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I am just trying to do a simple rocket performance analysis. I want to determine specific impulse (vacuum isp) as a function of oxidizer to fuel ratio (O/F) for a LOX/LH2 engine and LOX/CH4 engine. I cannot seem to find a clear explanation/approach and I am a little rusty on my thermodynamic relations. Any help would be appreciated, thank you.

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  • $\begingroup$ It's a little complicated - because isp depends on exit velocity, which depends on chamber pressure, which depends on efficiency, which depends on chamber pressure.... This page might help, though: web.mit.edu/16.unified/www/SPRING/propulsion/notes/node102.html $\endgroup$ – IronEagle May 8 '20 at 20:25
  • $\begingroup$ Why are you assuming that specific impulse is a function of mixture ratio? $\endgroup$ – Organic Marble May 9 '20 at 13:26
  • $\begingroup$ I have the formula in my office, unfortunately i am in home-office due to Corona, so I cannot provide you an answer for the next weeks. Just as a basic idea: changing the mixture ratio can change the mass of the exhaust gasses more, than it changes the exit velocity. so having "much havier" exhaust gases while having "slightliy lower" velocities will increase your efficinecy. $\endgroup$ – CallMeTom May 11 '20 at 6:31

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