Ignition of AP-based composite solid propellants containing nitramines exposed to CO 2 laser radiation at subatmospheric pressures

Abstract

Ignition characteristics of AP-based composite solid propellants that contained either HMX or RDX, were investigated using a CO 2 laser at relatively low heat flux and subatmospheric pressures, in order to simulate space environments and also simplify our understanding of the nature of ignition phenomena. The results are presented graphically as a function of surface heat flux and ambient pressure. The ignition process was found to be roughly divided into two regions of self-sustaining ignition (S.S.I.) and non-S.S.I., depending on whether the ignition reaction is self-sustained or extinguished when the external radiation is interrupted after ignition. Furthermore, the non-S.S.I. was composed of two regions of radiation-assisted combustion (R.A.C.) and radiation-assisted unstable combustion (R.A.U.). The addition of nitramines, in particular, HMX to AP-based composite propellants resulted in a marked reduction in the ignition times and made the self-sustaining ignition region shift to lower heat fluxes at each specific pressure. It was found, from the results of differential thermal analysis and thermogravimetry, that nitramine-containing propellants exhibit an exothermic decomposition at relatively low temperatures that is independent of the presence of other ingredients. This triggers the ignition reaction of the AP-based propellants and lowers temperature of the main exotherm. Consequently, it appears that the improvement of ignitability by the addition of nitramines is due to the intense low-temperature exothermic reaction of nitramines. It was found from the time-temperature histories measured with fine thermocouples on the surface during the ignition process that ignition temperatures also are lowered by the addition of nitramines. Howevver, when a propellant sample containing nitramines was exposed to higher heat flux levels, the shorter ignition times reduced self-sustaining ignitability slightly, presumably because these propellants could not form a sufficiently thick thermal layer for steady-state burning.