Depressurization Extinguishment of Composite Solid Propellants: Flame Structure, Surface Characteristics, and Restart Capability

Abstract

During a rapid depressurization, the intensities of several spectral lines and the pressure are measured, simultaneous to the taking of high speed movie photographs of the propellant flame. The quenched surfaces of all samples are analysed with a scanning electron microscope. The 24% carboxyl terminated polybutadiene -+- 76 % 25 μ ammonium perchlorate propellant extinguishes permanently for initial depressurization rates exceeding ca. 7.5 × 10³ atm/s, when the initial pressure is 45 atm. At the highest depressurization rates (>16 × 10³ atm/s), the gaseous flame quenches immediately due to the adiabatic expansion. Lower rates of fall in pressure (7 × 10³ to 16 × 10³ atm/s) allow a second flame to develop after 5–7 ms, the relaxation time of the solid phase. This new flame is irregular and partially consumes the AP particles available on the propellant surface, depending on the imposed dP/dt. The driving mechanism is far from one-dimensional. The new flame goes out only at the end of the pressure transient. The restart capability of the propellant is most favorable when the imposed (dP/dt)₀ is considerably above (twice) that required for permanent extinction. The reduction of pressure is never an exponential function of time throughout the transient. Increased AP particle size of the propellant yields qualitatively similar results. Other methods of extinguishing a propellant flame give quenched surfaces with drastically different structural characteristics. The investigation shows that previous one-dimensional and/or linearized theories of propellant burning can not be valid for the phase where the new flame is developed.