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The pendulum rocket fallacy is a common fundamental misunderstanding of the mechanics of rocket flight and how rockets remain on a stable trajectory.
 
Gimblized stability ?
Strike two. (Hint #3: Look at where the motor nozzle is located on the rocket. Anything unusual about that? If you don't know where the motor nozzle is, look up the rocket - called Goddard's first liquid-fueled rocket of March 24, 1926 - on Wikipedia or some other source.)

Mark K.
 
It was believed that, in flight, the rocket would "hang" from the engine like a pendulum hanging from a pivot, and the weight of the fuel tank would be all that is needed to keep the rocket flying straight up. However, this belief is incorrect – such a rocket will turn and crash into the ground soon after launch. This is what happened to Goddard's rocket. In fact it can be shown using basic Newtonian mechanics that the rocket is exactly as unstable as when the rocket engine is mounted under the fuel tank, as is the case in most modern rockets.
 
Gravity*
The center of gravity is identical to the center of mass[2] and therefore gravity does not exert any torque. This is a general property of all systems in a uniform gravitational field.
Reaction force from the engine*
Due to the rigid construction of the rocket frame, the force is exerted on a line that is fixed with respect to the rocket. The unavoidable imperfection mentioned above means that this line does not contain the center of mass exactly. The amplitude of the reaction force depends on the thrust of the engine, which is always positive. Therefore, the torque is exerted with respect to an axis whose direction is fixed with respect to the rocket frame, and is of constant sign.
Given that torques are pseudo-vectors and hence add linearly, it follows that the rotation speed of the rocket around the aforementioned axis can only increase.
 
In plain (and simplified) terms, the pendulum theory assumes that gravity will always pull the weighted aft end of the rocket down toward the ground. This will bring the motor back up to a vertical orientation and insure that its line of thrust is perpendicular to the ground (so that the rocket goes straight up). The problem is that if the rocket motor's thrust is powerful enough to lift the rocket up against gravity and launch it into the air, then it is also strong enough to overcome the supposed stabilizing effect of gravity. Also, even if the rocket motor remains in an absolutely vertical orientation, its line of thrust will vary a little bit from perpendicular to the ground. That variance (or misalignment) will cause the thrust to "push" the rocket over. Since the thrust is stronger than the pull of gravity, it can easily overcome any pendulum effect and will keep forcing (torquing) the entire rocket over and turning the line of flight away from vertical until the rocket's trajectory eventually intersects the ground.

The rocket needs some other stabilizing mechanism because the pull of gravity will never be sufficient to keep the rocket aligned vertically in relation to the ground.

The mystery for me is how Jim Flis was able to tweak his scale version of it so that it would be stable without any fins. He launched Tony Vincent's Nell on the final day of NARCON from a scale version of Goddard's stand, apparently without any launch rod, and despite the gusty crosswinds, it went up straight without a hint of weathercocking. (The wind was strong enough to blow the stand over just as the count reached "2" during the first launch attempt.)

Mark K.
 
Mark

The basic criteria for stability it that the CG be ahead of the CP.

In the Fliskits Nell this is fairly easy because the motor is at the forward end of the focket and the aft end is draggy. This is the same type of stability you have with a bottle rocket, or simply a rocket motor on a stick.

Bob
 
Has more to do with flex of the connecting tube frame then the pendulum action.
My 1/3 scale Nell used lexan rod and tube which proved to be just a tad to limber for the model to remain stable during it's first flight at Goddard Space Flight Center in greenbelt on an Apogee 10.5mm B2-4 motor. By stiffenening the frame the CP/CG relation is enough to keep the model going in the Up direction now. That First flight was truely a Goddardesk flight though with same trajectory and "scale distance" :)
No ScrapDaddy not a mystery of history LOL!!!!

GoddardRocketLaunch-g_3rd scale Launch Pg_3-18-01.jpg
 
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Mark

The basic criteria for stability it that the CG be ahead of the CP.

In the Fliskits Nell this is fairly easy because the motor is at the forward end of the focket and the aft end is draggy. This is the same type of stability you have with a bottle rocket, or simply a rocket motor on a stick.

Bob
Yes, obviously, but somehow that principle didn't work for Dr. Goddard in March, 1926.

Mark K.
 
Yes, obviously, but somehow that principle didn't work for Dr. Goddard in March, 1926.

Mark K.

That's because the fuel was in the back in Goddard's, making the back end of his rocket much heavier than the back end of any model. This moved the cg back far enough to cause it to be very marginal on the stability.
 
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