Investigations of aluminum behavior and its role using laminate propellants

In AP composite propellants, the composition and structure of the fuel-rich matrix ("pocket") between coarse AP particles affect combustion behavior. Laminates of ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) propellants intermixed with powdered aluminum, nominally 15 um, at 15% and 31% loading, by mass, are used to study the behavior of aluminum in AP composite propellants where the AP is characterized by two disparate length scales, fine (2 μm) and coarse (hundreds of μm). The AP laminate takes the place or plays the role of the coarse AP particles but simplifies the system by creating a nearly 2-D, steady outer (canopy) flame structure. Aluminized oxidizer/hydrocarbon mass ratios of 60/40 and 76/24 are tested in two configurations: matrix and laminate (a matrix layer between coarse AP slabs). A high speed camera images experiments at pressures from 3 atm to 30 atm. In matrix only formulations, the higher oxidizer containing propellants exhibit a transition from burn to no-burn as pressure increases whereas when the oxidizer/fuel ratio is reduced, the oxidizer/hydrocarbon matrix burns leaving behind unburned, unagglomerated aluminum. Laminate investigations indicate a non-protruding fuel layer for 76/24 formulations whereas protrusion occurs in 60/40 formulations. An increase in aluminum agglomerate diameter is observed in 60/40 laminates as compared with 76/24 laminate formulations. Burn rate data for varying pressures and fuel layer thicknesses is also examined.