No abstract

The contribution of pressure-sensitive interfacial reactions to the acoustic admittance of composite solid propellants has been studied experimentally and theoretically. Using a carboxy-terminated poly butadiene-ammonium perchlorate propellant, the effects of various coatings on the oxidizer crystals were investigated with a T-burner over a range of frequencies from 150 to 5000 cps. The results show that, of the coatings used (Kel-F 800, Hypalon 30, Viton A, and ethyl-cellulose), only the Kel-F and Viton reduced the apparent reactivity of the propellant binder-oxidizer interface sufficiently to produce a significant reduction in the maximum value of the observed acoustic admittance. These effects are consistent with the greater resistance to oxidation of Kel-F and Viton A compared to the other coating materials and the basic propellant binder. However, all of the coatings produced a significant shift in the acoustic frequency at which the maximum admittance is observed, even when burning rate changes are considered. Theoretical studies were conducted using a combustion model that incorporates the effects of pressure-dependent surface reactions, pressure-dependent gas-phase heat transfer, and surface pyrolysis reactions. The predicted acoustic admittance frequency relation is characterized by three independent parameters using a perturbation approach. Parametric studies reveal that high ratios of the maximum acoustic response function to the burning rate pressure exponent are predicted when the net heat release at the propellant surface (pressure-dependent reactions plus pyrolysis reactions) is nearly zero or is exothermic. The stability of the combustion process in self-excited modes has also been considered theoretically and the stability bounds determined as a function of combustion parameters. Nomenclature ai= I + a 1 c = sonic velocity of the combustion gases EI = activation energy of exothermic surface reactions Ei = activation energy of over-all pyrolysis rate at surface Ez = activation energy of solid-phase combustion reactions Ef = activation energy of gas-phase reactions g , fd =. frequency of the acoustic pressure oscillations at the point where a g and ad were measured FQ = steady-state heat flux from gas-phase combustion zone to solid surface F = instantaneous heat flux from gas-phase combustion zone to solid surface k = thermal conductivity mi =-}[! + (! + s)i"] m l = order of gas-phase reaction n = steady-state burning rate exponent n l -pressure exponent on gas-phase heat flux P = instantaneous pressure PO = steady-state pressure P = amplitude of the acoustic pressure oscillation r = instantaneous linear burning rate of propellant r 0 = steady-state linear burning rate of propellant f = amplitude of the burning rate oscillations R = (/3 La Placiaii variable time temperature steady-state temperature propellant flame temperature initial temperature of the propellant distance into the solid from the surface Arrhenius pre-exponential factor for exothermic surface reactions Arrhenius pre-exponential fac...