The Ignition of Ultra-Fine Aluminum in Ammonium Perclorate Solid Propellant Flames

Abstract

A preliminary experimental investigation was carried out to investigate the ignition of ultrafine aluminum (UFAl) compared to conventional sized aluminum (CSAl) particles in ammonium perchlorate (AP)-polybutadiene acrylonitrile acrylic acid (PBAN) solid propellants. To evaluate the temperature criteria for igniting UFAl, matrix samples (binder, fine AP and Al only) were prepared with various loadings of 10μm fine AP (fAP) as a means of varying the AP-binder flame temperature, and with Al coarse-to-fine ratio of 0(30μm)/20(UFAl) and 80(30μm)/20(UFAl). Preliminary results showed that UFAl sized particles ignite at lower gasphase flame temperatures than CSAl particles and that the UFAl particles tend to affect the combustion processes close to the propellant surface depending upon the availability of oxidizer. * Senior Combustion/Propulsion Engineer. AIAA Member. E mail: allandokhan@hotmail.com † ‡ Regent Professor Emeritus. Fellow Member. Associate Professor. Senior Member. E mail: jerry.seitzman@ae.gatech.edu Senior Research Engineer. INTRODUCTION In recent studies of ultra-fine aluminum (UFAl~0.1μm) in ammonium perchlorate (AP) solid propellants with bimodal AP (10μm:400μm and 82.5μm:400μm) and bimodal aluminum (Al) distribution (30μm:UFAl), the burning of UFAl was found to create a very dense aluminum burning region (ABR) directly above and some distance beyond the propellant surface (~3000-4000μm) compared to the burning of conventional sized aluminum (CSAl~12-100μm). The density of the ABR was found to be the product of the number of burning Al particles/droplets/agglomerates leaving the propellant surface per unit area (one 30μm Al particle is the mass equivalent of 10 UFAl particles), which is very dependent upon the propellant microstructure (see later). The combustion of fine Al (~3μm) and UFAl particles occur much closer behind the AP-binder flame because of their near equilibrium state with the gas flow (temperature & velocity) compared to CSAl. As a result, they ignite quickly as they pass through the flame surface and burn close behind the convoluted flame canopy. This dense luminous bright ABR was found to be responsible for a significant amount of heat feedback to the propellant surface and to the AP-binder flame array in the form of either radiation and/or conduction, which resulted in high burning rate propellants. Copyright© 2003 A. Dokhan, E. W. Price, J. M. Seitzman and R. K. Sigman. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission 1