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Wikipedia references the ratio of liquid oxygen to RP-1 as 2.56. Is this based on mass or volume? Are they using some typical combustion chamber pressure?

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It's always by mass but there's one additional problem that rockets pretty much never use stoichiometric oxygen to fuel ratio (O/F) that you'd expect if you calculated molar mass of propellant components an their ideal exhaust product(s). So the source should mention that somewhere if they're using stoichiometric ratios, typical O/F or even quoting some specific system's ideal O/F (and for where, usually given for sea-level and vacuum).

As a note, quoting volumetric ratio wouldn't make sense without also quoting storage pressure and temperature. And in case of mixtures and otherwise non-pure (e.g. non-refined) propellants also their molar composition. And ullage volume. And so on.

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I believe that's the mass ratio. The liquid rocket propellants page on Wikipedia gives 2.77:1 as the mass ratio; the optimum varies with the exact engine design.

There's some interesting discussion of fuel ratios in this old usenet article.

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  • $\begingroup$ Ah, old usenet articles - always a find! $\endgroup$ – kim holder Oct 20 '14 at 14:28
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It is a mass ratio as the ratio itself is based on the molecular masses of the fuel and oxidizer. Check out http://www.braeunig.us/space/propuls.htm and there "Combustion and Exhaust velocity" for details.

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