Grain Geometry Effects on a Hybrid Rocket Engine Performance

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. Usually thrust does not remain constant during operation. An important factor affecting this trend is the varying fuel mass flow rate, caused by the unevenness of the grain burning process: the burning area changes and the port area variation causes a possible variation of the fuel regression rate, causing both a mixture ratio and propellant mass flow change. This is a well-known issue in HRE, called “mixture ratio shifting”. Another cause of this thrust trend may be the non-uniform combustion process along the grain length during the test, as it has been noticed by the post burn analysis of the spent fuel grain. These variations influence not only the thrust level, but also the characteristic velocity c*. In some applications, it may be advisable to keep the thrust level constant during operation, while avoiding mixture ratio shifting. Nevertheless, due to the one-lever control feature of the HRE, it is not possible to control both thrust level and mixture ratio. On the other hand, an optimization of the grain geometry may mitigate this issue. The choice of the propellant combination has a very important role. In the setup used in this study at Cal Poly, N2O is the oxidizer. Despite its limited performance, particularly in terms of specific impulse, this oxidizer is commonly used in hybrid rockets because of its self-pressurizing capabilities. As far as the solid grain is concerned, several fuels were considered in the preliminary phase of the study, and PMMA was chosen for its simplicity of manufacturing, its cost and availability. The Marxman model was used to describe the combustion process. Thrust depends on the fuel and oxidizer mass flow rates, the characteristic velocity and the thrust coefficient. At the present time, the oxidizer mass flow rate cannot be controlled on the Cal Poly experimental test stand, so several geometries are analyzed to tailor c* and the fuel regression rate. A promising configuration appears to be a cylinder liner with a helical internal surface.