The subject of this paper is the study of the performance of a Hybrid Rocket Engine (HRE) in the frame of a research program conducted in a collaboration between the California Polytechnic State University, San Luis Obispo (USA) and Politecnico di Torino (Italy). Specifically, the present study investigates the combustion process of the solid fuel, through experimental tests using poly-methyl-methacrylate (PMMA) as the solid fuel and Nitrous Oxide (N2O) as the oxidizer. In the past, many tests have been conducted on the hybrid rocket engine of Cal Poly using different materials for the solid fuels, like PMMA and hydroxyl-terminated polybutadiene (HTPB) and different geometric configurations, including straight cylindrical grain with circular port, double D, and an annular configuration with a circular pin along the axis of the motor.