RDX/AP composite propellant

When some portion of the AP particles contained within an AP composite propellant is replaced with nitramine particles, an AP-nitramine composite propellan-tis formulated. However, the specific impulse is reduced because there is an insufficient supply of oxidizer to the fuel components, i. e., the composition becomes fuel-rich. The adiabatic flame temperature is also reduced as the mass fraction of nitramine is increased. Fig. 7.49 shows the results of theoretical calculations of and Tf for AP-RDX composite propellants as a function of Irdx- Th propellants are composed of jjxpb(0-13) and the chamber pressure is 7.0 MPa with an optimum expansion to 0.1 MPa. Both I p and T)-decrease with increasing Irdx- The molecular mass of the combustion products also decreases with increasing Irdx due to the production of Hj by the decomposition of RDX. It is evident that no excess oxidizer fragments are available to oxidize this H2. 

The burning rates of AP-RDX composite propellants are dependent on the physicochemical properhes of the AP, RDX, and fuel used, such as particle size, as well as on mixture raho and the type of binder. The results of burning rate measurements are reported in AlAA Paper No. 81-1582.125] Various combinahons of AP and RDX parhcles are used to formulate AP-RDX composite propellants, as shown in Table 7.6.125] pjjg particles incorporated into the propellants have bimodal combinations of sizes, where large RDX particles (RDX-I), small RDX particles (RDX-S), large AP particles (AP-I), and small AP particles (AP-S) are designated by d, d, dj, and da, respectively. HTPB binder is used in all of the propellants shown in Table 7.6. 

Fig. 7.53 Burning rate characteristics of AP-RDX composite propellants composed of coarse RDX and fine AP particles.

Fig. 7.55 Effects of particle size and mixture ratio of AP and RDX on the burning rates of AP, RDX, and AP-RDX composite propellants.

The addition of aluminum powder to AP-nitramine composite propellants increases the specific impulse, as in the case of AP composite propellants. Fig. 7.50 shows the theoretical fp and 7 -values for AP-RDX composite propellants containing as a function of The propellants are composed of... 

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 ...

The combustion wave structure of RDX composite propellants is homogeneous and the temperature in the solid phase and in the gas phase increases relatively smoothly as compared with AP composite propellants. The temperature increases rapidly on and just above the burning surface (in the dark zone near the burning surface) and so the temperature gradient at the burning surface is high. The temperature in the dark zone increases slowly. However, the temperature increases rapidly once more at the luminous flame front. The combustion wave structure of RDX and HMX composite propellants composed of nitramines and hydrocarbon polymers is thus very similar to that of double-base propellants composed of nitrate esters.[1 1... 

Fig. 7.58 Burning rate characteristics of AP, AP-RDX, and RDX composite propellants (HTPE).

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. 

Since rocket propellants are composed of oxidizers and fuels, the specific impulseis essenhally determined by the stoichiometry of these chemical ingredients. Ni-tramines such as RDX and HMX are high-energy materials and no oxidizers or fuels are required to gain further increased specific impulse. AP composite propellants composed of AP particles and a polymeric binder are formulated so as to make the mixture ratio as close as possible to their stoichiometric ratio. As shown in Fig. 4.14, the maximum is obtained at about p(0.89), with the remaining fraction being HTPB used as a fuel component. 

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-... 

Fig. 12.21 Mole fractions of the combustion products formed by AP-HTPB and RDX-HTPB composite propellants.

Fig. 12.21 shows the combustion products of AP-HTPB and RDX-HTPB composite propellants. Large amounts of H2O, HCl, and CO2 are formed when an AP-HTPB propellant composed of a.p(0.85) is burnt. The molecules of H2O, HCl, and CO2 each emit infrared radiation. On the other hand, no COj or C(g) is formed when an RDX-HTPB propellant composed of ri3x(0.85) is burnt. Instead, large amounts of CO, H2, and Nj molecules are formed as its major combustion products. However, no infrared radiation is emitted from H2 or N2 molecules. Though CO molecules are formed at ri3x(0.85), the infrared radiation emitted from these is less than that from H2O or CO2 molecules. 

Fig. 13.18 shows DC and AC pressure curves for an RDX-AP composite propellant having a six-pointed-star geometry burning at 9 M Pa. The combustion sud-... 

Fig. 13.21 shows another example of oscillatory burning of an RDX-AP composite propellant containing 0.40% A1 particles. The combustion pressure chosen for the burning was 4.5 MPa. The DC component trace indicates that the onset of the instability is 0.31 s after ignition, and that the instability lasts for 0.67 s. The pressure instability then suddenly ceases and the pressure returns to the designed pressure of 4.5 MPa. Close examination of the anomalous bandpass-filtered pressure traces reveals that the excited frequencies in the circular port are between 10 kHz and 30 kHz. The AC components below 10 kHz and above 30 kHz are not excited, as shown in Fig. 13.21. The frequency spectrum of the observed combustion instability is shown in Fig. 13.22. Here, the calculated frequency of the standing waves in the rocket motor is shown as a function of the inner diameter of the port and frequency. The sonic speed is assumed to be 1000 m s and I = 0.25 m. The most excited frequency is 25 kHz, followed by 18 kHz and 32 kHz. When the observed frequencies are compared with the calculated acoustic frequencies shown in Fig. 13.23, the dominant frequency is seen to be that of the first radial mode, with possible inclusion of the second and third tangential modes. The increased DC pressure between 0.31 s and 0.67 s is considered to be caused by a velocity-coupled oscillatory combustion. Such a velocity-coupled oscillation tends to induce erosive burning along the port surface. The maximum amplitude of the AC component pressure is 3.67 MPa between 20 kHz and 30 kHz. - ...

Fig. 13.21 DC- and AC-pressure curves of the combustion instability of an RDX-AP composite propellant containing 0.4% Al particles.

Fig. 13.26 Stable and unstable combustion regions for an RDX-AP composite propellant.

Kubota, N., Yano, Y, and Kuwahara, T, Particulate Damping of Acoustic Instability in RDX/AP Composite Propellant Combustion, AlAA-82-1223, AlAA, New York (1982). 

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