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

There’s a handy analogy that’s perhaps a bit odd, and certainly very American: imagine firing a gun at a range. The gun pushes the bullet, which is relatively light, out at enormous speeds. But it also pushes back on you, sometimes quite hard. If you were to mount yourself on a rolling platform, you could propel yourself backwards with sufficient ammunition and firing rate. Rockets do a similar thing: they throw mass(propellant, or reaction mass) out the back of a spacecraft and take advantage of the “recoil” to accelerate the spacecraft itself.