Classification of Angled-Hole Oxidizer Injector Designs for Hybrid Rockets, and Experimental Study of Hole Angle on Discharge Coefficients

The McGill Rocket Team is a student-run rocketry team based in McGill university that specializes in the design of hybrid-engine-propelled rockets. In the team’s objective to design rockets capable of attaining higher apogees, more stringent requirements are placed on engine efficiency and performance. Within hybrid rocket engines, the oxidizer injector is a component that promises high potential for improvement, given supporting evidence in existing literature highlighting the effect that various designs have on efficiency and thrust generation. While the McGill Rocket Team currently employs an axial-hole orifice-plate (showerhead) oxidizer injector design, there is growing interest within the team of investigating the potential relative combustion stability and/or efficiency improvements afforded by various angled-hole orifice-plate oxidizer injector designs, namely vortex, hollow-cone, and impinging — and to develop a comprehensive framework through which the angular components of any orifice-plate-injector holes, and their anticipated effects on combustion stability and efficiency, can be understood in terms of key aspects of these designs. This paper aims to serve as a general resource in the design, analysis, and modeling of orifice-plate oxidizer injectors designs, and summarizes relevant literature to provide sources for further research. Furthermore, The effects of swirling, diverging and impinging oxidizer flow in hybrid engines on various combustion performance parameters are compared, in order to establish a qualitative classification system for their corresponding angled-hole injector designs. To aid the practical sizing of injectors in delivering target mass flow rates, this paper also aims to contribute experimental data examining the influence of angled holes on discharge coefficients in the single-phase incompressible flow regime. An experimental campaign characterizing injector discharge coefficients as a function of hole angle, across a range of pressure differentials within a subcritical flow regime, is presented. Orifice-plate oxidizer injectors with identical net orifice area but varying hole angles were flow tested with water on a specialized testing bench. Plumbing and instrumentation diagrams of flow testing apparatus using nitrogen-pressurized water as a rough analogue for nitrous oxide are described, and curves plotting the calculated discharge coefficients as a function of time and pressure drop for a set of angled-hole injectors, as well as video footage of post-injector fluid streams, is presented and discussed. Irregular flow separation effects, as well as abrupt changes in flow direction with increasing hole angles resulted in discharge coefficients that remained nearly constant as a function of hole angle. Further testing is required to ascertain whether this result is representative of reality, or influenced by measurement errors.