Oscillatory combustion of fine-AP/HTPB propellants: Disproportionate pyrolysis response

The combustion of hydroxyl-terminated polybutadiene (HTPB) propellants containing fine ammonium perchlorate (AP) was investigated using laser-excited, combustion recoil at 1-5 atm. At 1 atm, binder-rich (70-75% AP), monomodal fine-AP (2-50 μm) propellants exhibit a prominent non-quasi-steady gas and surface reaction zone, homogeneous propellant, one-dimensional flame 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) fluctuations that occur in the fuel-rich, premixed gas-phase reaction zone adjacent to the fine-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 fine-AP propellants and corroborate the disproportionate pyrolysis interpretation of the laser-excited resonant response. This finding of a strong disproportionation response in composite propellants with fine AP or binder-rich, fine-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.