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.
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