Propellants oscillatory burning

Since the pressure exponent of burning rate is less than unity for conventional propellants, a becomes negative, and the burning becomes stable. In the case of n being greater than unity, a becomes positive, and increasingly oscillatory burning may occur. When n is very close to unity, a becomes approximately zero, and co can be determined from the approximation... 

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. - ...

Kubota, N., and Kimura, J., Oscillatory Burning of High Pressure Exponent Double-Base Propellants, AlAA Journal, Vol. 15, No. 1,1977, pp. 126-127. 

Heterogeneous solid propellants possess additional mechanisms with potentials for producing oscillatory burning. For example, certain metalized composite propellants have been observed to burn in the laboratory with identifiable ranges of frequencies of oscillation [118] in this case, the mechanism may involve chemical interactions between the metal and the oxidizer. It was indicated in Section 9.1.5.5 that heterogeneities introduce at least local periodicities and that, in the presence of a mechanism for synchronizing the phases of the oscillations over the surface of the propellant, sustained coherent oscillations of the combustion will occur. A review is... 

OSCILLATORY BURNING IN LIQUID-PROPELLANT ROCKET MOTORS... 

Oscillatory Burning in Liquid-Propellant Rocket Motors... 

A. G. Smith, A Theory of Oscillatory Burning of Solid Propellants Assuming a Constant Surface Temperature, in Solid Propellant Rocket Research, vol. 1 of Progress in Astronautics and Rocketry, M. Summerfield, ed.. New York Academic Press, 1960, 375-392. 

Fig. 13.16 Stable, oscillatory, and unstable burning zones for a lead-catalyzed double-base propellant.

When an energetic material burns in a combustion chamber fitted with an exhaust nozzle for the combustion gas, oscillatory combustion occurs. The observed frequency of this oscillation varies widely from low frequencies below 10 Hz to high frequencies above 10 kHz. The frequency is dependent not only on the physical and chemical properties of the energetic material, but also on its size and shape. There have been numerous theoretical and experimental studies on the combustion instability of rocket motors. Experimental methods for measuring the nature of combustion instability have been developed and verified. However, the nature of combustion instability has not yet been fully understood because of the complex interactions between the combustion wave of propellant burning and the mode of acoustic waves. 

Combustion of propellant in a rocket motor accompanied by high-frequency pressure oscillation is one of the most harmful phenomena in rocket motor operation. There have been numerous theoretical and experimental studies on the acoustic mode of oscillation, medium frequency (100 Hz - 1 kHz) and high frequency (1-30 kHz). The nature of oscillatory combustion instability is dependent on various physicochemical parameters such as the burning rate characteristics, mechanical properties, and energy density of the propellants, and the physical shape and dimensions of the propellant grains 10111. 

---