Oxidizer Enhanced Hybrid Rocket Engine: Regression Rates and Performance

Abstract

Hybrid rocket motors have a significant advantage over solid and liquid rockets in that they are intrinsically safer. Keeping the liquid oxidizer separate from the solid fuel grain inhibits the two propellants from mixing rapidly and eliminates the potential of a serious explosion. However, the inability of the oxidizer and fuel to mix quickly in a typical hybrid rocket motor results in low propellant burning rates causing a reduction in performance. This is a result of a fundamental difference in the combustion process. A classic solid rocket motor produces a premixed (faster burning) flame, whereas a hybrid rocket engine produces a diffusion (slower burning) flame due to the oxidizer and fuel mixing mainly through diffusion processes. The addition of solid oxidizers into the hybrid fuel grain has the potential to increase the burning rate of the fuel grain. This causes the combustion zone to act more like that of a solid rocket motor. Also, if the solid oxidizer levels are kept below stoichiometric values, a rapid combustion reaction cannot take place in the solid fuel, preserving the safe characteristics of hybrid. This paper will explore the performance and safety implications associated with the oxidizer enhanced fuel grain by the use of a laboratory scale hybrid rocket engine test stand. The test stand has the ability to measure thrust, chamber pressure, line pressure and gaseous oxidizer flow rates. The thrust is measured via a strain gauge mounted on a spring steel thrust stand. Laboratory scale Hydroxyl-Terminated Polybutadiene (HTPB) /Ammonium Nitrate (NH4NO3) /Oxygen (O2) hybrid rocket engine performance is compared verses their solid oxidizer content. Analysis of the data produces empirical regression rates. To compare the relative safety of the hybrid propellants, samples of HTPB hybrid propellant were mixed with various percentages of ammonium nitrate. The samples were exposed to an ignition source and their ability to self sustain combustion was recorded.