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When designing a rocket engine, what degrees of freedom do you have to get the combustion chamber to a desired pressure? I assume you have mass flow rate, type and mixture ratio of fuel and oxidizer, and geometry, but what aspects of the geometry matter?

With a given geometry, is there a way to calculate the minimum pressure to cause choked flow at the nozzle?

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  • $\begingroup$ The existence of choked flow is solely a function of the pressure ratio for a given gas. en.wikipedia.org/wiki/… $\endgroup$ – Organic Marble Jul 24 '16 at 0:04
  • $\begingroup$ The pressure is less of a problem than the temperature. $\endgroup$ – SF. Jul 24 '16 at 7:58
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    $\begingroup$ And -from my observations - two "schools" - ALL aspects of the geometry matter - the whole path of flow, including the combustion phase is modeled and the chamber is shaped to that; general data like overall pressure go to PR publicity papers, while the actual plans have a precise layout of localized conditions and every centimeter of the chamber built to match and guide the local conditions it is facing - and NO geometry aspects matter; it's just a rough bottle shape where chaotic processes mix the gasses and they escape through the only opening to a (reasonably complex) nozzle. $\endgroup$ – SF. Jul 24 '16 at 8:05
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The combustion chamber is typically designed to ensure the combustion reaction goes to completion. This is often thought of in terms of characteristic length (L*). Where L* is equal to chamber volume / throat area. In practice, the smallest L* that will allow complete combustion can be found experimentally by measuring chamber temperature and characteristic velocity for different chamber lengths.

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