# Could the lunar module have moved to a different spot?

Suppose Neil Armstrong had landed with plenty of spare fuel, only to decide it really wasn't a good spot after all. Could he have lifted back off and landed again, perhaps a couple of hundred feet away? I just confirmed that the engine was restartable (they had to do that for Apollo 13, I believe). But are there reasons it wouldn't have worked? Was it even discussed?

A possible scenario where this would be needed is realizing that you have landed on a thin crust above a hollow, and are starting to break through. Not very likely, I realize.

In addition to the "single-serving" mechanical features that Uwe's answer describes, the guidance programming for the LM's onboard computer doesn't support any ascent flight on the descent stage.

In particular this means you'd have to go to the manual throttle mode, P67, to lift off, then switch back to P66 to land again. The switch to P66 would have to be done while the spacecraft was descending, not ascending (I don't have a citation, but I've been told that the LM would attempt to turn upside down if it was moving upward in P66). Here's a nice overview of the Apollo LM guidance programs.

Getting off the surface, to an altitude and velocity safe enough to switch guidance programs, then landing again, with whatever propellant was remaining after the initial landing would be extremely risky, and it was never considered or trained for -- but I believe it would have been physically possible.

I disagree with Uwe that the contact probes and landing gear "crush cores" would be needed to land.

The probes were actually intended to keep the engine from being shut off too late, not too early; there was concern that running the engine all the way to touchdown would risk kicking surface material up into the spacecraft and causing damage, so the mission plan called for shutting off the engine a few feet above the surface. On A11 in particular, Armstrong didn't consciously react to Aldrin's "contact light" call, and ran the engine all the way to touchdown, or very close to it, and no particular problems occurred as a result:

Aldrin: "I called contact light."

Armstrong: "I'm sure you did, but I didn't hear it, nor did I see it. I heard you say something about contact, and I was spring loaded to the stop engine position, but I really don't know whether we had actually touched prior to contact or whether the engine off signal was before contact. In any case, the engine shutdown was not very high above the surface."

Lifting off and landing again would have been mechanically fine, assuming that the initial touchdown hadn't done any damage to the engine bell. Without the probes, the crew would rely on the radar altimeter and the physical impact of the footpads on the surface; it would be another very soft landing.

As a result of Apollo 11's soft touchdown, the crush cores in the landing gear were barely affected on the landing. This is the reason the LM crew ladder ended so high above the footpad -- it was designed to stay just above the surface if the entire shock absorber core was collapsed.

In a situation such as your "breaking crust" hypothetical, or the LM starting to tip over, or any other situation that would call for an urgent liftoff, the done thing would be to stage the LM, lift off with the ascent stage, and abort the mission.

• "I've been told that the LM would attempt to turn upside down if it was moving upward in P66." I know, it's so frustrating when that happens in Kerbal Space Program. :D – Ilmari Karonen Mar 15 '20 at 21:07
• I suspect that in any circumstance that might call for moving the LM, the response would have been an abort to orbit, dropping the descent stage and using the ascent module. Trying to move the LM on the fly would have been going way off plan. – Howard Miller Mar 15 '20 at 23:29
• @HowardMiller That's the upshot of my third and last paragraphs. – Russell Borogove Mar 15 '20 at 23:45
• These were two very good answers (that is, Uwe's answer and this one, in case some other answers come along). Together I think they are pretty complete. I accepted this one since it sounds like the correct answer is "yes, it is barely possible, but would never have been done." – Mark Foskey Mar 16 '20 at 16:55

The Lunar Module was designed for a single landing.

The contact sensors under the footpads of the landing gear did bend and stick out sideways during a landing. They could not be reused for another landing. The contact sensors were needed to prevent stopping the engine too early. The distance between footpads and ground could not be seen through the LM windows.

See this related question.

The shock absorbers within the landing struts were made using a honeycomb structure. The landing shock did compress them permanently. Shock absorbers of the reusable pneudraulic type would have been much heavier than the single use aluminum honycombs. Two very soft landings might be possible, but without the contact sensors, the second landing could be too hard.

See this related question.

Armstrong did a very soft landing, two such landings seem tolerable to the shock absorbers, but not two harder landings like Apollo 15.

More details about the landing gear in this Pdf.

• I don't think the guidance system was made to do it either. – Organic Marble Mar 14 '20 at 22:14
• @OrganicMarble They might be able to do it just in ATT HOLD mode, but I agree, they would have to do it largely without guidance support. – Polygnome Mar 15 '20 at 7:38

# The descent engines of Apollo 5, 9, and 13 were re-lit multiple times

You don't need to run a descent program on the AGC to fire the descent engines. It was fired manually during Apollo 9 and 13, and even remotely from Mission Control on Apollo 5.

