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

Get on a skateboard with a big rock or bag of sand or something heavy. Stand on the skateboard and Throw the heavy thing as hard as you can off the back. You’ll find that you will move in the other direction.

Try it again with a bucket full of stones and throw them out one by one. Same thing happens, just instead of one big push, you get a bunch of small ones.

Same thing happens with rockets, but instead of one heavy thing, it’s ‘thowing’/pushing out lots of expanding gas out the back. It all adds up and the rocket goes!

Newton’s 3rd law of motion is the result.