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I was watching the Apollo 15 landing and I noticed that after the LM surface sensing probes (contact probe) touched the surface, the view was clear which meant no dust was being raised, but all of a sudden, dust was being raised again which I initially though that the engine was throttled up. It's hard to explain it in words so here's the video clip for reference.

So what was the reason for dust being raised up and the LM shaking after the contact probes touched the surface but before the LM footpads touched the surface?

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    $\begingroup$ Title question is about the LM violently shaking, body question is about dust raised. Can you converge them one way or the other? $\endgroup$
    – Russell Borogove
    Nov 15, 2019 at 23:45
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    $\begingroup$ The landing proper on 15 was the hardest of any of the Apollos; Dave Scott was apparently very quick to hit the engine stop button when the contact light came on, and they hit at over 6 feet per second, so it definitely wasn’t an engine throttle-up that kicked up dust. $\endgroup$
    – Russell Borogove
    Nov 15, 2019 at 23:48
  • $\begingroup$ @RussellBorogove I remember reading that the Apollo 15 landing was the hardest (because it was the first J-class mission meaning more payload), but I am curious about why after the contact light, the visibility increased, and then dust started to raise before the LM actually landed. $\endgroup$
    – Star Man
    Nov 16, 2019 at 1:11
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    $\begingroup$ If you think about it, while the engine is running, exhaust velocity is pretty high; perhaps it caused the dust to be mostly blasted radially outward. Engine shutdown might not be instantaneous, so as the exhaust flow falls off and becomes less energetic, instead of blasting out and away, it rebounded more upward. It's just a thought, would be interesting to get an expect take on the matter. As for the violent jolt, the video says the contact light came on at 1.5m (probe length), so the LM would have fallen that height if the engine was shut down promptly. $\endgroup$
    – Anthony X
    Nov 16, 2019 at 1:23
  • $\begingroup$ I think engine cut off is pretty fast. Never seen a slow responsive engine cut off in any if space x landing $\endgroup$
    – zephyr0110
    Nov 16, 2019 at 17:43

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Looking at this photo https://en.m.wikipedia.org/wiki/File:Apollo_15_Engine_Bell.jpg It seems there is a local terrain elevation (a hill) right in the middle of LM final landing spot with engine bell resting on the surface (note also how the bell is damaged: they must have had a decent bump on touchdown). Having some horizontal velocity, they would approach this hill some seconds before touchdown (see Uwe's answer here Why is the Apollo LEM ladder so far from the ground?)

Here's another photo clearly showing local surface curvature (follow rover tracks in the middle-left of the image) https://upload.wikimedia.org/wikipedia/commons/1/1e/As15-86-11600_%2826069559865%29.jpg

It might have ben that the engine plume with dust just naturally bounced up off the curved surface of this hill.

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That was a very controlled 1fps decent at the end. Scott wrote in his report that he shut down shortly after the contact light illuminated but there is some discussion that the bent nozzle was due not from impact but from an overpressure caused by thrusting too close to the lunar surface (the nozzle was 10 inches longer than previous missions). In any event, the LM straddled a crater and the upslope leg would therefore trigger the contact light while the center of the LM was higher above the surface than if it was flat terrain. Additionally, the downslope legs would have a slapdown component which likely made the landing feel firmer than desired.

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    $\begingroup$ Look at space.stackexchange.com/questions/37762/… it was not 1fps, it was much faster, about 7 fps. Do you have a source for 1 fps? $\endgroup$
    – Uwe
    Jun 9 at 21:55
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    $\begingroup$ From Apollo 15 Mission Report see history.nasa.gov/alsj/a15/a15mr.htm part 2: Landing on the lunar surface occurred with estimated velocities of 6.8 ft/sec in the minus X direction, 1.2 ft/sec in the plus Y direction, and 0.6 ft/sec in the plus Z direction. The descent rate at probe contact was 0.5 ft/sec. Following probe contact, the descent engine was shut down while the footpads were still about 1.6 feet above the surface, resulting in the 6.8 ft/sec velocity at footpad contact. $\endgroup$
    – Uwe
    Jun 9 at 23:57

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