Combustion Efficiency in Single Port Hybrid Rocket Engines

High combustion efficiency is a critical attribute of a successful rocket engine. Reported combustion efficiency results from ground tests of single port hybrid rocket engines show measured characteristic velocity efficiency ranging from 55 to 97 percent. In many single port hybrid configurations, the efficiency starts out high at the beginning of a burn and decreases as the fuel port opens up over the course of the burn. Differences in engine interior geometry, propellents, injector design, post combustion chamber design, mixing devices and measurement techniques have made it difficult to elucidate the factors that lead to high combustion efficiency. Using current concepts in fluid mixing and chemical kinetics of combustion along with data from the open literature, this paper attempts to shed some light on the factors that have a direct impact on combustion efficiency of single port hybrids. It is shown that turbulence within the combustion chamber and propellant chemical kinetics are important to achieving high efficiency.