Investigation of aluminum agglomeration in AP/HTPB composite propellants

Aluminum flame conditions, burning surface structure, and surface regression rate of composite propellant laminates of aluminum powder, fine and coarse ammonium perchlorate (AP), and hydroxyl-terminated polybutadiene (HTPB) are experimentally investigated. An aluminum matrix consisting of 15 μm Al powder mixed with fine-AP/HTPB (76:24) is sandwiched between two AP slabs. The Al matrix, or fuel, thickness ranges from 0.5 mm to 1.2 mm. The laminate sandwiches are burned in a combustion chamber with a nitrogen shroud at pressures of approximately 4 atm and 21 atm. Observation of aluminum agglomerates including separation from the burning propellant surface is conducted using a high-speed, digital video camera. Once separated from the surface, the burning agglomerates suggest the location and structure of the stoichiometric flame. A backlight is used to provide clear definition of the propellant surface profile. Preliminary results indicate a merged (premixed) flame at low pressures transitioning to a split (diffusion) flame at higher pressures.