As I understand it, in rocket propulsion a converging/diverging nozzle is used to convert the random velocity vectors of the combustion chamber gases into a mostly unidirectional velocity field at the exhaust nozzle. What are the physics of this process?
closed as too broad by Kyle Kanos, ACuriousMind♦, Sofia, John Rennie, tom Mar 14 '15 at 20:17
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In a rocket, propulsion occurs when the combustion products are expelled in a direction opposite to the direction of intended thrust. Sending it sideways does not move you in the right direction - so it's important to have a coherent output.
You would also like the velocity of the exhaust gas to be as high as possible. It turns out that if your exhaust gas velocity is supersonic, then the shaped nozzle will cause further acceleration of the gas - in the process producing more thrust.
This is nicely explained on this NASA website from which I copy the following for ease of reference - but I recommend you read the original (which includes a detailed description):