Influence of AP/HTPB flame structure on aluminum behavior in laminate propellents

High speed video imaging of aluminum and surface flame structure in ammonium perchlorate/hydroxyl-terminated poiybutadlene (AP/HTPB) laminate propellants was used to characterize the influence of AP/HTPB flame structure on aluminum behavior in these oxtdizer/hydrocarbon laminates. Spherical aluminum powder, nominally 15 μm, intermixed with 76/24 (mass ratio) 2 μm AP/HTPB was used as the fuel layer between two AP slabs. Fuel layer thicknesses ranging from 0.25 mm to 1.25 mm and pressures of 3 atm to 30 atm were investigated. Video images indicated a non-protruded surface layer for all conditions tested including thick fuel layers for which non-aluminized laminates showed a protruding fuel layer. Similar to non-aluminized laminates, Increased pressure and fuel layer thickness resulted in a transition in flame structure from merged (premixed) to split (diffusion). Ignition of aluminum occurred in the stoichiometric AP/HTPB diffusion flame where adiabatic flame temperatures were suitable for oxide layer cracking or melting. Due to lack of surface protrusion (which was used for non-aluminized laminates), the criterion for merged vs. split flame AP/HTPB flame structure was based on traces of alumina smoke produced by aluminum ignition and combustion. Laminate burning rate increased with increasing pressure but remained relatively constant with fuel layer thickness. This was in contrast to non-aluminized laminates that showed a clear optimum burning rate at conditions where the diffusion flames merge. Aluminum agglomerate size typically ranged between 25-40 μm.