What is this beige, pill shaped object on the back of the Apollo Lunar Module?

Question

Is it an antenna, or something else?

Image from https://upload.wikimedia.org/wikipedia/commons/0/04/Apollo_11_Lunar_Lander_-_5927_NASA.jpg

Answer 1

This is kind of speculative, but bear with me. Reading Apollo Experience Note - Lunar Module Communications System (TN D-6974) I think The Feature has to do with mitigation of corona discharge failures in the electronics of the aft equipment bay, most notably the S-band equipment. My best guess at the moment is that it is a cover for a vent or a pressurization umbilicus (or both).

This feature is apparently absent on LM-2 per the photos at Historic Spacecraft. Page 49 of the Apollo Experience Note notes that

Corona and pressurization. - In September and October 1967, four failures occurred in the S-band transceiver because of corona in the RF multiplier chain. The prime contractor recommended that the multiplier chain be foamed and that the circuits be sealed with combinations of epoxy and foam and an overcoating of 0.004-inch aluminum foil. Long-term thermal vacuum tests were performed to evaluate effects of temperature, vacuum, and humidity on the sealing materials. The tests provided information on the leak rate and time-to-failure for this type of seal.

The delta-qualification test of the seal began in May 1968. On May 18 and 19, 1968, a corona failure occurred in both transceiver channels. On May 29, 1968, this problem was presented to management, and it was decided that the S-band transceiver case would be pressurized for LM-3 and subsequent lunar modules.

This establishes that a change was made post-LM-2 to things in the Aft Equipment Bay. Unfortunately, I haven't been able to find a suitable photo of LM-3 (Spider) to determine whether or not LM-3 has The Feature or if it's an even later design change.

It would make sense for it to be a vent, though. Page 12 of the same document says

In the original design concept, the pressure environment of the aft equipment bay was planned to be either atmospheric pressure or absolute vacuum (except during the short period when earth orbit was being achieved). However, as the LM design developed, the aft equipment bay was found to maintain a partial pressure that was in the corona region for electronic equipment. This finding brought about a revision of system requirements and necessitated extensive redesign in three of the ERA's [ERA = Electronic Replaceable Assembly].

A caveat to this, though: per Figure 5 in the document, the Feature isn't particularly near the S-Band parts that most needed the corona mitigation. Those are all on the right side; the feature is about one-fifth in from the left. Page 35 notes that the PCMTEA ERA, which is further left, was redesigned in 1966 with "provision for external power for the ground-support-equipment (GSE) buffer-signal outputs"; the Feature could equally be covering a penetration for that or other GSE connections, though my impression is that LM-2 would have come later than the 1966 redesign so I doubt this particular connection is the culprit.

The Feature is almost certainly not an antenna; there are abundant diagrams of the communications and radar antennas on the LM, and The Feature doesn't show up in any of them.

The only other candidate I have so far is from Engineering Development of the Apollo Lunar Module by Joseph G. Gavin Jr., which notes that "provision was also made to sample the condition of the [glycol] coolant in an assembled lunar module." The Feature is near enough to the cooling rails to be associated with them, but I don't have any additional evidence that it could be covering a sample port.

The same question on Quora has answers that I don't find very credible; the first claim is that it has to do with cabling, and not only is there not an evident bundle for such in the assembly photo they cite but LM-2 lacks The Feature and should have extremely similar cabling. Another answer claims it has to do with fuel tankage but that's obviously nonsense because those aren't in that part of the LM.