Theoretical and experimental study of composite solid propellant combustion

A burning rate law is proposed to relate the overall burning rate of composite solid propellant to the burning rates of individual components and the geometry of the burning surface. The complex three dimensional burning surface structure is simplified into unit cells in order to allow computational investigation of the mixing mechanism near the burning surface. The conservation equations of mass, momentum and mixture fraction have been solved numerically to obtain the velocity and-mixture fraction field above the surface. The averaged mixture fraction on the burning surface of oxidiser and of the binder is correlated with both the surface geometry and Reynolds number. Experimental results for detailed surface structure and burning rate for specially formulated ammonium perchlorate (AP) composite propellant grains have been obtained at different pressure. The experimental results are explained using the averaged mixture fraction on the surface. It is concluded that the combustion of AP composite propellant is strongly influenced by the diffusion process near the surface; the large AP particles burn essentially as that of monopropellant AP at high pressure; the burning rate of the mid-size AP particles is augmented by the diffusion process at all the pressures investigated; and the overall burning rate is controlled by the burning rate of binder and that of mid-size AP particles.