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

Imagine a scuba tank. You have a top end, a tube, and a bottom end. Inside is gas compressed to 200 times normal air pressure. It’s pushing extremely hard on the inside of the tank.

It’s pushing with several tons of force on the top end, and also several tons of force on the bottom end.

Now, imagine the bottom end stops existing. Poofs away. Now the pressure inside is just pushing against the top end. The tank starts rising rapidly because the pressure isn’t counteracting itself by pushing on both ends.

A rocket’s combustion chamber is a scuba tank that’s missing the bottom. The huge pressure pushes the rocket in the direction you want to go.