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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.