How does a rocket push against its exhaust? Does the reaction come from the rocket nozzle or as a result of the imbalance of forces in the combustion chamber?

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    $\begingroup$ "How does a rocket push against its exhaust?" The same way you push on a ball when you throw it. $\endgroup$
    – knzhou
    Dec 3, 2018 at 14:28
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    $\begingroup$ Just think about the rocket and matter that comes out of its nozzle as a single system having a constant or 0 momentum and then you will came to know that what is happening $\endgroup$
    – Sourabh
    Dec 3, 2018 at 14:32

1 Answer 1


Yes, the imbalance of force in the combustion chamber explain how a rocket works. The hot expanding gases push against the top of the chamber, but not the bottom since it's open, and so the rocket is pushed upward. (The gases push against the sides too, but those forces balance each other out).

That's just the gist of it, though. The shape of the nozzle is very important to the performances of the rocket. If there was simply a hole at the bottom of the combustion chamber, the gases would spray out in every direction, carrying a lot of wasted momentum with them. But by pushing against the nozzle, which redirects them downward, more of that momentum is transmitted to the rocket.

  • $\begingroup$ Thanks a lot, but I'm still confused; which one of them is the true action force, is it the extra thrust produced by the nozzle, or the imbalance of forces in the combustion chamber? $\endgroup$
    – Taofeek
    Dec 3, 2018 at 15:25
  • $\begingroup$ @Taofeek: It's a good question. The trick in designing a combustion camber is to get the exit the right size, so as to get the right amount of pressure, both at the exit of the chamber and at the exit of the bell, so as to maximize the velocity of exhaust leaving the whole thing. $\endgroup$ Dec 3, 2018 at 15:29
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    $\begingroup$ @Taofeek: it's both. The pressure inside the combustion chamber and against the nozzle both contribute to the rocket's thrust. There's no single "true action force", but a sum of forces across the surfaces of the engine. $\endgroup$
    – Aetol
    Dec 3, 2018 at 15:34
  • $\begingroup$ @Aetol: I still don't understand why rocket propulsion is linked to Newton's third law, I mean an explosion took place in the combustion chamber, and due to an imbalance of forces in the chamber,the rocket is propelled, so how does Newtons law apply here? Pls I'd really appreciate a comprehensive explanation. Thanks $\endgroup$
    – Taofeek
    Dec 4, 2018 at 8:38
  • $\begingroup$ @Taofeek: if they were not contained by the rocket engine, the gases would just expand in all directions. Since the combustion chamber and the nozzle change their direction of expansion to exclusively downward, that means (according to Newton's first law) they exert forces on those gases. In return, the gases exert equal and opposite forces on them, and by proxy, the rocket, pushing it upward. $\endgroup$
    – Aetol
    Dec 5, 2018 at 8:44

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