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I was reading about calculating thrust chamber geometry, and a key tool seems to be the characteristic length, L*. The engine I'm modelling (hypothetically! please don't take down the question) uses HTP, and not only have I found no values for a corresponding characteristic length, but that, as it is defined, the minimum length for complete combustion, doesn't really apply as there is none. Does this concept still apply to monopropellants, and if not, are there any other means to approximate combustion chamber geometry?

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It's worthwhile to note that some sources make a distinction between monopropellants based on the mechanic - Barrère et al. (1960) indicate that there are combustion monopropellants such as propyl nitrate, where the propellant takes both roles of fuel and oxidizer. Then, there are decomposition monopropellants, which include the well-known HTP and hydrazine. Because both mechanics include the breaking of reactant bonds and one or more steps of product formation in finite time, it is reasonable to assume the characteristic length ($L^*$) concept, which is really a shorthand for reaction residence time, exists for both.

There is a master's thesis from UT El Paso, Hogge (2017), which attempts to characterize $L^*$ for ionic liquid monopropellants. Wada, Watanabe, & Takegahara (2018) have also done so in an AIAA paper for ammonium compound-based monopropellants. I haven't found data for HTP specifically, but knowing the fair amount of interest and the historical uses (gas generators in V-2/RD-107/108, etc.), it likely exists.

As such, the answer is that it still applies, but the usual caveats surrounding $L^*$ still exist - it is a property that both depends on the individual propellants as well as their operating conditions.

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