Why HB's think the LM is top heavy

[raven]

The Moon Machines episode on the subject, though apparently it gets a few things wrong, is also worth watching.
I just wish the narrator didn't sound like he was always. about. to fall. a. sleep.

[Sus_pilot]

I've always liked the LM. Something so thoroughly optimized for its function has to be beautiful.

Form follows function, but then again I'm an engineer. Non-engineers may disagree, but they wouldn't have made a working lunar lander either.

Watching the episode Spider from the television series From Earth to the Moon gave me some real appreciation of what an outstanding feat of engineering the design and construction of the LM really was!

Also, that episode had the best humor of the series - did anyone notice how Kelly bounced the ball off the wall as he walks back into the plant?  Especially after the movie reference and the music from said movie started playing...

[smartcooky]

I've always liked the LM. Something so thoroughly optimized for its function has to be beautiful.

Form follows function, but then again I'm an engineer. Non-engineers may disagree, but they wouldn't have made a working lunar lander either.

Watching the episode Spider from the television series From Earth to the Moon gave me some real appreciation of what an outstanding feat of engineering the design and construction of the LM really was!

Also, that episode had the best humor of the series - did anyone notice how Kelly bounced the ball off the wall as he walks back into the plant?  Especially after the movie reference and the music from said movie started playing...

The Great Escape did not go unnoticed our household, though my youngest at the time thought it was a reference to The Simpsons...

[Noldi400]

It might also be that the LM looks top-heavy to theignorant uninitiated because they intuitively think that the center of pressure is at the back of the nozzle where the (invisible) flame comes out, instead of in the combustion chamber a meter or so farther forward.

It's the same old "I don't need to know anything about science; I have the evidence of my eyes" argument.

[cjameshuff]

It might also be that the LM looks top-heavy to theignorant uninitiated because they intuitively think that the center of pressure is at the back of the nozzle where the (invisible) flame comes out, instead of in the combustion chamber a meter or so farther forward.

It doesn't matter where the center of pressure is, as long as the direction of thrust is through the center of mass.

I think it's more just the common misunderstanding of rockets behind the pendulum fallacy...seeing the vehicle as being balanced on a rocket engine firing upward.

[Noldi400]

It might also be that the LM looks top-heavy to theignorant uninitiated because they intuitively think that the center of pressure is at the back of the nozzle where the (invisible) flame comes out, instead of in the combustion chamber a meter or so farther forward.

It doesn't matter where the center of pressure is, as long as the direction of thrust is through the center of mass.

I think it's more just the common misunderstanding of rockets behind the pendulum fallacy...seeing the vehicle as being balanced on a rocket engine firing upward.

True; I was speaking only of appearance.

Although it's hard to see how the short, squat LM "looks" more topheavy than a full Saturn V stack.  Or pretty much any rocket sitting on a pad, for that matter.

[JayUtah]

It doesn't matter where the center of pressure is, as long as the direction of thrust is through the center of mass.

True, but that's rarely the case.  And when it's not the case, the moment arm length dictates in part how tolerant the system can be.

[Echnaton]

It doesn't matter where the center of pressure is, as long as the direction of thrust is through the center of mass.

True, but that's rarely the case.  And when it's not the case, the moment arm length dictates in part how tolerant the system can be.

It strike me that a broader vehicle like the LM would be more tolerant to off center trust.  Is this correct?

[JayUtah]

It strike me that a broader vehicle like the LM would be more tolerant to off center trust.  Is this correct?

Generally yes; the vehicle's moment of inertia about the relevant axis is another factor that dictates the tolerance for error moments.  However, visually "broad" does not automatically translate to a higher moment of inertia.  That said, the LM had a suitably high moment of inertia.  Further, you want the moments of inertia in all three axes to be within a certain margin of each other, otherwise rotation along the minimum moment-of-inertia axis tends to migrate to another axis.

[ka9q]

It might also be that the LM looks top-heavy to theignorant uninitiated because they intuitively think that the center of pressure is at the back of the nozzle where the (invisible) flame comes out, instead of in the combustion chamber a meter or so farther forward.

The center of pressure is actually somewhere between those two points. There's pressure on every interior surface of the rocket, though it varies with position. The engine has rotational symmetry so the lateral components all cancel and can be ignored. Most of the components parallel with the thrust vector are forward, though there's a little in the reverse direction where the combustion chamber necks down to the throat.

But it doesn't really matter how the engine actually develops thrust, it simply applies a forward force through its mount on the vehicle, and I think that's what you meant. The effect on vehicle orientation depends on the moment arm between the mounting point and the vehicle and the vehicle's moments of inertia in the pitch and roll axes.

[ka9q]

Further, you want the moments of inertia in all three axes to be within a certain margin of each other, otherwise rotation along the minimum moment-of-inertia axis tends to migrate to another axis.

If I understand this stuff the moments of inertia wouldn't matter if the thrust were directly through the center of mass and the vehicle were perfectly rigid with no internal energy dissipation mechanisms. But real vehicles are never like this. Even if the vehicle itself is rigid, interesting modes can develop in the propellant flow. I've heard of roll moments introduced by swirling of the propellants as they flow out of the tanks and I suspect this could happen even in the gas flow in a solid fuel rocket. One reason for the baffles in liquid fuel tank sumps is to keep the propellants from swirling and setting up quite a bit of angular momentum.

[ka9q]

That said, the LM had a suitably high moment of inertia.

Which decreased enormously through the mission as the propellants were depleted and the descent stage was jettisoned. IIRC, they decreased by an order of magnitude between undocking and docking after ascent. Each RCS thruster remained at 100 pounds so vehicle response got pretty zippy. I think some crews compared it to a sports car.

I've had hoaxers claim that the movements in the 16mm films of rendezvous and docking are physically impossible. But when you take into account the actual film speed (6x real time), depleted ascent stage moments of inertia and the RCS engine thrusts, what we see is exactly what we should see.

Of course I have yet to meet a hoaxer who wasn't totally innumerate so this never seems to faze them. They never understand that claims like "it's too fast" are inherently quantitative and can be checked with math and well established physics. If they don't understand it, and they never do, it can't possibly be relevant.

[JayUtah]

Any of Marcel Sidi's books can show you the real nuts and bolts of spacecraft guidance and stabilization.  It's not for the faint-hearted, but it's the actual techniques we use.  And yes, things like fuel-slosh are accounted for and discussed.

[JayUtah]

I think some crews compared it to a sports car.

"It was ... sporty."  --Ed Mitchell

Most of them had the RCS in pulse mode in undocked flight.

If they don't understand it, and they never do, it can't possibly be relevant.

Yes, pretty much every hoaxer follows this path.