The mechanism of super-rate burning of catalyzed double base propellants

Abstract

Previous investigators have offered qualitative explanations for the large, pressure-dependent, burning behavior changes seen in nitrate ester propellants when lead or copper salts are added. The most developed qualitative models are those of Camp and of Powling and co-workers but both exhibit certain discrepancies with experiment. New evidence reported here derives from radiation-assisted burning tests in which the spectral content (particularly ultraviolet) of the impinging radiation was varied; contrary to the original hypothesis of Camp, the UV component of the radiation yielded no special burning rate enhancing effect. Experimental evidence recently presented by the authors shows that the burning rate enhancement by lead or copper compounds is a result of acceleration of the fizz zone reactions; this is accompanied by an increased production of carbonaceous material at the burning surface. Therefore it is hypothesized that the acceleration is due to a shift in equivalence ratio toward the stoichiometric when potentially burnable fuel molecules are instead carried through the fizz zone as solid carbon. A simplified mathematical model of this hypothesis is developed based on representing the equivalence ratio change as a shift from a single normal reaction pathway toward a second, more reactive pathway. This hypothesis successfully explains the appearance of a region of enhanced burning rate and its disappearance at higher pressures but it is shown that sudden disappearance (mesa burning), sometimes seen experimentally, requires a further mechanism whose nature is not yet clear.