So here's a fairly basic question. I understand that rocket thrusters work by expelling hot gas into empty space, which results in motion in the opposite direction. This is because of the equal and opposite reaction to that force right? In my mind it's completely unintuitive that a thruster would work without pushing against something. So is this equal and opposite reaction explanation all there is to it? Is there a way of breaking this down into something that could be understood more intuitively or is it just a 'brute fact' that I need to accept?

Pick up a full 5 litre plastic (distilled water etc) flagon, go into the garden, and with your feet together and with your weight on your heels throw it at chest height away from you; you will stagger back or fall over. Empty it and throw at roughly the same speed. Both have the same surface breaching the air and if it were a matter of pushing back off the air then the result would be the same - but the full one will cause you to stagger back the empty one not at all.

Imagine standing on a skateboard (or, better, an air-hockey puck) and throwing oranges backwards. You will move forwards, not because the oranges push on the air (they have left your hand by then so that can't have any effect) but because you "psuh" against the oranges and they push back. Or, conservation of momentum - same thing.

Thank you. That helps!

The rocket engine pushes on the gas it ejects, that's what accelerates the gas. Conversely, the ejected gas pushes on the rocket engine.

 

You can also observe that a rocket engine has a hole at the bottom. There, the gas pressure can't compensate the effect it has on the top of the chamber. The result is a net force forward.

 

Why both concepts would lead to the same computed force, the equivalent but easier way is to evaluate the momentum acquired by the expelled gas.

OK so something really is pushing against something then. It's not just magic