Oscillatory Combustion of Fine-AP/HTPB Propellants: Disproportionate Pyrolysis Response

Abstract

The combustion of hydroxyl-terminated polybutadiene (HTPB)propellants containing e neammonium perchlorate (AP) was investigated using laser-excited, combustion recoil at 1 ‐5 atm. At 1 atm, binder-rich (70‐75% AP), monomodal e ne-AP (2‐50 µm) propellants exhibit a prominent non-quasi-steady gas and surface reaction zone, homogeneous propellant, one-dimensional e ame response peak at 100 ‐300 Hz, with frequency varying inversely with AP size. Adding coarse AP (resulting in a wide, bimodal AP distribution ) causes this response to disappear at 1 atm. Raising the pressure to 2 atm causes the response to reappear and increase in frequency to 600 ‐800 Hz (2-µm AP). This oscillatory combustion behavior is attributed to time-varying selective or disproportionate pyrolysis of AP and HTPB (unsteady accumulation and depletion of AP at the propellant surface ) and the associated compositional (stoichiometric ) e uctuations that occur in the fuel-rich, premixed gas-phase reaction zone adjacent to the e ne-AP/HTPB solid region. A low Peclet number appears to be a requirement of achieving this condition. Combustion recoil and thermocouple measurements at 1 atm without laser excitation exhibited spontaneous oscillations in the monomodal e ne-AP propellants and corroboratethe disproportionate pyrolysis interpretation of the laser-excitedresonant response. This e nding of a strong disproportionationresponsein compositepropellants with e ne AP or binder-rich, e ne-AP matrix regions has important implications for pressure-coupled response and solid rocket motor stability in that the same response mechanism could operate under oscillatory pressure conditions and at elevated pressures.