Pendulum Rocket Fallacy
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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.
Many Liquid fuel rockets constructed by the early pioneers of rocketry in the 1920s and 1930s differed significantly from modern rockets in that the rocket engine was placed at the top and the fuel tank at the bottom of the rocket. The first liquid-fueled rocket, constructed by Robert Goddard, was typical of this.
It was believed that, in flight, the rocket would "hang" from the engine like a pendulum hanging from a pivot. The weight of the fuel tank would keep the rocket flying straight up as long as the fuel lasted. However, this belief is incorrect - such a rocket will never fly in a straight line and will always turn and crash into the ground soon after launch. This is what happened to Goddard's rocket.
The pendulum belief is a fallacy because (aside from engine thrust) a rocket in flight is in a state of free-fall. This is true regardless of whether the rocket is going up or down - if the air resistance is ignored, the only forces acting on the rocket are gravity and the thrust of its engine.
Consider a rocket like Goddard's that has become slightly misaligned from its vertical trajectory. It requires a rotational force or torque to turn it back onto its correct path. The engine is fixed to the frame of the rocket so it can't provide any sort of steering torque; the only other force available is gravity. If gravity is to steer the rocket, it must pull harder on the heavier bottom part of the rocket than it pulls on the lighter top part. However, it was shown by Galileo, both theoretically and experimentally, that all objects are affected equally by gravity, regardless of mass. Gravity will affect both ends of the rocket equally and can't provide any sort of torque. Therefore, there are no forces available to the rocket that will act to steer it.
No rocket can be perfectly constructed. Inevitably, the engine's direction of thrust will be imperfectly aligned with the rocket's axis so the rocket will have a slight inbuilt tendency to turn. As there is nothing to correct this, such a rocket will always turn around and strike the ground.
To fly correctly, rockets must have another means of stability. The fins of model rockets and the sticks of firework rockets act aerodynamically to keep the axis of the rocket pointing in the direction of flight. Larger rockets can do without fins by using sophisticated guidance and control systems that actively steer the rocket and keep it flying in the intended direction.
Even a Goddard-type rocket, with the engine at the front, will fly correctly if fitted with fins or another means of control. An example of this is the Launch Escape Systems fitted to some crewed spacecraft. These are aerodynamically stable; indeed, in the case of the Apollo spacecraft, engineers had to fit several hundred kilos of Depleted uranium ballast to the nose of the escape rocket in order to make sure this was so.
