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Pendulum rocket fallacy
The pendulum rocket fallacy is a common fundamental misunderstanding of the mechanics of rocket flight and how rockets remain on a stable trajectory. The first liquid-fuel rocket, constructed by Robert Goddard in 1926, differed significantly from modern rockets in that the rocket engine was at the top and the fuel tank at the bottom of the rocket. 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—such a rocket will turn and crash into the ground soon after launch, and this is what happened to Goddard's rocket. Use of basic Newtonian mechanics shows that Goddard's rocket is just as unstable as when the engine is mounted below the fuel tank, as in most modern rockets.
==Practical explanation==
No rocket can be constructed perfectly. Inevitably, the engine's direction of thrust will not be perfectly aligned with the rocket's center of mass, so the rocket will have an inbuilt tendency to turn. When this happens, the engine starts rotating with the rest of the rocket (regardless of its shape) and the direction of thrust rotates as well. Except for air resistance, there is no rotational force or torque available to turn a simple aerodynamic rocket back onto its correct path, as can be shown from the classical Newtonian physics reasoning in the next section. Consequently, the initial deviation from a vertical path will increase over time, and a rocket constructed in this way will always turn around and strike the ground eventually.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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