What is the combustion efficiency of liquid fuel rockets?

[Cato]

How efficiently is the fuel burned in a typical liquid fuel rocket engine? I've heard numbers ranging from 95% to below 50%.

 

[anorlunda]

This is not a direct answer to your question. However, it indicates that combustion efficiency is not the right question.

As such, we find that an optimum sp I is achieved at a value of O:F (oxidiser to fuel ratio) that is significantly lower than the stoichiometric ratio (please refer to Reference 7) for complete combustion. This leads to the presence of combustible products in the plume. These combustibles are here by design to enhance sp I , i.e. they should not be regarded as unburnt fuel. sp I is the main driver of hybrid rocket design rather than combustion efficiency.

 

[cjl]

As anorlunda indicates, combustion efficiency really isn't the correct figure of merit. Instead, rocket engineers look at things like impulse density (how much momentum change can you get from a given volume of fuel), specific impulse (how much momentum change can you get from a given mass of fuel), and thrust per engine diameter (different fuels will require different sizes of engines to produce the same thrust).