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Thread: Lunar Module Structural Strength

  1. #1
    I was just reading Clavius' arguments against Sibrel (http://www.clavius.org/bibsibrel.html) and I have a question...

    Here I quote from that page:
    "The lunar module was never tested on earth. It couldn't have been. Its engine was too weak to lift the spacecraft against earth gravity, and the lunar module's structure can barely hold up its own weight on earth."

    Ok, fine. But if the LM's structure could barely hold up its own weight, then how did it support itself during the Saturn V launch? Did it get extra support from being stowed within the SIVB?

    (I'm not proposing this as a conspiracy--far from it. I'm just curious...)

  2. #2
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    There's an easy answer to that question: I wrote the sentence in question and I should probably fix it because it's more hyperbole than is probably appropriate. The LM considered as a whole quite adequately supports its own dry weight in earth gravity. One's been doing just that for years in the Smithsonian.

    The technical answer is that the LM structure provided essentially two different support systems, one for Saturn V boost and one for managing its own thrust.

    Certain elements of the lunar module had to be protected from overloading while on earth. The landing struts, for example, would probably not have borne the load of a fully fueled LM in earth gravity. They would have buckled below the knee. But certain key elements of the structure -- the cabin frame, the docking apparatus, certain elements of the descent stage -- were sufficiently strong.

    If you look at how the LM was mounted inside the Saturn V -- primarily on four hardpoints -- it makes a bit more sense. The struts didn't have to take any load, except the "knee" struts. This is a different statics problem than if the LM's full weight were supported by the engine thrust structure, such as would have to happen in a hypothetical earth powered test.

    Under its own thrust the descent stage cantilevers. Under boost from the Saturn V, it's more of a pier-span system. But if you look at where the LM mass was concentrated during Saturn V boost, you realize that the hardpoints closely coincide with the heavy fuel tanks. They were more directly supported than in the cantilever arrangement.

    Similarly with the ascent stage. The heaviest parts were directly coupled to the descent stage, not far from the descent stage's own hardpoints. It cantilevers under its own thrust, and is directly supported during boost.

    It's more accurate to say that testing the LM in earth gravity would have required heavily modifying certain parts of the LM, and that would largely invalidate the test.

    But if I've conveyed the notion that the LM would collapse under earth gravity if you merely sneezed on it, then I've been to cutesy and not factual enough. Sorry about that.

    <font size=-1>[ This Message was edited by: JayUtah on 2002-05-15 14:24 ]</font>

  3. #3
    Thanks Jay,

    I figured the LM could stand up under Earth's gravity--I was just wondering how it could withstand the multiple g's it would have to sustain during launch. What you've said about passing the loads through hardpoints being different than the loads it would sustain using the descent engine makes sense.

  4. #4
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    Thanks for your patience. I like to write, and I like to find interesting ways of saying things. However, more than once I've found that in trying to be cute, I end up causing confusion.

    Obviously the LM can stand up under its own weight. They had to build the darn thing on earth, didn't they? It had to dock, and sustain inertial loads from MCC and LOI. It had to survive the Saturn V boost.

    Imagine a long pole. Lay it across your shoulders, with the midpoint behind your neck. That's a cantilever.

    Now get two big buckets and fill them with water, and hang one from each end of the pole. That's how a lot of people carry water. The loads balance, and the strength of the system depends on the strength of the support (you) and the shear (not sheer) strength of the pole. The expected failure mode is that the pole will snap at its midpoint.

    That's how the descent stage's structure behaves when its own engine is firing.

    Now go find a friend and have him hold up one end of the pole while you hold up the other end. The buckets are in the same places. You and your friend are each pretty much supporting the full load of the bucket nearest you.

    The key concept is that in this setup the beam (pole) strength is largely irrelevant. If the load was at the center of the beam, or distributed across it, it would matter more. But since the loads are concentrated at each end, very close to the supports, the load is better supported.

    The statics concept that applies here is "moment". Since the buckets are only inset from the support by a few inches, probably just enough to provide you a handhold on the beam, the length of the beam between load and fulcrum is very short. Thus the moment arm is very short, and the magnitude of the moment is very small, and the beam is not likely to react to the small moment by shearing.

    This is a highly simplified analogy to the LM structure.

    The descent stage structure was two sets of two parallel beams, arranged transversely (like a tic-tac-toe board), with the fuel tanks fastened to them outboard (edge and corner squares), and the engine contained within them inboard (center square). At each end of the beam pair (edge squares) is the hardpoint for connecting it to the Saturn V.

    How the ascent stage fits into this is left as an exercise for the reader.

  5. #5
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    How the ascent stage fits into this is left as an exercise for the reader.

    It fits onto the top of the descent stage, I think. [img]/phpBB/images/smiles/icon_smile.gif[/img]

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