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

From my understanding, it’s still pushing away from something. It’s just that the something is its fuel. Same thing as when you let go of the end of an air filled balloon. It’s newton’s 3rd law, I think. It works better in space because there’s no air resistance to push back.

You can do the same thing by getting in an office chair and throwing a basketball ball or using a foaming fire extinguisher.

Summary: Rockets work because, in space, farting propellants is the same as pushing away from a wall.