The Lunar Lander and the Pendulum Fallacy

[skeptic]

MeteorWayne,

That is a very interesting video on your page of the Mock Lunar Lander Success. There is a discussion on another forum about whether the pendulum fallacy applies to a lunar lander scenario.

The pendulum fallacy has to do with the belief held by Robert Goddard and other scientists that the nozzle of the rocket should be at the top with the rest of the rocket dangling beneath. The belief was that the body of the rocket, dangling below the nozzle like a pendulum, will tend to keep the nose pointed upwards. This is false because to an accelerating rocket, gravity is generally one of the lesser forces the rocket experiences and generally doesn’t have enough effect to offset other unstabilizing forces.

However in the case of a lunar lander such as in the video which is either motionless or moving slowly either upwards or downwards, might not having the center of mass below the nozzle have a stabilizing effect on the lunar lander? Obviously in the video, that lunar lander has the center of mass well above the nozzle and there must be a fairly sophisticated guidance system on board to keep the nozzle pointed down. In fact one can see the nozzle constantly moving to keep the lander stabile.

I put it to the group whether the lunar lander scenario is an exception to the pendulum fallacy and whether having the center of mass below the thruster would tend to stabilize or destabilize the lander.

 

[MeteorWayne]

I haven't the slightest clue as to what you are talking about, since I don't have a page, And I have no idea of what you mean by the "pendulum effect"

Since descent/ascent engines are under active control to keep the thrust through the center of gravity, it makes no sense.

 

[skeptic]

http://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

 

[MeteorWayne]

I am still mystified what the heck that has to do with me!
And equally mystified what the point of the topic is.

 

[skeptic]

I am still mystified what the heck that has to do with me!
And equally mystified what the point of the topic is.

I'm sorry MeteorWayne, the first place I saw the video was on your page and I thought that perhaps you had a special interest in it. I see now it's at the bottom of all the pages, so it really has nothing to do with you.

The point of the topic is that although the first rule of rocketry is that the center of mass must be higher than the center of pressure or the rocket will be unstable, the case of the lunar lander may be an exception. Goddard believed that the weight of the rocket would serve to stabilize the rocket without using fins, if the weight was below the thruster. He was wrong because the thrust of the rocket so overpowers the effect of gravity that the stabilizing effect is negligible.

However with a lunar lander of the type shown in the video and without significant acceleration, the situation is different. The lunar lander can be thought of as being suspended from the thruster similar to what Goddard believed. If the center of mass is below the thruster and symmetrical around the vertical axis, it may tend to keep the thruster aligned to the vertical axis without an active controller. If true, the designers of the lunar lander in the video might have been able to simplify their active controller by just shifting some of the weight downwards.

I was hoping members of this group could offer their opinions as to whether a center of mass below the thruster would tend to stabilize or destabilize a lunar lander and why.

 

[kelvinzero]

Hi skeptic,

The pendulum fallacy is not to do with the relative strength of the rocket. It is more fundamental.

That link you provided actually gives some good explanations of this.

The reason it does not work is that the nozzle will twist if the rocket twists, the force it exerts will always be along the axis, unlike a pendulum where the force on the top is always opposing gravity.

 

[drwayne]

Tangent:

I am surprised at times the number fo folks that trot out the simple (though incorrect) image of
a finger and a pencil in discussions such as this.

End Tangent