Author Topic: Flimsy LM  (Read 620 times)

Offline Dandy

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Flimsy LM
« on: July 17, 2019, 06:37:06 PM »
I keep reading (and hearing) that the LM is so flimsy. I know it's flimsy but I also read somewhere that it could withstand a drop to the Lunar surface from 50 feet if the crew had run out of fuel. Where did I get that from? I do not remember but I'm pretty sure it was in regards to Neil running low on fuel. A book or program on television, who knows... Now I know the legs were designed to collapse and absorb some impact but haven't been able to find any sort of "drop tests" information which is what I'm looking for. I have been thumbing through Chariots for Apollo looking for anything regarding that kind of test. I could swear I read something regarding the collapse feature.

Offline Allan F

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Re: Flimsy LM
« Reply #1 on: July 18, 2019, 01:38:16 AM »
Not 50 feet. 10 feet, maybe. It was designed to drop from the height of the extended probes - around 5 feet. The pressure-bearing skin of the LM was thicker than the pressure-bearing skin of a commercial airliner, but had to contain less pressure-differential. Look at a sodacan. It will comfortably hold around 50-60 psi for ever. The LM had to hold just about 4 psi. The problem with the LM was that it had to be launched into orbit on the Saturn V, and that was a rough ride. So the skin was supported by rigid stringers quite close to eachother.

Yes, you could punch a screwdriver through it, but you can do that to the wing of a car made of steel too. Solution: Don't have loose screwdrivers around.
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Online Zakalwe

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Re: Flimsy LM
« Reply #2 on: July 18, 2019, 07:14:08 AM »
I keep reading (and hearing) that the LM is so flimsy. I know it's flimsy but I also read somewhere that it could withstand a drop to the Lunar surface from 50 feet if the crew had run out of fuel. Where did I get that from? I do not remember but I'm pretty sure it was in regards to Neil running low on fuel. A book or program on television, who knows... Now I know the legs were designed to collapse and absorb some impact but haven't been able to find any sort of "drop tests" information which is what I'm looking for. I have been thumbing through Chariots for Apollo looking for anything regarding that kind of test. I could swear I read something regarding the collapse feature.

Page 28 of this document shows the capabilities for the leg structures.
mirror.heroicrelics.org/www.cs.indiana.edu/sudoc/image_30000061709352/30000061709352/pdf/lmnr2.pdf#page=9

The NASA Technical Report TN D-6850 details the development of the legs
https://ntrs.nasa.gov/search.jsp?R=19720018253
Page 34 onwards details the drop testing that was completed.
"The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.' " - Isaac Asimov

Online Zakalwe

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Re: Flimsy LM
« Reply #3 on: July 18, 2019, 08:16:06 AM »
This document (page 5 onwards) describes the drop tests that were carried out on the legs.
http://www.lpi.usra.edu/lunar/documents/SP-2013-605.pdf

Page 6 describes the parameters within which the legs were designed for.

Also "The landing gear system energy absorption capability was more than adequate to absorb the touchdown kinetic and potential energies. The maximum energy absorption capability of the landing gear system was 162,000 foot-pounds compared to the touchdown kinetic energy of the Apollo 11 Lunar Module of 1960 foot-pounds." (page12)
"The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.' " - Isaac Asimov

Offline Peter B

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Re: Flimsy LM
« Reply #4 on: July 18, 2019, 09:25:43 AM »
This document (page 5 onwards) describes the drop tests that were carried out on the legs.
http://www.lpi.usra.edu/lunar/documents/SP-2013-605.pdf

Page 6 describes the parameters within which the legs were designed for.

Also "The landing gear system energy absorption capability was more than adequate to absorb the touchdown kinetic and potential energies. The maximum energy absorption capability of the landing gear system was 162,000 foot-pounds compared to the touchdown kinetic energy of the Apollo 11 Lunar Module of 1960 foot-pounds." (page12)

I figured that Apollo 11 must have had the most benign landing, being a light earlier series LM, and the descent engine continuing to fire until touchdown. Therefore the later landings must have pushed the envelope at least a bit.

