Circumstantially fired combustion port geometry: analysis of hybrid rocket motor solid fuel grain

Efficient hybrid rocket engine designs are highly desirable due to the inherent safety of these systems when compared to the premixed solid rocket motor or bi-propellant rocket engines. Hybrid rocket motors use a pressurized tank to impel liquid oxygen or other oxidizers into the solid rocket fuel combustion chamber, where an ignition source is used to start the combustion reaction. An actuated valve can be placed between the oxidizer tank and the solid fuel grain, allowing throttle ability of the hybrid engine designs. Solid fuel grains are inert and will not combust without an oxidizer. The combination of throttle ability and a non-premixed inert solid fuel grain result in the increase safety of the hybrid motor system. Hybrid rocket motors lack the high regression rates that solid or liquid rockets possess and therefore are not as commercially viable. This paper focuses on a complex solid fuel grain geometry that leverages rapid prototyping to increase the effective regression rate of the solid motor. Traditional cylindrical fuel port geometries have long length to diameter aspect ratios that increases the fuel port burn surface area but results in a poor volumetric efficiency. Advanced designs such as triangular wagon wheel configurations or helical fuel ports have been successful at increasing the regression rate of the engine but can result in a decreased volumetric efficiency. In place of the previously mentioned fuel port configurations, it is proposed that by orienting the combustion ports laterally on the circumference of the solid fuel grain that the internal surface burn area and internal flow path of the oxidizer would be increased, resulting in an increased fuel regression rate. Preliminary theoretical analysis shows that there is a slight increase in the time to peak thrust using a circumferentially fired combustion port, but a significant decrease in the volumetric efficiency of solid fuel grain. One initial advantage of the design is an increased number of combustion ports, which allows for a smaller solid fuel grain length to diameter aspect ratio because of the increased internal flow path. The smaller aspect ratio gives more flexibility when tailoring the design to envelope limitations.