This work presents a combined experimental and computational investigation of the Cascaded Multi-Stage Impinging Jet (CAMUI) solid-fuel configuration for hybrid propulsion. CAMUI departs from conventional solid-fuel architectures by arranging orthogonal fuel blocks that force sequential oxidizer-jet impingement, thereby intensifying mixing and extending residence time, which could be of interest for solid-fuel ramjet and hybrid rocket applications. This study provides the first schematic and theoretical interpretation of CAMUI internal ballistics, combining firing experiments and computational fluid dynamics (CFD) simulations to explain its internal flowfield and the resulting postfiring fuel-grain geometry. Four oxygen/High-Density Polyethylene firing tests were performed while varying the oxidizer mass flow rate and interblock spacing. Combustion efficiencies exceeded 90%, with peak regression consistently occurring at the impingement point on each block’s front surface. CFD simulations reproduced internal velocity, temperature, and mixture-fraction fields, explaining the postfiring geometry of the front, back, and port surfaces. Front-surface regression was strongly nonuniform and governed by impinging temperature and port geometry. Back-surface regression was dominated by two counter-rotating vortices whose heat transfer intensified as the gap length decreased, while low-regression “hump” regions formed where the vortices merged and the flow separated from the wall. Inside the ports, regression was highest near the motor axis due to preferential oxidizer flow in the axis-adjacent region.
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