How does a rocket take off?

Answer

Suppose you put a cannon on a cart, a cannon of 1000 kilograms, with the barrel pointing backwards. Then you shoot a bullet of 1 kilogram at a speed of 500 km/h. What will happen then? You will see that “because of the impact” the cannon also moves forward. Not as fast as the bullet that flies backwards, of course, because that cannon is 1000 times heavier.

If you measured the speed of the bullet and the cannon, you would see that the bullet is actually flying backwards at 499.5 km/h, and the cannon is flying forward at 0.5 km/h! Because the gun is 1000 times heavier, it also moves 1000 times slower due to recoil.

Now that’s what happens with a rocket. In the engine, fuel is burned and converted into very hot gases. They have only one way out: the rocket nozzle. They are thus “fired” backwards along that exit at a very high speed, of a few thousand km/h. Just as the cannon moves forward when firing the bullet, now the missile will also move forward when “shooting” the hot gases behind. The gas molecules are, as it were, all small cannon balls that are fired from behind.
Of course, that rocket weighs much heavier than those gases, which is why they must also have such a high speed.

But because the rocket is getting lighter, it is also accelerated forward better and better. So the engine keeps getting better and better.

This also works in a vacuum. It has to be, because the rocket has nothing to push against, like the wheels of a car push against the road it is driving on.