In footage of the LM undocking from the Command Module, the footpad looks like it was "fluttering":

At first, I thought it was from the RCS thrusters but then I noticed that none of the RCS thrusters are in line with the footpads. enter image description here

Furthermore, any exhaust that leaves the RCS would rapidly dissipate as it is in a vacuum. So what's the reason for the cellophane on the LM footpad to flutter? Judging by the way it flutters, it certainly does look like RCS exhaust.

  • 2
    $\begingroup$ I believe the CSM thrusters are closer to in-line with the LM pads; perhaps the CSM was performing the back-off maneuver. $\endgroup$ Nov 29, 2019 at 2:14
  • $\begingroup$ Yeah, I thought about that too but aren't the CSM thrusters too far? $\endgroup$
    – Star Man
    Nov 29, 2019 at 2:39
  • 3
    $\begingroup$ Plume impingment effects can almost certainly occur that far from the jets. $\endgroup$ Nov 29, 2019 at 2:57
  • $\begingroup$ @StarMan Remember that there is no atmosphere to impede the exhaust from the RCS. The molecules in the exhaust plume are (almost) all moving roughly in the same direction and once the exhaust expands enough there will be very little interaction between them anymore. The RCS exhaust is more akin to molecular shotgun pellets than a puff of gas cloud. $\endgroup$
    – tylisirn
    Nov 29, 2019 at 14:46

1 Answer 1


The film on the foot pads are likely kapton, from Wikipedia:

Aluminized kapton, with foil thickness of 50 and 125 µm, was used e.g. on the Apollo Lunar Module. The polyimide gives the foils their distinctive amber-gold color. Space blankets are made by vacuum-depositing a very precise amount of pure aluminum vapor onto a very thin, durable film substrate.

I suggest that the exhaust from the RCS does not "dissipate" so much as expand. The particles contained within the exhaust would not necessarily be visible and could be deflected by portions of the structure, sufficiently to impact the kapton film on the foot pads.

The deflection of the foil is consistent with small-force thrust movements shown in the video. Considering the extremely thin material, a sneeze in our atmosphere would shake the film violently, while a tiny bit of deflected exhaust can explain the flutter in the foot pads.

I'd expect there may be a more technical explanation available, perhaps even thrust vector diagrams performed during the design of the LEM, showing that the impact on the pads was within acceptable limits.


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