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How can I calculate the propulsive force produced by ejection of a gas? I suppose it depends on the gas, the velocity with which it ejects, the amount of gas, and some other parameters, like the geometry of the nozzle for example. I would like a calculator to play with the gas density, the gas velocity etc, and see the various propulsive forces generated.

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    $\begingroup$ It's a little simpler than you might think - it all comes down to conservation of momentum. Are looking for an answer on a system with steady flow/constant mass (e.g. a leaking valve on a pipe), or on a system that is losing mass as it ejects fluid (e.g. a rocket)? $\endgroup$ – Alecg_O Jul 27 '15 at 14:49
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It's pretty basic conservation of momentum kind of problem. A converging diverging nozzle is used to accelerate the gas such that it's supersonic leaving the nozzle. These problems are pretty well-defined. There are lots of calculators out there that let you monkey around with varying different parameters without getting into a full on mathematical understanding of compressible flow. Here's one:

https://engineering.purdue.edu/~wassgren/applet/java/cdnozzle/

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