Dynamic burning rate measurements of metalized composite propellants using the laser-recoil technique

Abstract

The combustion mechanisms of metalized AP/HTPB/Al composite propellants have been investigated using a newly developed pulsed laser-recoil technique to measure dynamic propellant burning rate. The results demonstrate that aluminum has a stronger influence on the combustion of AP/HTPB propellants than is indicated by the small changes observed in the macroscopically steady burning rate or the mean burning rate during macroscopically unsteady burning. The addition of 18% Al in AP/HTPB propellant causes only a 10% increase in the mean burning rate at 1 atm pressure but a 300% increase in the amplitude of the laser-perturbed fluctuating component of the burning rate at 40Hz. The prominent resonance observed at 40 Hz does not correspond to the classical thermal conduction-relaxation mechanism but appears to be related to the processes of unsteady accumulation, agglomeration, ignition, and departure of aluminum at the burning surface of the propellant. It was also found that a small quantity (1%) of chopped Kevlar fibers effectively damped the 40 Hz resonance and increased the steady burning rate at pressure below 5.4 MPa (800 psia). These observations are attributed to a flame-holding effect of the Kevlar fibers which protrude through the surface of the propellant and anchor the partially premixed, leading edge flame near the propellant surface.