Experimental studies on low pressure deflagration limit of ammonium perchlorate with additives

Abstract

This paper deals with the low pressure deflagration limit (LPDL), which is the minimum pressure below which burning of a propellant ceases without the assistance of an external energy source. The LPDL of AP monopropellant reported in literature is around 20 bar and it is reported to shift to different higher pressures with the addition of small fraction of impurities or additives. Most of these additives (such as iron oxide and copper chromite) are known to catalyze AP combustion, but yet their reported LPDL is significantly higher than the LPDL of AP itself. In present study, reasons for this behavior have been explored experimentally. Three additives (each of 1% by mass) namely iron oxide (IO), copper chromite (CC) and activated charcoal (AC) were used. The LPDL of AP with these additives was determined using three methods. Two of these methods (method I and method II) include the effect of ignition dynamics on LPDL while in method III (slow depressurization) ignition dynamics does not play a role. In method I, hot nichrome wire was used for ignition, while in method II, nichrome wire ignition in conjunction with an in-situ propellant was used to ignite the pellets. The effect of convective heat loss on LPDL was studied with the use of silica grease whose role through carefully thought out experimentation has been identified as an insulation on sides of pellet. The LPDL of AP with additives was found to be higher for method I compared to method II. Lastly, method III (slow depressurization) was found to result in lowest LPDL for these additives. The LPDL of pure AP, which has been reported as around 20 bar, was revisited and method III along with the use of silica grease has lowered the LPDL of pure AP to 14 bar.