AP Composite Propellants

Fig. 1. The postulated flame stmcture for an AP composite propellant, showing A, the primary flame, where gases are from AP decomposition and fuel pyrolysis, the temperature is presumably the propellant flame temperature, and heat transfer is three-dimensional followed by B, the final diffusion flame, where gases are O2 from the AP flame reacting with products from fuel pyrolysis, the temperature is the propellant flame temperature, and heat transfer is three-dimensional and C, the AP monopropellant flame where gases are products from the AP surface decomposition, the temperature is the adiabatic flame temperature for pure AP, and heat transfer is approximately one-dimensional. AP = ammonium perchlorate.

Fig.4.n Rocket flight trajectories assisted by (a) an NC-NG double-base propellant and (b) an aluminized AP composite propellant. 

When aluminized AP composite propellant burns, a high mole fraction of aluminum oxide is produced as a combustion product, which generates visible smoke. If smoke has to be avoided, e. g. for miUtary purposes or a fireworks display, aluminum particles cannot be added as a component of an AP composite propellant In addition, a large amount of white smoke is produced even when non-aluminized AP composite propellants bum. This is because the combustion product HCl acts as a nucleus for moisture in the atmosphere and relatively large-sized water drops are formed as a fog or mist This physical process only occurs when the relative humidity in the atmosphere is above about 60%. If, however, the atmospheric temperature is below 260 K, white smoke is again formed because of the condensation of water vapor with HCl produced as combustion products. If the HCl smoke generated by AP combustion cannot be tolerated, the propellant should be replaced with a double-base propellant or the AP particles should be replaced with another... 

Azide polymers such as GAP and BAMO are also used to formulate AP composite propellants in order to give improved specific impulses compared with those of the above-mentioned AP-HTPB propellants. Since azide polymers are energetic materials that burn by themselves, the use of azide polymers as binders of AP particles, with or without aluminum particles, increases the specific impulse compared to those of AP-HTPB propellants. As shown in Fig. 4.15, the maximum of 260 s is obtained at (AP) = 0.80 and is approximately 12 % higher than that of an AP-HTPB propellant because the maximum loading density of AP particles is obtained at about (AP) = 0.86 in the formulation of AP composite propellants. Since the molecular mass of the combustion products. Mg, remains relatively unchanged in the region above (AP) = 0.8, decreases rapidly as (AP) increases. 

As in the case of double-base propellants, various types of materials, such as plasticizers, burning rate modifiers, and combustion instability suppressants, are added to mixtures of AP and a binder. Table 4.12 shows the materials used to formulate AP composite propellants. 

Table 4.12 Chemical materials used to formulate AP composite propellants.

Nitramine composite propellants composed of HMX or RDX particles and polymeric materials offer the advantages of low flame temperature and low molecular mass combustion products, as well as reduced infrared emissions. The reduced infrared emissions result from the elimination of COj and H2O from the combustion products. To formulate these composite propellants, crystalline nitramine monopropellants such as HMX or RDX are mixed with a polymeric binder. Since both HMX and RDX are stoichiometrically balanced, the polymeric binder acts as a coolant, producing low-temperature, fuel-rich combustion products. This is in contrast to AP composite propellants, in which the binder surrounding the AP particles acts as a fuel to produce high-temperature combustion products. 

The temperature in the condensed phase increases from the initial propellant temperature, Tq, to the burning surface temperature, Tj, through conductive heat feedback from the burning surface. Then, the temperature increases in the gas phase because of the exothermic reaction above the burning surface and reaches the final combustion temperature, Tg. Since the physical structure of AP composite propellants is highly heterogeneous, the temperature fluctuates from time to time and also from location to location. The temperature profile shown in Fig. 7.2 thus illustrates a time-averaged profile. This is in a clear contrast to the combustion wave... 

Fig. 7.2 An overall view of the reaction process and the temperature profile in the combustion wave of an AP composite propellant.

Structure for double-base propellants shown in Fig. 6.3. Thus, the burning rate of AP composite propellants depends largely on the particle size of AP.F b the mass fraction of AP, and the type of binder used.F.i i... 

In order to clarify the combustion wave structure of AP composite propellants, photographic observations of the gas phase at low pressure are very informative. The reaction rate is lowered and the thickness of the reaction zone is increased at low pressure. Fig. 7.3 shows the reduced burning rates of three AP-HTPB composite propellants at low pressures below 0.1 MPa.FI The chemical compositions of the propellants are shown in Table 7.1. The burning rate of the propellant with the composition ap(0-86) is higher than that of the one with ap(0-80) at constant pressure. However, the pressure exponents are 0.62 and 0.65 for the ap(0-86) and Iap(0.80) propellants, respectively that is, the burning rate is represented by r for the p(0.86) propellant and by r for the p(0.80) propellant. 

Fig. 7.14 shows the effect of the AP particle size on the burning rate of mono modal AP composite propellants.1 1 The burning rate increases as the AP particle size, do, decreases. However, the effect of the particle size on the burning rate diminishes with increasing pressure. Though the propellants shown in Fig. 7.14 are all fuel-rich, with ap(0-65), the effect of AP particle size on burning rate is clearly evident. 

The burning rates of AP composite propellants are not only dependent on the AP particles, but also on the binder used as a fuel component. There are many types of binders, with varying physicochemical properties, as described in Section 4.2. The... 

Various types of binders are used to formulate AP composite propellants. Binders such as HTPB and HTPE decompose endothermically or exothermically at the burning surface. The burning rates of AP composite propellants thus appear to be dependent on the thermochemical properties of the binders used. Figs. 7.17 and 7.18 show In r versus In p plots for AP composite propellants made with five differ-... 

Table 7.2 Chemical compositions of AP composite propellants % by mass).

The burning rate of propellants is one of the important parameters for rocket mo-tordesign. As described in Section 7.1.2, the burning rate of AP composite propellants is altered by changing the particle size of the AP used. The diffusional mixing process between the gaseous decomposition products of the AP particles and of the polymeric binder used as a fuel component determines the heat flux feedback from the gas phase to the condensed phase at the burning surface. - This process is a... 

As shown in Fig. 7.20, the burning rate of the AP composite propellant is increased approximately twofold by the addihon of 1 % BEFP. In general, the degree of the burning rate increase is proportional to the amount of catalyst added when the catalyst conshtutes less than about 3 % of the total mass, and the effect of the catalyst addihon shows saturation behavior at about 5% by mass. Fig. 7.23 shows the burning rates of AP-HTPB composite propellants composed of ap(0-80) and... 

Fig. 7.24 Burning rate of an n-HC-cat-alyzed AP composite propellant, showing that the burning rate is increased drastically but that the pressure exponent remains unchanged by the addition of the catalyst.

Fig. 7.27 Burning rates of LiF-catalyzed AP composite propellants, showing that the burning rate decreases and the pressure of self-inter-mption increases with increasing concentration of LiF.

Fig. 7.28 Temperature gradients in the gas phase just above the burning surfaces of non-catalyzed and 0.5% LiF-catalyzed AP composite propellants.

Fig. 7.29 Scanning electron microphotographs of quenched AP composite propellant burning surfaces without LiF (a) and with 0.5% LiF (b), obtained by a pressure decay from 2 MPa to 0.1 MPa the width of each photograph is 0.60 mm.

Fig. 7.34 Flame photographs of an AP composite propellant (a) and an RDX composite propellant (b) showing that the luminous flame front of the RDX composite propellant is distended from the burning surface ...

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