Is "chamber pressure" the pressure at the tip of the injector, or the pressure before combustion inside the chamber, or the pressure after combustion inside the chamber?
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2$\begingroup$ I'm betting that the pressure of everything behind the throat of the combustion chamber is at about the same pressure. Flow in the combustion chamber will be subsonic (and will be sonic right at the throat) so pressure differences can iron themselves out. The flow will then become supersonic in the expansion part of the nozzle and at that point you can have parcels of fluid that aren't talking to each other any more. $\endgroup$– zeta-bandCommented Oct 19, 2018 at 17:26
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2$\begingroup$ This is a good question! After an admittedly brief search I can't find any reference that says definitively "This is where chamber pressure is measured." But the pressure in the chamber's pre-convergent section must decrease, albeit the ∆P is small compared to the absolute pressure in the chamber. Otherwise you'd get no nozzle-ward flow except as propelled by the momentum of the injected propellants, and that specific momentum is smaller than that at the nozzle entrance. In the nozzle's converging section the pressure drop will be more dramatic, accelerating the flow to Mach 1 at the throat. $\endgroup$– Tom SpilkerCommented Oct 19, 2018 at 18:09
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2$\begingroup$ This answer:space.stackexchange.com/questions/28134/… shows how it works in the real world. It's the static pressure measured in a small cavity connected to the combustion chamber. $\endgroup$– Organic MarbleCommented Oct 19, 2018 at 19:10
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1 Answer
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The F-1 Engine Familiarization Manual tells us, on page 1-8, that the Rocketdyne engineers worked with only 1 pressure in the combustion chamber: the injector end pressure, nominally at 1125 psia (7.757 MPa in SI units, or 76.55 atm). The nominal exit pressure was 9.6 psia (66 kPa in SI units, or 0.65 atm).
