at 20:47
• @FGreg all photos are from NASA Apollo Lunar Surface Manual, see hq.nasa.gov/alsj/main.html searching the images descriptions for the word footpad. – Uwe Jul 30 '19 at 20:52

According to W. David Woods' excellent "How Apollo Flew to the Moon", p. 262:

Originally a probe had been attached to all four footpads but Neil Armstrong had pointed out the possibility that his descent down the ladder might be impeded by a large length of metal probe that had been bent in some unpredictable way during the landing. The probe below the ladder was therefore removed from Eagle and all subsequent landers.

• Thanks! As I commented on the question a while back, I thought I had read that Armstrong made this request, but I couldn't remember where. – Organic Marble Aug 3 '19 at 13:16

According to the Apollo 11 Lunar Surface Journal, the probe on the forward/ladder footpad was still installed on Eagle in April of 1969:

Aldrin is quoted in the notes at 102:45:40 as saying "we [presumably the crew] asked that they take it [the probe] off."

• See this Apollo 9 LM image with four visible contact probes. More info here. – Uwe Jul 31 '19 at 11:14
• Apollo 11 LM in lunar orbit with three visible contact probes on this image. – Uwe Jul 31 '19 at 11:21

According to NASA Technical Note TN D-6850 the fourth probe was removed

because of a concern that the failed probe could inter­fere with crewmen descending the LM ladder.

I seem to remember from Gene Kranz' Failure Is Not an Option that Bill Tindall was involved in this somehow, but I can't find the reference to it there or in the Tindallgrams.

• I didn't find any relevant references to probes, footpads, or contact light in my ebook of Failure Is Not An Option. – Russell Borogove Jul 31 '19 at 21:39