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In November 2020, a SpaceX rocket suffered damage due to a problem with the launchpad, and future missions to the Moon and Mars could suffer similar problems since there won't be a launchpad at all.

During the Apollo missions, the lunar module landed and took off from the surface of the Moon. This likely projected regolith could have damaged the engines, somewhat like what happened to the SpaceX rocket. How did NASA manage this problem?

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3 Answers 3

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The lunar module had a descent and an ascent stage, each was equipped with a single engine. During descent and landing, the ascent engine was covered and protected against flying regolith by the descent stage.

enter image description here

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    $\begingroup$ Basically, the LM brought its own "launch pad" to the surface of the moon in the form of the descent stage. $\endgroup$
    – kgutwin
    Oct 11, 2021 at 16:49
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    $\begingroup$ apollo50kiosk.com/apollo/ascent.html - artist's rendition of an ascent stage at launch, next to footage of the actual thing. $\endgroup$
    – Mazura
    Oct 11, 2021 at 20:20
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    $\begingroup$ @Mazura it looks like there is a bit of debris that goes flying there at takeoff. But am I seeing it right, that the engine was only firing for a very short time? Could that be a relevant difference vs an Earth launch? $\endgroup$
    – craq
    Oct 12, 2021 at 6:13
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    $\begingroup$ @craq No, the engine is firing continously. The hot gas jet of the engine is invisible. At first when the jet touches the surface of descend and ascend stage, it separates hot particles from there. These glowing particles are visible, not the hot gas itself. $\endgroup$
    – Uwe
    Oct 12, 2021 at 7:09
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In practice, there simply wasn't a problem with flying regolith to begin with.

The Moon doesn't have an atmosphere. One implication of this -- and one that surprised a whole lot of people -- is that you don't get debris flying around when you fire a rocket engine. You get sheets of dust flowing radially away from the engine nozzle.

Of the Apollo missions, Apollo 11 had the most debris-prone landing, since Neil Armstrong left the engine running clear until touchdown. Neither Buzz Aldrin's inspection nor the post-flight analysis of photos of the lander makes any mention of debris damage to the descent stage. (In contrast, both of them call out thermal damage from various exhaust plumes impinging on the vehicle structure.)

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    $\begingroup$ Do you have any sources to confirm your assertions? For example " you don't get debris flying around when you fire a rocket engine" on the Moon. Some examples of the people who were surprised would be nice too. Your answer seems to focus on the landing - the question is about taking off. $\endgroup$ Oct 12, 2021 at 1:41
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    $\begingroup$ @OrganicMarble, I got it from reading the Apollo 11 mission report, and the Apollo 11 and Apollo 12 annotated transcripts, both of which call out the sheets of dust flowing away from the lander. There's not much difference between landing and taking off -- both involve firing rocket engines in close proximity to the ground. $\endgroup$
    – Mark
    Oct 12, 2021 at 1:50
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    $\begingroup$ Taking off had a "launch pad" - the descent stage. Landing didn't. That's a very large difference. $\endgroup$ Oct 12, 2021 at 1:52
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    $\begingroup$ @nick012000 That pebble that comes bouncing back is now driving into a headwind of exhaust gases and other dust particles, which might slow it somewhat... Also, this sounds a lot like Rutherford's scattering problem (a cosec^4 probability function) - not very likely. $\endgroup$ Oct 12, 2021 at 12:19
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    $\begingroup$ Indeed. On the Moon, there's no atmosphere to form the toroidal vortex that carries debris back toward a rocket launched from Earth. $\endgroup$
    – John Doty
    Oct 12, 2021 at 15:35
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Indeed NASA did bring a launchpad to the Moon; it's still there, together with further 5 launchpads of the other Apollo missions. They have been portrayed from orbit by Lunar Reconnaissance Orbiter.

Descent stage from orbit:

Descent stage from orbit

Ascent and descent stage coupled:

Ascent and descent stage coupled

Descent stage (=launchpad for ascent stage):

Descent stage (=launchpad for ascent stage)

Damage from regolite is a problem on the Moon: it was suspected, and then confirmed upon visually inspecting Surveyor 3 remains by Apollo 12's crew, which also brought back to Earth some Surveyor3 pieces for further analysis.

Ideal vs Real moon landing

NASA did suspect this could be a problem while landing: this is why the landing procedure requires ideally a "virtual landing" (=0 vertical speed) at 5.6 feet (1.7m) above surface: this is the length of "foot probes", three booms hanging from LEM landing gear, which detect surface proximity by physically touching it and triggering a "contact light" onboard. The commander is supposed to shut off the engine upon this ground contact, to avoid damage from regolite. Starting from 0 speed and accelerating at 1.62 m/s2, maximum terminal velocity on ground would be sqrt(2 * 1.62 * 1.7) = 2.3 m/s = 8.4 km/h

(Or you can also interpret this as "8.4 km/h will be added to the vertical speed at engine cutoff at 1.7 m altitude")

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