Dark zone of double-base propellant

Metallic nickel is known to be a catalyst in promoting the reduction of NO when NO reacts with aldehyes or hydrocarbon gases1221, since the primary reaction in the dark zone of double-base propellants is NO reduction to N2. The small amounts of fine Ni particles or organic nickel compounds are added to a double-base propellant to increase the reaction rate in the dark zone. 

The flame standoff distance Li defined in Eq. (3.70) decreases as pressure increases, and the pressure exponent of the flame standoff distance is d= -1.9 —2.3 for RDX and HMX propellants. The overall order of the reaction in the dark zone is determined to be 2.5 - 2.8. This is approximately equal to the overall order of the reaction in the dark zone of double-base propellants, m= 2.5, which indicates that the reaction pathway in the dark zone of nitramine composite propellants is approximately equal to that of double-base propellants. 

Thus, the combustion wave structure of double-base propellants appears to showa two-stage gas-phase reaction, taking place in the fizz zone and the dark zone. The thickness of the fizz zone is actually dependent on the chemical kinetics of the... 

Fig. 6.11 Determination of the activation energy in the dark zone at different energy densities of double-base propellants.

The temperature profile in the combustion wave of a double-base propellant is altered when the initial propellant temperature Tq is increased to Tq -i- ATq, as shown in Fig. 6.15. The burning surface temperature is increased to -i- AT, and the temperatures of the succeeding gas-phase zones are likewise increased, that of the dark zone from Tgto Tg-t- ATg, and the final flame temperature from 7 to Tf-t- ATf If the burning pressure is low, below about 1 MPa, no luminous flame is formed above the dark zone. The final flame temperature is Tg at low pressures. The burning rate is determined by the heat flux transferred back from the fizz zone to the burning surface and the heat flux produced at the burning surface. The analysis of the temperature sensihvity of double-base propellants described in Section 3.5.4 applies here. 

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... 

The combustion wave of a double-base propellant consists of the following five successive zones, as shovm in Fig. 6.3 (I) heat conduction zone, (II) soHd-phase reaction zone, (III) fizz zone, (IV) dark zone, and (V) flame zone-l i -i -i ]... 

The thermal structure of the combustion wave of a double-base propellant is revealed by its temperature profile trace. In the solid-phase reaction zone, the temperature increases rapidly from the initial temperature in the heat conduction zone, Tq, to the onset temperature of the solid-phase reaction, T , which is just below the burning surface temperature, T. The temperature continues to increase rapidly from T to the temperature at the end of the fizz zone, T, which is equal to the temperature at the beginning of the dark zone. In the dark zone, the temperature increases relatively slowly and the thickness of the dark zone is much greater than that of the solid-phase reaction zone or the fizz zone. Between the dark zone and the flame zone, the temperature increases rapidly once more and reaches the maximum flame temperature in the flame zone, i. e., the adiabatic flame temperature, Tg. 

The burning rate of a double-base propellant can be calculated by means of Eq. (3.59), assuming the radiation from the gas phase to the burning surface to be negligible. Since the burning rate of a double-base propellant is dominated by the heat flux transferred back from the fizz zone to the burning surface, the reaction rate parameters in Eq. (3.59) are the physicochemical parameters of choice for describing the fizz zone. The gas-phase temperature, Tg, is the temperature at the end of the fizz zone, i. e., the dark-zone temperature, as obtained by means of Eq. (3.60). 

Fig. 6.17 Dark zone temperature, burning surface temperature, surface heat release, and temperature gradient in the fizz zone for high- and low-energy double-base propellants as a function of initial propellant temperature.

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