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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.

user167195
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  • See this similar question – Uwe Jun 04 '19 at 10:30
  • I am trying to design a vacuum nozzle so it is a little bit non relevant. but thanks mate:) – user167195 Jun 04 '19 at 10:39
  • that's my question only but didn't find an answer for it so thought of asking the question more properly. – user167195 Jun 04 '19 at 10:44
  • 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. – TooTea Jun 04 '19 at 12:17
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    Also, it's much better to edit your original question to improve it instead of asking a new one. – TooTea Jun 04 '19 at 12:19
  • if you are familiar with equations used to design the throat area and exit area of a nozzle and also the equation for exit mach number you need a value for exit pressure in it. if i use a value of pressure at 100km its impossible and way too big, if I use 50km its possible and good and if I use use value of 25km it is also possible and good but how do I know which one is correct? any value between 10km and 100km can be chosen but how do I find out which one to use? – user167195 Jun 04 '19 at 12:23
  • If you know what size of nozzle is "impossible to design and use", start with a size of nozzle which is practical and work the problem in the other direction. – Russell Borogove Jun 04 '19 at 17:12

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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.

Bob Jacobsen
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