Numerical simulations of the flowfield in a hybrid rocket engine are carried out with a multispecies chemically reacting Reynolds-averaged Navier–Stokes (RANS) solver which includes detailed gas–surface interaction (GSI) modeling based on surface mass and energy balances. The oxidizer is gaseous oxygen which is homogeneously fed into single-port cylindrical grains. The modeling of GSI already developed and validated for pyrolyzing fuels such as hydroxyl-terminated polybutadiene (HTPB), is extended to the case of liquefying fuels, such as paraffin wax. A simplified two-step global reaction mechanism is considered for the gas-phase chemistry to model the combustion process inside the chamber. Numerical simulations performed at different gas/melt-layer interface temperatures and oxygen mass fluxes show a considerable increase of fuel regression rate, in the range of 3 up to 5 times, for the liquefying fuel with respect to the pyrolyzing one. Results show that the regression rate enhancement is significant only when the gas/melt-layer interface of the liquefying fuel is close to the melting temperature. At increasing gas/melt-layer interface temperatures, the regression rate decreases following an inverse power law and gets close to that of a pyrolyzing fuel for the same operating conditions. Finally, regression rate behavior at varying oxygen mass flux of liquefying fuels is not substantially altered from that of pyrolyzing fuels as the oxidizer flux exponent remains rather constant.