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This question already has an answer here:

What value of exit pressure should I use in the design equations for a small liquid rocket engine nozzle for 200km-500km operating conditions? Since using zero as a value is not possible or else nozzle size will be infinite what is the exit pressure that I should use for my nozzle and how do propulsion engineers do it? I guess engineers use a rather low altitude value and let the nozzle be under-expanded at vacuum conditions. I have checked out these atmospheric properties tableslink for the tables but using those values gives a huge nozzle which is impossible to design and use. So please help me find a value that I can use.

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marked as duplicate by JCRM, DrSheldon, Mark Omo, user259412, Sean Jun 5 at 0:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ See this similar question $\endgroup$ – Uwe Jun 4 at 10:30
  • $\begingroup$ I am trying to design a vacuum nozzle so it is a little bit non relevant. but thanks mate:) $\endgroup$ – user167195 Jun 4 at 10:39
  • $\begingroup$ that's my question only but didn't find an answer for it so thought of asking the question more properly. $\endgroup$ – user167195 Jun 4 at 10:44
  • $\begingroup$ What do you mean? The linked duplicate question has an answer that's perfectly valid here. You have also partially answered your question yourself. You don't design to a particular pressure, you just use the biggest nozzle you can afford in terms of dimensions, mass, mechanical properties etc. $\endgroup$ – TooTea Jun 4 at 12:17
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    $\begingroup$ Also, it's much better to edit your original question to improve it instead of asking a new one. $\endgroup$ – TooTea Jun 4 at 12:19
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Your goal is to design the best possible engine. Only you can define “best”.

The exit pressure is a parameter of that design: you can pick the value you use.

A priori, there’s no way to know which value to use to get that best design, because it enters in multiple ways: cost, weight, thrust, fit, etc. So you need to use multiple values to home in on the one you choose to use in your design.

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