How efficiently is the fuel burned in a typical liquid fuel rocket engine? I've heard numbers ranging from 95% to below 50%.
https://www.raeng.org.uk/publications/other/22-rocket-engine-bssc said: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.