Modeling the Regression Rates of Protrusion-and-Lattice Hybrid Rocket Fuel Grains

Hybrid rockets based on neat paraffin fuel grains often suffer from poor combustion efficiency due to fuel sloughing and propellant stratification. Fuel sloughing occurs when unburned fuel fragments detach because of the weak mechanical properties of paraffin, reducing thrust and risking motor failure. Embedding lattice structures within the fuel grain enhances structural integrity and mitigates sloughing. Stratification refers to the layering of fuel-rich and oxidizer-rich zones, resulting from the diffusion flame formed at the interface between the solid fuel and gaseous oxidizer that leads to incomplete combustion. To address this, prior studies have introduced passive mixing devices, such as protrusions, to enhance turbulent mixing and combustion efficiency. The first objective of this study is to develop a one-dimensional analytical model that predicts the regression rates of protrusion-and-lattice-augmented fuel grains, accounting for enhanced convective heat transfer induced by protrusions. The second objective is to evaluate the effect of lattice volume fraction on regression rates for values ranging from 5% to 25% and compare predictions with experimental data. Results indicate that increasing lattice volume fraction reduces regression rates, and the model accurately captures this trend, matching experimental data within the error margin for lattice volume fractions up to 10%.