eli5: If space is a vacuum, how can rockets work? What are the thrusters pushing *against* if there is nothing out there?


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


Rockets do not rely on pushing against something else in order to move as they use the principle of conservation of momentum to propel themselves forward based on Newton’s third law….every action has an equal and opposite reaction.

They’re pushing against the rocket. Remember Newton’s 3rd Law of Motion: every action has an equal and opposite reaction. If you throw stuff out the back of the rocket, it pushes the rocket forward. That’s what a rocket does: it pushes some stuff very quickly out the back of the rocket.

Cool concept: Start with a stationary rocket and fire its engine so it’s moving. Then back way up and look at the whole system of the rocket and all of the exhaust that got it moving. You will see that the whole rocket + exhaust system is still holding still.

You would see a (relatively) slow moving and heavy rocket moving away from very fast-moving, lightweight rocket exhaust.

They are sending the exhaust gasses out of the nozzle at extremely high speed. You know how if you jump out of a small boat, you push the boat the opposite direction of your jump? The same phenomena is happening here, just in a continuous manner as fuel is combusted and pushed overboard.

Imagine you have a row boat filled with bowling balls. If you throw a bowling ball out of the backwards out of the boat, Newton’s 3rd law says that there is an equal and opposite reaction which pushes you and the boat forwards.

In a space rocket, the bowling balls are atoms of fuel. You throw the fuel out of the back of the rocket. You get an equal and opposite reaction which pushes the rocket forward.