Apollo 5 was the first spaceflight with a lunar module, an unmanned mission in low Earth orbit to test the LM. It fired its descent engines three different times:

After checking out the spacecraft for two revolutions, ground control signaled the descent engine to fire for 38 seconds. [...] The descent engine was fired twice more (once for a full 33 seconds). There were two ascent engine firings, one for the fire-in-the-hole abort maneuver.

Chariots for Apollo, chapter 10-3 "Apollo 5: The Lunar Module's Debut"

Apollo 9 was the first manned flight with a lunar module.

• On the third day, the descent engine was tested while the LM was still docked to the CSM:

As soon as he was sure the systems were operating properly, McDivitt asked Scott to put the command module into neutral control, so he could check out the lunar module's steering system. McDivitt then operated the small thrusters to get the docked vehicles into the correct position for firing the lunar module's throttleable descent propulsion system. Seconds after starting the large descent engine, McDivitt shouted, "Look at that [attitude] ball; my God, we hardly have any errors." Twenty-six seconds later, at full thrust, he reported that errors were still practically nonexistent. In fact, things were going so smoothly that halfway through the 371.5-second exercise, the commander felt hungry - not an uncommon sensation with him.

Chariots for Apollo, chapter 12-5 "Apollo 9: Earth Orbital trials"

• On the fifth day, the LM undocked from the CSM. The descent engine was fired twice to increase the separation of the spacecraft:

After drifting around within 4 kilometers of the command module for 45 minutes, McDivitt fired the lunar module's descent propulsion engine to increase the distance to nearly 23 kilometers. The motor was smooth until it achieved 10-percent thrust. When McDivitt advanced the throttle to 20 percent, the engine chugged noisily. McDivitt stopped throttling and waited. Within seconds, the chugging stopped. He accelerated to 40 percent before shutting down and had no more problems. McDivitt and Schweickart checked the systems and fired the descent engine again, to a 10-percent throttle setting; this time it ran evenly.

• and once again to bring the two spacecraft closer:

To begin the rendezvous, McDivitt and Schweickart flipped their craft over and fired the thrusters against the flight path to slow their speed enough to drop below the command module's orbital path. Below and behind the command module, they would begin to catch up. They fired the pyrotechnics to dump the descent stage and leave it behind.

Apollo 13 fired the descent engine manually three times: the second midcourse correction at 61:29:43, Earth orbit injection at 79:27:39, and the third midcourse correction at 105:18:28. (The first midcourse correction was done by the service module, before the explosion.)

# However, it's better to avoid such a maneuver

• My very first thought reading the question title (had not even been to the question page) was that the landing gear contact sensors cannot be reset. This is noted in Uwe's answer.

• Uwe is also correct that the shock absorbers in the landing struts are one-use honeycomb.

• Either you succeed in landing (so why would you need to try again?) or you fail (which probably would damage the LEM enough to prevent or discourage another attempt).

• Imperfect landings were tolerated. Apollo 14 was tilted 7 degrees to the starboard, so much that the crew could not sleep because they were constantly afraid that the LM was tipping over!

The lunar module crewmen received little, if any, sleep between their two extravehicular activity periods. The lack of an adequate place to rest the head, discomfort of the pressure suit, and a 7-degree starboard list of the lunar module on the lunar terrain were believed responsible for the lack of sleep. The crewmen looked out the window several times during the sleep period for reassurance that the lunar module was not starting to tip over.

Apollo Program Summary Report, p. 8-7

• I've stated before that once you pop the valves on the pressurization system for the LEM, its reliability decreases with time. As seen in the quotes above, Apollo 9's descent engine made an abnormal "chugging" sound after being restarted after two days. And Apollo 13 also had problems with pressurization:

With the exception of supercritical helium system performance, descent propulsion system operation, including engine starts and throttle response, was normal.

Apollo 13 Mission Report

• If you don't run a guidance program, what keeps the ship from toppling over in flight? Where does attitude control come from? Don't think anyone suggested restarting the engine was an issue. It even says so in the question. – Organic Marble Mar 17 '20 at 12:07
• @OrganicMarble: Attitude would be manually controlled, using the rotational hand controller. I'm not saying it's a prudent idea, just that it is theoretically possible. – DrSheldon Mar 17 '20 at 12:18
• I'm not a GNC expert so I'm going to ask a followup on this. – Organic Marble Mar 17 '20 at 12:27