Using 30 J.1Dl Pt and Pt 13 per cent Rh thermocouples, flame temperatures of uncatalysed and catalysed ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/lffPB) composite solid propellants were measured under subatmospheric conditions. Ferric oxide F e 20 J and copper chromite (CC) were the catalysts used. The study demonstrates that Fe2O3 catalysed propellant, notwithstanding its least combustion efficiency undt;r subatmospheric conditions and weak gas-phase flame, has the maximum burning rate enhancement. This is argued to be due to the increased surface and subsurface reactions caused by Fe2O3. CC-catalysed propellant burns to the least subatmospheric pressure with minimum loss in combustion efficiency indicating that this class of propellant may be more suitable for base-bleed applications.

Subatmospheric combustion characteristics of ammonium perchlorate (AP)/hydroxyl-terminatedpolybutadiene (HTPB) propellants prepared using two different catalyst mixing procedures were studied. In the rst one, identi ed as the wet-mixing process, copper chromite catalyst (CC ) was mixed with the binder (HTPB 1 di-2-ethylhexyl adipate); and to this CC -binder mix, AP and the curing agent [toluene di-isocyanate (TDI)] were then added. In the second, identi ed as the dry-mixing process, CC was rst mixed with AP, and to this dry-mixture, the binder and toluene di-isocyanate were then added. For such propellants of different mixing procedures, with various AP particle sizes, the measurements of burning rates up to 1 bar, low-pressure de agration limit (LPDL), and the scanning electron microscope (SEM) study of extinguished samples at LPDL were carried out. Results of the study show a higher burning rate for dry-mix propellants, and this higher burning rate is AP particle-size and pressure dependent. LPDL is also AP particle-size dependent, and for catalyzed propellant it is lower than that of an uncatalyzed one. The two procedures of CC mixing do not demonstrate any signi cant difference in LPDLs. The micrographs of wet-mix and dry-mix propellants look similar; however, the strong effect of low pressure in increasing the surface and subsurface reactions is brought out by the SEM study. Table 1 Propellant ingredients Ingredients Mass fractions a AP -oxidizer 0.7300 HTPB -binder 0.2160 DOA -plasticizer 0.0405 TDI -curing agent 0.0135 a CC fraction in catalyzed propellant = 0.02; the balance fraction of 0.98 has the same ingredients' mass fractions as shown. IntroductionB ETTER understanding of subatmospheric and low-pressure combustion of solid propellants is necessary for the development of missile base -bleed propellant grains 1 -4 and stop -start solid motors, and for the accurate prediction of early stages of solid motor ignition transients. Further, the subatmospheric combustion of composite solid propellants (by its premixed ame-dominated gas-phase reaction zone, less severe temperature gradient at the burning surface, and better spatial and temporal resolutions), offers for mechanistic study a simpler perspective of the complex phenomenon.Subatmospheric burning rate studies on composite solid propellants were rst reported by Silla. 5 Other low-pressure combustion studies on composite propellants conclude the following:1) Low-pressure de agration limit (LPDL) generally increases as the fuel/oxidizer ratio moves away from the stoichiometric value. 6,7 2) The addition of a catalyst decreases the LPDL, whereas that of a depressant augments it. 83) With a thicker and slower thermal wave the gas phase ame processes are at a larger standoff distance, frequently with the dark zone above the burning surface, a few tenths of a millimeter to a few millimeters thick. 9 -11 4) With the gas-phase ame dominated by the premixed ame processes, the value of a pressure exponent in the burning rate equation r = ap n is higher, say, 0...

The subatmospheric burning with its higher catalytic effectiveness, lower temperature gradient, and slower combustion wave offers a tting environment to study the effect of catalysts on the burning surface temperature of composite propellants. Using platinum and platinum-13% rhodium 7.5-m thermocouples in uncatalyzed as well as copper-chromite-catalyzed ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants, the subatmospheric-burning surface temperatures were measured. The results of the present experimental study are in close agreement with the established trend; the surface temperature increases with the increase in pressure. Some experimental studies of others failed to give an observable change in surface temperature with pressure. This is argued to be because of the dimensional inadequacy of detectors in the very high-temperature-gradient environment. The measured surface temperature of the catalyzed propellant is signi cantly higher than that of the uncatalyzed one. The study shows that the increased surface and subsurface heat release caused by catalyst addition causes this temperature enhancement. Nomenclature c = speci c heat, J/(kg K) p = pressure, bar Q l = heat loss, J/kg Q s = heat release, J/kg r = steady burning rate, m /s T = temperature, K T amb = ambient temperature, K T s = burning surface temperature, K t = time, s x = length or distance, m = thermal diffusivity, W/(m 2 K) = thermal conductivity, W/(m K) = density, kg/m 3 Subscripts amb = ambient g = gas phase p = propellant s = surface or solid phase tra = transition

No abstract