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I understand that rockets create a lot of gas which is pushed out of the back and because of equal and opposite reactions the rocket is propelled forwards.

What I don’t quite understand is this:

The interstellar space is a vacuum so the gas that the rocket creates is going to naturally want to try to fill this empty space. If this is the case then how is it creating a force that pushes the rocket?

So in the classic gun example where a bullet is shot which in turn pushes back on the gun, there is a gas pressure pushing them both. In space, there is nothing for the gas to push on. It seems that it is simply moving to fill an empty space so I don’t understand where the force is coming from.

Would it be that the rockets just create so much gas that pressure is able to build up in the back of the rocket even though the gas is trying to leave naturally?

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    $\begingroup$ Do you know about conservation of momentum? $\endgroup$ Jul 6 at 6:06
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How do you move on Earth? You push the Earth in one direction and you accelerate in the other direction.

How do you move in space? You don't have an Earth handy, so you carry whatever you will push with yourself. The thing you push in a rocket is called "rocket propellant". You can't carry much, that's why you push it really hard. And you can't carry much of anything in a rocket, that's why you get a propellant that can push itself out of the engine nozzle.

In both cases, you rely on "momentum conservation". You + Earth (as a whole) don't accelerate, a rocket + its propellant don't accelerate as well.

On the other hand, the Earth with its really great mass builds our intuition that pushing it is the only practical way of accelerating.

This is not true. You can accelerate by interacting with anything.

p.s. In space, sometimes, you in fact do have a planet handy, because some forces (e.g. gravity) act pretty well at a distance. That's how gravity-assisted maneuvers are done and that's how satellites stay in orbit in the first place.

p.s2 In regard to the gas that left the combustion chamber: you can leave it alone. It doesn't interact with the rocket anymore. It already did its job by pushing the combustion chamber walls and not pushing the opening.

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I believe you already gave the explanation: in the combustion chamber, the pressure is very high and pushes back. While the pressure at the outlet of the nozzle is much lower. The fact that gas escapes into a vacuum does not remove the pressure difference.

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  • $\begingroup$ So essentially the emptiness of space is causing some gas to go out freely but the rocket is creating gas faster meaning a pressure can build up? Like gas escapes into space at a rate of 1 lb/s but the rocket is creating gas at a rate of 3 lbs/s meaning a pressure is built up because gas building at a rate of 2 lbs/s? If that makes sense. $\endgroup$
    – Aeon
    Jul 6 at 6:29

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