Are there any photos of the Apollo LM descent engine bell and the lunar soil beneath it disturbed by the engine exhaust?
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$\begingroup$ hq.nasa.gov/alsj/a11/AS11-40-5921.jpg $\endgroup$– user20636Sep 2, 2019 at 16:43
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$\begingroup$ see also space.stackexchange.com/questions/1691/… $\endgroup$– user20636Sep 2, 2019 at 16:53
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3$\begingroup$ Would you accept video of the dust being disturbed? ->Apollo 11 landing showed this. $\endgroup$– HobbesSep 2, 2019 at 18:09
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1$\begingroup$ If it were a hoax, they'd present what you'd expect to see, and part of that would be lunar dust flying everywhere. $\endgroup$– RonJohnSep 3, 2019 at 16:04
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2 Answers
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Most of the Apollo photo libraries have a few shots of the surface under the descent engine bell; I think A14 has some interesting ones:
The disturbance of the soil is very subtle; compared with the surface further out, I see more 1-2cm-sized pebbles, suggesting that smaller particles have been blown away from under the engine.
The lack of a massive blast crater under the LM engine is often pointed to as evidence of a hoax, but there are several factors involved, which are not obvious or not intuitive, which minimize the effect of the engine exhaust:
- At touchdown, the engine is running at only about 25% throttle (~2500 lb-f or 11kN), as the descent fuel tanks are nearly empty, making the LM fairly light;
- The area at the end of the engine bell is about half that of the four LM footpads, so pressure at that point while hovering should be about twice as much as the ground pressure of the footpads with the LM on the surface, which of course displace only a few cm of soil;
- As Schwern points out, without atmospheric pressure constraining the exhaust plume, it spreads out more rapidly than the familiar image of a rocket exhaust flame at liftoff, thus is very diffuse. The LM engine is normally shut off while the footpads are still a meter or so above the surface (the contact probes extend about 1.6m below the footpads, but it takes a moment for the crew to react to the contact light) so the exhaust has quite a distance to disperse before reaching the surface soil. (On Apollo 14, however, per the annotated transcript at 108:15:12, Al Shepard left the engine firing all the way to the surface and for a couple of seconds after touchdown, so this doesn't apply to the above picture.)
answered Sep 2, 2019 at 16:47
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7$\begingroup$ Re "...lack of a massive blast crater...", one would think that anyone doing a halfway competent hoax would fake a rather dramatic crater, no? $\endgroup$– jamesqfSep 3, 2019 at 14:47
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2$\begingroup$ @jamesqf If you manage to fire a whole Saturn V rocket stack into the sky which is actually capable of lifting the gear to perform a lunar landing/return mission it's ok if you slack off on managing the details. After all Kubrick also got the shadows and lighting wrong in "2001: A Space Odyssey" .. $\endgroup$ Sep 3, 2019 at 17:44
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$\begingroup$ In addition, in a vacuum you don't get the dramatic tail of fire like in atmosphere. Without air pressure to keep it together it rapidly dissipates once it leaves the cone. If you watch a rocket launch you'll see the exhaust fan out as it increases in altitude and air pressure decreases. $\endgroup$– SchwernSep 3, 2019 at 23:28
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$\begingroup$ @Schwern Thanks, I have incorporated that. $\endgroup$ Sep 4, 2019 at 1:35
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$\begingroup$ @Schwern: Isn't there video of at least one of the ascent modules leaving the moon? Or of the descent burn taken from the command module? $\endgroup$– jamesqfSep 4, 2019 at 14:33
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There is a very nice Apollo 14 image, a combination of the two images 9254 and 9255:
From the Apollo 14 lunar surface journal, see.
Another Apollo 12 ALSJ image:
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4$\begingroup$ You might add a note that the dramatic crater in the first shot is outside the footprint of the LM, i.e. it is not an exhaust crater. $\endgroup$ Sep 3, 2019 at 17:06
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$\begingroup$ What do I see on the second picture? Is that one of the "feelers", broken off? $\endgroup$ Sep 3, 2019 at 18:16
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2$\begingroup$ @DavidTonhofer it is a contact probe, bent upwards, but still fixed to the footpad on the right image border. $\endgroup$– UweSep 3, 2019 at 20:48
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$\begingroup$ @RussellBorogove I am not shure about the first shot, "The area under, and slightly behind the engine bell shows evidence of disturbed soil resulting from the Descent Engine exhaust." But where is the evidence shown? $\endgroup$– UweSep 3, 2019 at 22:15
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$\begingroup$ About halfway between the bottom of the bell and the Réseau cross on the rim of the foreground crater, if I had to guess, where the ground is a little lighter colored. The foreground crater is in front of the nearest footpad, so that definitely isn't it. From the shadows, incidentally, your first picture must be almost 180º from the picture in my answer. $\endgroup$ Sep 3, 2019 at 22:19