Aero Eng question. Why is the flow through a jet/rocket engine

>>8975093
It is not, however as a first approximation you can assume it is for some rough values.

In reality, every component has an "adiabatic efficiency". On a temperature-entropy diagram, the non-ideal components will have curved lines instead of going straight up or down when changing temperature. Adiabatic efficiency of a component allows you to characterize how this increase in entropy translates to the T-s diagram and subsequently the effects on pressure ratio etc.

Furthermore, every combustion chamber has a "pressure ratio" across it in the 0.9-0.98ish range which describes the loss in stagnation pressure compared to the stag. pressure that would be achieved if processes were isentropic.

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>>8975093
>>8975106
Was just about to stride in and put on my aero eng pants but this guy gave a pretty good explanation, kudos.

Basically, isentropic efficiency is the maximum that isphysically possible, with no heat losses, bleeds, efficiency losses due to pumps, bearings and flow friction etc etc etc. It gives a good first approximation to know what your ceiling will be, and takes relatively little time to calculate. Relatively.