Experiments were conducted with using liquid  oxygen and also with gaseous oxgyen. In this study, c* the efficiency in hybrid rocket engines (defined as the ratio between the expermetal efficiency, based on combustion chamber pressure and propellant mass flow rate and correspong theoretical efficiency based on equivalence ratio and mass flow rate of propellant) was varied with different values of equivalence ratio.  The efficienciew with swirling gaseous oxygen were nearly 100%, but those with swiriling liquid oxyen were considerably lower at the values equivalence ratio nealy equal to 0.5. In addition, equivalence ratio with liquid oygen were much smaller than those with gaseos oxygen. These results indicate that fuel regression rate with swirling liquid state oxygen were considerably low and combusiton reaction proceeded insufficiently.

Development of small hybrid rocket with swirling oxidizer flow type engine. 

Saburo et al., J. Space Tecnology and Science, 21(1), 1-11, 2005

www.repository.lib.tmu.ac.jp/dspace/bitstream/10748/.../1/10638-001.pdf

in hybrid rocket motor，diffusion combustion depends on local flow field. so I think the reaction can not occur on entire fuel surface because of the low temperature of LOx. Maybe this is the reason.

Figure shows that average combustion efficiency obtained through the tests carried out in this experiment is as a function of the equivalence ratio (the ratio between the actual oxidizer to fuel ratio and the stoichiometric one).Nevertheless data are almost dispersed, it seems that at the stoichiometric mixture ratio (equivalence ratio equal to 1) the efficiency has a peak; this may be due to the fact that at the stoichiometric ratio the reaction rate, at a given pressure, is expected to be maximum. However, other factors certainly influence the combustion efficiency, such as the propellant mixing efficiency. Indeed, what should play a main role is the ratio between the propellant residence time in the chamber and the sum of the mixing time and the characteristic reaction time. Those characteristic times can differ a lot with distinct propellants,

Ref: Basic Aspects of the Hybrid Engine Operation; 

C. Carmicino,O. Orlandi, A. Russo Sorge, F. Dauch, R. De Amicis and  M. De Rosa

45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit

2 - 5 August 2009, Denver, Colorado