Pendulum fallacy - hovering rockets, still applies?

[michaeldk]

Hi all!

I've come here to seek your expertise because I've ran into a bit of a heated discussion (well, heated from the other side ;-) about rockets, hovering and center of gravity.

Basically people are referencing to the pendulum fallacy when I say that a rocket which is hovering would be better off having a payload hang below the rocket, rather than having a payload at the same level or above the thrusters. I am by no means an expert, not even something distantly related to one! So it might well be that I am wrong, but everything that I do know about helicopters and general physics seems to indicate that having the payload hanging below the thrusters when hovering should be more stable.

Can anyone comment on this? I'd be greatly helped! I don't mind being proven wrong at all, but certainly would enjoy it of course if the logic I applied in the end made sense :)

- Michael

 

[Filip Larsen]

Welcome to PF!

In short, since the direction of the force from the rocket engine is "rigidly" attached to and turning with the rocket body it does not in itself give any additional stability to mount the rocket engine at the top.

I recommend that you search for "rocket pendulum fallacy" and then return with specific questions if you come up empty handed or if the explanations you find do not make sense to you.

 

[michaeldk]

Welcome to PF!

In short, since the direction of the force from the rocket engine is "rigidly" attached to and turning with the rocket body it does not in itself give any additional stability to mount the rocket engine at the top.

I recommend that search for "rocket pendulum fallacy" and then return with specific questions if you come up empty handed or if the explanations you find do not make sense to you.

Filip, I did not specifically think of a rigidly or non-fixed configuration. I was actually referring to a situation where a payload would be attached in some way to a rocket-stabilized platform (with the intent to safely deliver the paypload to the terrain below). E.g. similar to how a helicopter would deliver a payload.

To be precise, I was talking (in the original discussion) about the latest schiaparelli layout versus a sky crane configuration as used by the NASA.

 

[Filip Larsen]

Well, the pendulum fallacy applies to rockets and other rigid objects with thrusts and not to applications like the Sky Crane technique where the payload really is hanging in wires that obviously have to be below the body doing the lifting.

If this does not help, perhaps you can find some specific questions or arguments from that discussion you had?

 

[Mech_Engineer]

The thing about a simple pendulum subject to gravity is the force is always acting in the same direction, and is independent of the location of the system's C.G.; this is what makes it an inherently stable system. A rocket's force vector changes during flight, and so regardless of the thrust location a control system is required to maintain flight in a particular direction. Once a rocket is up to speed, the fins provide a level of stability as well.

See here: https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

It was believed that, in flight, the rocket would "hang" from the engine like a pendulum from a pivot, and the weight of the fuel tank would be all that was needed to keep the rocket flying straight up. This belief is incorrect. In actuality, the stability of such a rocket is dependent on other factors. Basic Newtonian mechanics shows that Goddard's rocket is just as stable (or unstable) as it would be if the engine had been mounted below the fuel tank (as it is in most modern rockets).[1]

Reference in Wikipedia Article: Jim Bowery, the Pendulum Rocket Fallacy

One of the two main places where intuition goes awry is in forgetting that a rocket engine is rigidly connected to the rest of the vehicle, so the engine's support of the vehicle changes direction along with the center of mass.

 

[michaeldk]

The thing about a simple pendulum subject to gravity is the force is always acting in the same direction, and is independent of the location of the system's C.G.; this is what makes it an inherently stable system. A rocket's force vector changes during flight, and so regardless of the thrust location a control system is required to maintain flight in a particular direction. Once a rocket is up to speed, the fins provide a level of stability as well.

See here: https://en.wikipedia.org/wiki/Pendulum_rocket_fallacyReference in Wikipedia Article: Jim Bowery, the Pendulum Rocket Fallacy

I have obviously read the article and read a lot more about it. However 99.999% of the cases what is discussed is a rocket in flight, not a rocket that is hovering or descending. And when we look at hovering vehicles suddenly the center of gravity does matter for stability, and I wonder why with rockets that wouldn't be the case. Thanks for your reply anyway!

 

[Mech_Engineer]

What matters is the direction of the thrust vector w.r.t. the rocket's center of gravity. If the rocket's CG does not lie perfectly on a line defined by the thrust vector, there will be a small moment applied which will spin the rocket. This moment is never "naturally balanced" by the thrust vector; if the thrust is not vectored to compensate for angular deviation, the rocket will continue to spin and eventually crash.

In some sense, it is akin to balancing a pendulum from underneath; it can be done, but requires a control system since it is not an inherently balanced system. Sure in theory a pendulum balanced from underneath should be able to stay in place, but in reality it can never be "perfectly" balanced and so will always spin downward. Similarly regardless of the rocket's CG location, if the thrust vector's line of action does not perfectly correspond with the CG, residual forces from the thrust vector will spin the craft.

 

[Baluncore]

During WW2 the DMWD in Britain developed a rocket based landing system.
https://en.wikipedia.org/wiki/Hajile