Plasma Propellant Interactions: RDX Films in Hydrogen, Argon, and Mixed Composition Plasmas

Abstract

The interaction of the components in electrothermal chemical (ETC) ignition have been studied due to its advantages over conventional ignition. In ETC ignition, a piece of polyethylene replaces the primer found in conventional ignition. The plastic is attached to a large capacitor that when high transient current results, due to the high temperature, the plastic dissociates into ions, radicals, electrons, and light. The individual and synergistic effects of these components have been studied and are presented in this dissertation. Erosion rates of RDX films have been measured under a variety of conditions. Experiments were performed to understand the roles of light, electrons, argon ions, hydrogen ions, and hydrogen radicals. The most significant erosion rate was found in high-energy hydrogen plasmas where the synergistic effect of ions and radicals appeared to be the greatest contributor. The roles of ions and radicals were investigated further through exposing RDX films to mixed hydrogen-argon plasmas. In theory, if the erosion rate of propellants behaved like semiconductor etching, then the addition of argon to a hydrogen plasma should greatly enhance the erosion rate due to the shear size of the argon ions. This is not observed. As a result, a new mechanism for the decomposition of RDX involving the embedment of hydrogen has been described. To test the roles of carbon atoms and ions in the plasma generated by ETC ignition, an apparatus was designed to obtain a sample of pure carbon atoms. The design incorporates an arc welder, water-cooled electrodes, graphite rods, and liquid nitrogen cooled glass. Once turned on, a sample of carbon was obtained under vacuum onto a glass slide. Although the design was successful, the integration of the apparatus to the erosion studies was not attempted. Using a nebulizing spray technique, RDX particle size was changed under a variety of conditions. When a sample of RDX contains particles that are uniform in size, ignition cannot be achieved due to lack of friction. Ideally, RDX would display a size ratio of 10:1 to allow for jagged edges and hot spot growth. A nebulizer can achieve this optimal ration simply by depositing a film of certain size and by changing a few variables, depositing a film that is ten times larger or smaller. The Scherrer Equation was used to obtain the particle size of RDX measured by x-ray diffraction. Performing a multivariate analysis, using concentration, flow rate, gas flow, and spraying distance as independent variables and particle size as the dependent variable, an equation was determined to account for the change in particle size for RDX. All of this work is preliminary and continues.