To identify the cause of backfiring in an axial-injection end-burning hybrid rocket using liquid oxygen, combustion experiments were conducted with a solid fuel tube of 2.0 mm port diameter and liquid oxygen as oxidizer. Results showed that under high chamber pressure or low oxidizer port velocity, the regression mode could not be maintained, and flame spread occurred. This is attributed to a reduction in quenching distance and increased heat flux to the fuel surface due to increased chamber pressure. Additionally, decreased oxidizer port velocity shortens the quenching distance, increases heat flux to the fuel, and reduces heat transfer to liquid oxygen. These effects are believed to promote flame spread onset.
Yamaguchi et al. (Wed,) studied this question.