CMDBS

CMDB Composite modified double-base (propellant)... 

Since the energetics of nitropolymer propellants composed of NC-NG or NC-TMETN are limited due to the limited concentration of oxidizer fragments, some crystalline particles are mixed within these propellants in order to increase the thermodynamic energy or specific impulse. The resulting class of propellants is termed composite-modified double-base (CMDB) propellants . The physicochemical properhes of CMDB propellants are intermediate between those of composite and double-base propellants, and these systems are widely used because of their great potential to produce a high specific impulse and their flexibility of burning rate. 

Though the physical structures of CMDB propellants are heterogeneous, similar to those of composite propellants, the base matrix used as a binder burns by itself and the combushon mode of CMDB propellants appears to be different from that of composite propellants and double-base propellants. The burning rate ofa CMDB propellant is dependent on the type of crystalline parhcles incorporated. 

When crystalline AP particles are mixed with nitropolymers, ammonium perchlorate composite-modified double-base (AP-CMDB) propellants are formulated. A nitropolymer such as NC-NG or NC-TMETN double-base propellant acts as a... 

Fig.4.24 Specific impulse and adiabatic flame temperature of AP-CMDB propellants.

When nitramine particles such as HMX or RDX particles are mixed with a doublebase propellant, nitramine composite-modified double-base propellants are formulated. Since HMX and RDX are stoichiometrically balanced materials, the use of these nitramine particles leads to a somewhat different mode of combustion as compared to AP-CMDB propellants. Since each nitramine particle can burn independently of the base matrix at the burning surface, a monopropellant flamelet is formed in the gas phase from each particle. The monopropellant flamelet diffuses into the reactive gas of the base matrix above the burning surface and a homogeneously mixed gas is formed. 

Fig. 4.25 Specific impuise and adiabatic flame temperature of HMX-CMDB propellant.

Triple-base propellants are made by the addition of crystalUne nitroguanidine (NQ) to double-base propellants, similar to the way in which nitramine is added to CMDB propellants as described in the preceding section. Since NQ has a relatively high mole fraction of hydrogen within its molecular structure, the molecular mass of the combustion products becomes low even though the flame temperature is reduced. Table 4.13 shows the chemical composition, adiabatic flame temperature, and thermodynamic energy,/ as defined in Eq. (1.84), of a triple-base propellant at 10 MPa (NC 12.6% N). 

Since the energy contained within double-base propellants is limited because of the limited energies of nitrocellulose (NC) and nitroglycerin (NG), the addition of ammonium perchlorate or energetic nitramine particles such as HMX and RDX increases the combustion temperature and specific impulse. Extensive experimental studies have been carried out on the combustion characteristics of composite-modified double-base (CMDB) propellants containing AP, RDX or HMX parhclesli- l and several models have been proposed to describe the burning rates of these pro-... 

When large spherical AP particles dg = 3 mm) are added, large flamelets are formed in the dark zone.Pl Close inspection of the AP particles at the burning surface reveals that a transparent bluish flame of low luminosity is formed above each AP particle. These are ammonia/perchloric acid flames, the products of which are oxidizer-rich, as are also observed for AP composite propellants at low pressures, as shown in Fig. 7.5. The bluish flame is generated a short distance from the AP particle and has a temperature of up to 1300 K. Surrounding the bluish flame, a yellowish luminous flame stream is formed. This yellowish flame is produced by in-terdiffusion of the gaseous decomposition products of the AP and the double-base matrix. Since the decomposition gas of the base matrix is fuel-rich and the temperature in the dark zone is about 1500 K, the interdiffusion of the products of the AP and the matrix shifts the relative amounts towards the stoichiometric ratio, resulting in increased reaction rate and flame temperature. The flame structure of an AP-CMDB propellant is illustrated in Fig. 8.1. 

When the mass fraction of AP is increased, the dark zone of the base matrix is eliminated almost completely and the luminous flame approaches the burning surface as shown in Fig. 8.2(a). For reference, the flame structure of an RDX-CMDB propellant is also shown in Fig. 8.2(b). Since RDX is a stoichiometrically balanced... 

Fig. 8.2 Flame photographs of AP-CMDB propellant (a) and RDX-CMDB propellant (b) showing that the luminous flame front of the RDX-CMDB propellant is distended from the burning surface ...

Region (2) is assumed to regress at approximately the same rate as the AP particles, as given by Eq. (8.1). Thus, the burning rate of an AP-CMDB propellant is expressed as the fractionally weighted sum of the two different regression rates ... 

Fig. 8.4 Computed and experimental burning rates of AP-CMDB propellants.

Fig. 8.7 shows the adiabatic flame temperatures and the heats of explosion of HMX-CMDB propellants as a function of KNOj) under conditions of thermal equilibrium. The adiabatic flame temperatures, Tg, and the heats of explosion increase... 

Fig. 8.6 Temperature profiles in the combustion wave structures of an RDX-CMDB propellant at different pressures.

Fig. 8.7 Adiabatic flame temperatures of HMX-CMDB propellants as a function of (N02) or heat of explosion.

Fig.8.9 Burning rates of HMX-CMDB propellants containing different mass fractions of HMX.

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