In the current study, an analytical model to estimate the fuel surface regression rate of hybrid rocket engines with head-end swirling flow oxidizer injection is established. The model is based on a convective heat feedback approach and, in conjunction with the corresponding boundary layer (or zone) concept which accounts for transpiration, effective hydraulic diameters, and wall friction. The effective tangential (swirl) velocity of the gas provides a positive augmentation effect to the fuel regression rate, above that due to the axial mass flux component of the core gas
flow. From the literature, a variety of propellant combinations, engine sizes, and flow swirl numbers are evaluated for engines having circular-port fuel grains, with sample results provided in this report for comparative purposes. The predicted fuel regression rates for the most part compare quite well with the corresponding experimental data. Additionally, the validity of the underlying assumption of a slowly decaying effective axial and tangential velocity of the gas as one moves downstream along the central fuel port is to some degree verified using a computational approach, based on a simplified engine flow model. As a final element of the overall study, the fuel regression rate model is evaluated for parameter sensitivity. The settings for some propellant and gas properties are found to have a significant influence on the quantitative predictive results.