Flame structure and burning rate of ammonium perchlorate/hydroxyl-terminated polybutadiene propellant sandwiches

The gas-phase flame structure that forms over a lamina of hydroxyl-terminated polybutadiene (HTPB) binder (50 to 450 μm) sandwiched between layers of ammonium perchlorate (AP) has been investigated through measurements of the surface regression rate of the interface region, ultraviolet emission and transmission imaging of the gas-phase flame zone, and numerical simulations using simplified kinetics. The results show that over the observed pressure range (0.2 to 3.2 MPa), for binder widths from 100 to 450/im, the regression rate is primarily a function of pressure (r_b ∼ p⁰⁴) and is relatively independent of binder width. At binder widths below 100 μm, the regression rate is reduced, presumably due to locally lean conditions created by diffusive mixing. The imaging results suggest the existence of relatively UV-inactive leading-edge flames that form on the AP side of the interfaces adjacent to the surface and probably dominate the regression rate. These leading-edge flames are followed by a relatively UV-active secondary flame region downstream. Both the leading-edge and secondary flames show a tendency to merge into a single flame for thin binder and at lower pressures (low Peclet and Damkohler numbers), and to split into two distinct flames for thick binder (Pe > 2) and at higher pressures (high Da). The modeling results show that simplified kinetics representing the AP monopropellant flame and the AP-binder non-premixed flame, in the framework of a species and energy transport analysis, can predict surface heat feedback distributions that correlate reasonably well with observed surface profiles.