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.
Read more about this topic: Pendulum Rocket Fallacy
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