MECHANICAL PROPERTIES OF COMPOSITE PROPELLANTS AND EFFECT OF PROPELLANT STRETCH ON ITS BURN RATE

Propellant stretching in combustion leads to an essential increase (up to 50% and more) in burn rate. Analysis of existing research regarding the influence of stretch deformations on propellant burn rate is carried out. It is shown that for composite solid propellants, the basic mechanism of increase in the burn rate under stretch conditions is detachment of the binder from particles of oxidizers, formation of additional burning surfaces, and change of structure in the burn zone. For a description of this effect, a rheological model of composite solid propellant is developed. The model takes into account a detachment of the binder from particles (oxidizers, coolers, metals, etc.) under action of stretch deformations. The criterion describing distinction in behaviors of propellant in stretch conditions with detachment of the binder from particles in comparison with a stretch of the same material without occurrence of internal defects is established. A method for experimental determination of the amount of defects arising in the propellant upon stretching is suggested based on analysis of the diagram of material stretch. A mathematical model of combustion of composite propellant under stretch conditions, taking into account a detachment of the binder from oxidizer particles, and the formation of an additional burning surface is developed. The model considers the heterogeneous structure of the propellant, the burn surface, and the gas phase. Analysis of ignition of the cracks formed on the binder-particle border as a result of detachment of the binder from particles in propellant stretch is carried out. When a burn rate shows little decrease with propellant stretch, an inverse effect can be observed under certain conditions. This effect is connected with incomplete burn-out of oxidizer particles on the propellant burn surface as a result of their detachment from the surface. Parametrical research of composite pro- pellant combustion under the action of stretch deformations is carried out. Results of calculations are compared with well-known experimental data. Correlation between changes in propellant burn rate under stretch conditions and the parameters of the propellant stretch diagram without combustion is established. A method is developed that allows for predicting the change in propellant burn rate under stretch conditions by using the diagram of propellant stretch without combustion.