Eli5, how does a rocket exert force?

Question

For example, if I took a butane lighter & fired it up, there’s no force in any direction. How does a rocket ignition work & propel itself upwards?

Thanks,

In: Engineering

Answer 1

Because you need a nozzle.

So you may have learned in high school that Newton’s third law says if you have a force in one direction you have an equal force in the opposite direction.

So how does this apply to rockets? Well, we have to push against something and in a rocket’s case it pushes against its exhaust. The way it does this is by rapidly burning its fuel and oxidizer in its combustion chamber, now these aren’t at particularly high pressure by themselves but when they ignite the sudden increase in temperature boosts their pressure to immense levels.

So this hot gas pushes against everything, mainly the rocket, and we give it only one way out, out through the nozzle. So it pushes against everything inside the combustion chamber, nozzle, and eventually pushes itself out, and pushes the rocket away.

Now your butane lighter has no nozzle, this is actually by design cause otherwise you would need to somehow provide it oxygen, but no nozzle means that energy of that high pressure expansion pushes against the air instead of against the lighter.