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

A butane lighter absolutely exerts a force – the burning butane expands, is less dense than the air around it, and the flame goes up because of it. In a rocket, the combustion wants to expand as well, but it’s not out on the open, it’s inside a space with only one way out. All that expanding gas going out the back of the rocket pushes the rocket the opposite direction.