23
$\begingroup$

In an answer to this question, the following photo was posted:

enter image description here

(source: NASA Apollo 11 Image Library)

This made me recall that the first thing Armstrong did after climbing down the ladder to the footpad is making sure he'd be able to get back up. When looking at the photo, it seems obvious to me that this might become an issue: the step is really high even if you're not wearing a bulky space suit.

Question: why did the ladder end so high above the surface? How did they assess if it was not too high?

Bonus question: what was the contingency plan if Armstrong had found out he couldn't get back up the ladder?

$\endgroup$
  • 9
    $\begingroup$ Regarding your bonus question, don't forget that the Moon has a significantly lower gravity than Earth. It'd be pretty easy to jump up from the lunar surface. $\endgroup$ – Infrisios Jul 29 at 9:06
  • 2
    $\begingroup$ 15 min explanation with lots of visualizations $\endgroup$ – Everyday Astronaut Jul 29 at 10:45
  • 5
    $\begingroup$ Bonus question: I guess the two astronauts would have helped each other out. Creepy to think about the Soviet plan to land a single person on the Moon. Although their ladder design was much more comfortable compared to that of the US :-) $\endgroup$ – Everyday Astronaut Jul 29 at 10:50
  • $\begingroup$ @EverydayAstronaut I will check that video after work :-) Looks like it answers the question perfectly (only checked the introduction). $\endgroup$ – Ludo Jul 29 at 12:36
  • 1
    $\begingroup$ I'm just thinking - why not just use a tree-house style rope ladder? Can be extended to any length, probably weighs considerably less, and can be packed away and stored inside the LM until needed. $\endgroup$ – Darrel Hoffman Jul 29 at 19:02
27
$\begingroup$

There were shock absorbing structures within the landing legs. If the primary shock absorbing strut would be maximally compressed by a hard landing, the lower end of the ladder would be directly above the foot pad.

enter image description here

enter image description here

enter image description here

PRIMARY STRUT The upper end of the primary strut is attached to the outboard end of the outrigger; the lower end has a ball joint for the footpad. The strut is of the piston-cylinder type; it absorbs the compression load of the lunar landing and supports the LM on the lunar surface. Compression loads are attenuated by a crushable aluminum-honeycomb cartridge in each strut. Maximum compression length of the primary strut is 32 inches. The aluminum honeycomb has the shock-absorbing capability of accepting one lunar landing. This may include one or two bounces of the LM, but after the full weight of the LM is on the gear, the shock-absorbing medium is expended. Use of compressible honeycomb cartridges eliminated the need for thick-walled, heavyweight, pneudraulic-type struts.

From APOLLO NEWS REFERENCE LUNAR MODULE QUICK REFERENCE DATA by Grumman.

102:19:19 Aldrin: Roger. AGS residuals: (X) minus 0.1, (Y) minus 0.2, (Z) minus 0.7 (feet per second). And we used the PGNS Noun 86 for Delta-VZ which was 9.5, versus yours which was 9.1, and I believe that may explain the difference (between the minus 0.7 residual in AGS versus minus 0.1 for the PGNS). Apogee 57.2, perilune 9.1; Sun check to three marks; Noun 20 minus Noun 22, plus 0.19, plus 0.16, plus 0.11. Over.

This quote from the Apollo 11 Lunar Surface Journal with the very low 3D landing speeds prove a remarkable soft landing.

The landing gear was designed for a much harder landing:

The landing gear can withstand: ( 1) a 10-foot/ second vertical velocity of the LM when the horizontal velocity is zero feet/second, (2) a 7-foot/second vertical velocity with a horizontal velocity not exceeding 4 feet/second, and (3) a vehicle attitude within 6° of the local horizontal when the rate of attitude change is 2°/second or less.

So Armstrong used only 7 % of the maximal vertical landing speed, that is less than 0.5 % of the maximal kinetic energy. This remarkable soft landing did not compress the honeycomb cartridges at all.

