Physical And Chemical Processes

The combustion in a solid propellant motor involves exceedingly complex reactions taking place in the solid, liquid, and gas phases of a heterogeneous mixture. Not only are the physical and chemical processes occurring during solid propellant combustion not fully understood, but the available analytical combustion models remain oversimplified and unreliable. Experimental observations of burning propellants show complicated three-dimensional micro-

structures, a three-dimensional flame structure, intermediate products in the liquid and gaseous phase, spatially and temporally variant processes, aluminum agglomeration, nonlinear response behavior, formation of carbon particles, and other complexities yet to be adequately reflected in mathematical models.

Some insight into this combustion process can be gained by understanding the behavior of the major ingredients, such a ammonium perchlorate, which is fairly well explored. This oxidizer is capable of self-deflagration with a low-pressure combustion limit at approximately 2 MPa, the existence of at least four distinct "froth" zones of combustion between 2 and 70 MPa, the existence of a liquid froth on the surface of the crystal during deflagration between 2 and 6 MPa, and a change in the energy-transfer mechanism (particularly at about 14 MPa). Its influence on combustion is critically dependent on oxidizer purity. The surface regression rate ranges from 3 mm/sec at 299 K and 2 MPa to 10 mm/sec at 423 K and 1.4 MPa.

The various polymeric binders used in composite propellants are less well characterized and their combustion properties vary, depending on the binder type, heating rate, and combustion chamber pressure.

The addition of powdered aluminum (2 to 40 jam) is known to favorably influence specific impulse and combustion stability. Photography of the burning aluminum particles shows that the particles usually collect into relatively large accumulaties (100 or more particles) during the combustion process. The combustion behavior of this ingredient depends on many variables, including particle size and shape, surface oxides, the binder, and the combustion wave environment. Ref. 13-1 describes solid propellant combustion.

Visual observations and measurements of flames in simple experiments, such as strand burner tests, give an insight into the combustion process. For doublebase propellants the combustion flame structure appears to be homogeneous and one-dimensional along the burning direction, as shown in Fig. 13-1. When heat from the combustion melts, decomposes, and vaporizes the solid propellant at the burning surface, the resulting gases seem to be already premixed. One can see a brilliantly radiating bright flame zone where most of the chemical reaction is believed to occur and a dark zone between the bright flame and the burning surface. The brightly radiating hot reaction zone seems to be detached from the combustion surface. The combustion that occurs inside the dark zone does not emit strong radiations in the visible spectrum, but does emit in the infrared spectral region. The dark zone thickness decreases with increasing chamber pressure, and higher heat transfer to the burning surface causes the burning rate to increase. Experiments on strand burners in an inert nitrogen atmosphere, reported in Chapter 1 of Ref. 13-1, show this dramatically: for pressures of 10, 20, and 30 atm the dark zone thickness is 12, 3.3, and 1.4 mm, respectively, and the corresponding burning rates are 2.2, 3.1, and 4.0 mm/sec. The overall length of the visible flame becomes shorter as the chamber pressure increases and the heat release per unit volume near the surface also increases. In the bright, thin fizz zone or combustion zone directly over the burning surface of the DB propellant, some burning and heat release occurs. Beneath

Typical temperature (°C)

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