Combustion in Solid Propellant Grain Defects. A Study of Burning in Single- and Multi-Pore Charges

摘要

The trend toward very high solids loading in high energy propellants leads to complexities in processing and to greater potential for ballistic anomalies. Present day high energy propellants are often more frangible than their predecessors and physical properties are now found, in many cases, to exert an inordinate effect on combustion. New failure modes are believed to be able to contribute to deflagration-to-detonation transition (DDT). For example, crumbling of a burning grain due to impact or shear represents vastly increased surface area which, if ignited, could lead to detonation. Combustion of porous or frangible charges is therefore of interest to the propulsion community. This study reduces the problem of convective combustion in porous charges and other grain defects to its ultimate simplicity (viz., the single pore), and then proceeds to reconstruct the more complicated geometrical and combustion situations leading to the porous bed or highly cracked (i.e., spiderweb) propellant. Flashdown (one of the more dramatic types of convective combustion), may be defined as runaway combustion rates (and pressure) associated with flame propagation into grain defects such as ducts, fissures, annuli, pores, etc. This type of anomalous combustion leads to flame spreading rates many orders of magnitude greater than normal. Pressures and pressurization rates exceeding design limits are common where such phenomena occur.