eli5: If space is a vacuum, how can rockets work? What are the thrusters pushing *against* if there is nothing out there?

Question

I’ve never really understood the physics of this. Obviously it works somehow — I’m not a moonlanding denier or anything — but my (admittedly primitive) brain continues to insist that a rocket thruster needs something to push *against* in order to work.

So what is it pushing against if space is essentially a void?

In: 7157

Answer 1

You know how cannons recoil because they shoot out the cannon ball so fast? Rockets work the same way only with exhaust gas. They react the fuel with stored oxygen and the released energy shoots it out the nozzle so fast that it pushes the rocket forward. If you sit on a wheeled office chair and fire an extinguisher you’ll go shooting backwards in much the same way.