Effect of pressure on optimum mixture ratio

The optimum mixture ratio for a given propellant combination is in turn influenced by the other primary design parameters, chamber pressure and expansion ratio. 

Even though recombination tends to increase the mean molecular weight, the increased enthalpy overrides. 

An even more pronounced effect of chamber pressure would be expected and is observed in the frozen flow expansion of these same propellants, as is shown in figure V.A.3. Unlike the equilibrium expansion case, no recovery of the energy of dissociation is possible so that the positive effect of the inhibition of dissociation in the combustion chamber brought about by higher pressures is evidenced strongly in the predicted specific impulse. The shift of optimum mixture ratio toward the stoichiometric value with increasing chamber pressure again is observed. 

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