What params and formulas do we need to calculate an engine's performance?

[Librazy]

I'm working on realistic engine performance calculation recently and read a few book about (liquid) engines,trying to find out how&what to do the calc.But I still don't know entirely what do we need.

Listed below are some params and formulas I think to be important:

Isp=Cstar*Ct
Cstar is a function of gamma,R and chamberTemp,all come from the propllants used
Ct is a function of gamma, nozzleExitPressure ,chamberPressure and nozzleExpansionRatio,ambientPressure
nozzleExpansionRatio is a function of gamma,nozzleExitPressure ,chamberPressure
chamberPressure is a function of massFlow, Cstar and nozzleThroatArea   

Which we should use as a "constant" to define an engine and which should be used as a var calculated from others?

Or anything more?

Edited June 25, 2015 by Librazy