So I read through the article to see what it said about the others. It turned out Apollo 15 had the most energetic landing - heavier, higher speed and onto the steepest slope any mission landed on. But even then apparently the touchdown kinetic energy of Apollo 15's landing was only 70% higher than for Apollo 11...so still way inside the limits of what the LM's landing gear was designed for.

Online Zakalwe

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Re: Flimsy LM
« Reply #5 on: July 18, 2019, 12:11:38 PM »
This document (page 5 onwards) describes the drop tests that were carried out on the legs.
http://www.lpi.usra.edu/lunar/documents/SP-2013-605.pdf

Page 6 describes the parameters within which the legs were designed for.

Also "The landing gear system energy absorption capability was more than adequate to absorb the touchdown kinetic and potential energies. The maximum energy absorption capability of the landing gear system was 162,000 foot-pounds compared to the touchdown kinetic energy of the Apollo 11 Lunar Module of 1960 foot-pounds." (page12)

I figured that Apollo 11 must have had the most benign landing, being a light earlier series LM, and the descent engine continuing to fire until touchdown. Therefore the later landings must have pushed the envelope at least a bit.

So I read through the article to see what it said about the others. It turned out Apollo 15 had the most energetic landing - heavier, higher speed and onto the steepest slope any mission landed on. But even then apparently the touchdown kinetic energy of Apollo 15's landing was only 70% higher than for Apollo 11...so still way inside the limits of what the LM's landing gear was designed for.

It's an interesting read, isn't it?

It also throws a lot of light on the "flimsy LM". It's not that it is flimsy, but that it was designed perfectly for it's environment (high vacuum, low-G) with suitable tolerances and overheads. As per the quote that i extracted from the report, the legs were built to withstand nearly two orders of magnitude more energy than A11 exposed them to.
"The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.' " - Isaac Asimov

Online Zakalwe

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Re: Flimsy LM
« Reply #6 on: July 18, 2019, 12:34:35 PM »
I also read somewhere that it could withstand a drop to the Lunar surface from 50 feet if the crew had run out of fuel.

I'm thinking out loud here.

Lunar gravity is 1.625 meters per second squared. That's 5.331 feet per second squared (damn the Imperial measurement system!).
The LM legs were designed to withstand a maximum of 10 feet per second with zero lateral velocity.

Lets assume a zero vertical speed at the point of engine cut-off. A fall from 50 feet would then give a vertical speed at impact of 23.09 feet per second, after a 4.3 second free-fall, which is over double what the leg structure was designed to withstand.

In Lunar gravity, a 10 feet per second velocity would result after just under 2 seconds of free-fall from a height of 9.4 feet.

So, no, the LM would not survive intact a fall from 50 feet.
"The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.' " - Isaac Asimov

Offline Dandy

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Re: Flimsy LM
« Reply #7 on: July 18, 2019, 04:02:00 PM »
wow thanks guys! That's exactly what I was looking for. I just read through the entire report. I wish I could locate the original photos of the development of each component that made up the LM. They sure love honeycomb! So many applications in the Apollo project used it.

Online Zakalwe

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Re: Flimsy LM
« Reply #8 on: July 19, 2019, 10:41:17 AM »
They sure love honeycomb! So many applications in the Apollo project used it.

Again, it was to a refined engineering decision to find the best solution for a particular problem. I believe that the very initial design had hydraulic dampers in the legs, as that was a familiar, commonplace solution (think car suspension, landing gear on aircraft and so on). Further thinking made them realise that the landing was a one-shot affair and there was no need for hydraulics (plus hydraulics were complicated and heavy). A single-use compressible core fulfilled the needs perfectrley and was the best compromise for the problem.
"The strain of anti-intellectualism has been a constant thread winding its way through our political and cultural life, nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge.' " - Isaac Asimov