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

Try grabbing a heavy object that you can carry (if you have kettlebell or maybe a sack of rice or flour), sit on an office chair with wheels with said object in your lap, and lift your feet off the ground. Now lift the object and push/throw it away quickly. You’ll notice that you’ll move in the direction opposite to your throw. That’s basically the principle.

In space, while a spaceship burns fuel, the fuel will expand, pushing out against the spaceship, and in reaction, the ship is pushed back.