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.
$\begingroup$ See this similar question $\endgroup$– UweJun 4, 2019 at 10:30
$\begingroup$ I am trying to design a vacuum nozzle so it is a little bit non relevant. but thanks mate:) $\endgroup$– user167195Jun 4, 2019 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. $\endgroup$– user167195Jun 4, 2019 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. $\endgroup$– TooTeaJun 4, 2019 at 12:17
2$\begingroup$ Also, it's much better to edit your original question to improve it instead of asking a new one. $\endgroup$– TooTeaJun 4, 2019 at 12:19
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.