Here is a comparison of the vertical and horizontal landing speeds of Apollo 11, 12, 14 and 15:

enter image description here

$\endgroup$
  • 3
    $\begingroup$ Good info! Looking at the photo, it seems that the strut has barely compressed (going by the length of the astronaut's leg) - still a big jump! $\endgroup$ – Ludo Jul 29 at 12:37
  • 2
    $\begingroup$ So if Neil had slammed it down a little harder, he wouldn't have had so far to jump. $\endgroup$ – A. I. Breveleri Jul 29 at 16:23
  • 10
    $\begingroup$ @Ludo, the landing was softer than anticipated. The flight plan called for shutting off the descent engine as soon as the contact light came on, five feet above the ground, and the shock absorbers were designed to handle a fall of ten feet or more. Armstrong kept flying until the point of actual touchdown. $\endgroup$ – Mark Jul 29 at 20:26
  • 6
    $\begingroup$ The Apollo LEM pilots were steely eyed module men. $\endgroup$ – Chris B. Behrens Jul 29 at 21:36
  • 3
    $\begingroup$ @A.I.Breveleri: Pete Conrad remarked that for him, in fact, the distance down from the bottom rung was significant. Something along this lines of "That might have been a small one for Niel, but a big one for me!". $\endgroup$ – dotancohen Jul 30 at 7:15
8
$\begingroup$

The ladder was designed not to be in the way even in a hard landing. The legs are (as stated here in the thread) designed to absorb the energy of the landing by crushing the honeycomb structure within. Now two things happend: 1) the astronauts did touch down much softer than they had planned for and 2) the lunar soil itself absorbed more energy than had been anticipated. => No much shortening of the legs => the ladder is more above the ground. And on museum displays - no crushing at all...

Everyday Astronaut has a good video on this topic:

$\endgroup$
2
$\begingroup$

The distance between the foot-pad and the bottom of the outer cylinder represents the maximum amount of compression that the strut can accommodate to absorb landing impact energy. Since it uses a deformable solid material to absorb impact energy (aluminum honeycomb), there is no rebound as you would get from a more conventional (and heavier) type of strut. The amount it compresses will depend on the amount of energy it has to absorb, which will depend on the LM's velocity at contact. It was designed with the amount of compression length it has in order to accommodate a certain maximum touchdown velocity, bringing the LM to a stop within a certain g force limit. The ladder ends where it does so as not to interfere with the foot-pad (be impacted/damaged by it) if the strut should compress to its maximum designed amount. If the LM had made the hardest landing for which it was designed, the bottom rung of the ladder would be right at the footpad. Apollo 11 landed very gently, barely compressing the struts, leaving the large gap.

$\endgroup$
1
$\begingroup$

For the bonus question, I think I found the answer.

They brought with them the Lunar Equipment Conveyor, a nylon strap with a hook and two carabiners:

NASA photo S69-37994

(NASA photo S69-37994)

Its intended use was transferring equipment from the LEM down to the surface or back up, but it was also a backup for transferring from the LEM to the CSM if docking had failed:

The waist tethers were intended for use during an EVA transfer to the Command Module, had there been a problem with docking.

It's not hard to imagine they would have used it to hoist Armstrong back up had he found he was unable to jump high enough (despite the lower gravity) to get back up the ladder.

The procedures for EVA in fact explicitly required Armstrong to be tether with the LEC:

CDR EVA procedure excerpt

$\endgroup$
-8
$\begingroup$

The ladder stops at that point because base on their theories of the Big Bang, the scientist were expecting several feet of dust. The fact the dust was only an inch or so deep forced them to reevaluate the age of the moon. That is also why it was the first thing Neil discussed when stepping onto the moon.

$\endgroup$
  • 9
    $\begingroup$ Do you have any citations for this? This is fairly inaccurate according to my understanding. IIRC, this was a fairly minor view at the time, but significant enough that they planned for it in case it turned out to be wrong. $\endgroup$ – Charlie Kilian Jul 29 at 21:35
  • 13
    $\begingroup$ Robotic spacecraft had already landed without sinking and sent back photos of solid terrain during the Surveyor program, as had the Soviet Luna 9 probe; Apollo was definitely not expecting several feet of dust. $\endgroup$ – Russell Borogove Jul 29 at 21:52
  • 16
    $\begingroup$ What does the Big Bang have to do with lunar dust? $\endgroup$ – Organic Marble Jul 29 at 21:57
  • 3
    $\begingroup$ Within NASA, the "deep dust" theory only had one (very vocal) proponent. Almost everyone else expected that the dust would be mostly consolidated into a firm, load-bearing structure. $\endgroup$ – Mark Jul 29 at 23:04
  • 2
    $\begingroup$ @OrganicMarble It's the best evidence for young-earth-creationism in a long time ;-) $\endgroup$ – hdhondt Jul 30 at 5:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.