Combustion mechanism of a negative pressure exponent composite solidpropellant

A study has been conducted on the combustion mechanism of the ammonium perchlorate (AP) based polyurethane (PU) composite solid propellant having a negative pressure exponent. Three types of sandwiches were investigated, one with calcium carbonate in the AP, one with the calcium carbonate in the binder, and one with no calcium carbonate. Laser-shadow high-speed cinemicrophotography was used to examine the burning surface and to determine the burning rate of the sandwiches. The surfaces of samples quenched by rapid depressurization were examined in scanning electron microscope. The composition and flow behavior of the molten binder over the AP crystal surface of the propellant were determined by X-ray photoelectron spectroscopy. During combustion, the CaCO3 enhances the burning rate of AP at low pressure ( 1.96 MPa). CaCO3 reacts with AP to produce CaCl2 and promotes melting of AP on the burning surface. The concentration of CaCl2 on the quenched surface increases with increasing pressure. Quenched samples show the molten binder covers part of the AP crystal surface in both the mesa pressure regime and the nonmesa regime. The fraction of the area covered increases with increasing pressure in the mesa pressure regime. Based on the experimental results, the combustion mechanism of the negative pressure exponent propellant is described.