Measurement and modeling of pressure-driven transient burning of solid propellants

The overall goal of this research is to improve the understanding and predictive capability of combustion-driven instabilities in solid rocket motors. Transient burning rates of solid propellants are not well characterized; better combustion diagnostics and theoretical models are needed. This work covers both of these areas. A new diagnostic technique using ultrasound echo-location for precisely measuring the unsteady burning rate of a solid propellant is described. Also, new methods for modeling transient burning in heterogeneous solid propellants are developed.; In the experimental section of this study, ultrasound is used to measure the burning-rate response of several solid propellants to an oscillatory chamber pressure with a frequency of up to 300 Hz. The technique described here is among the first to make wholesale use of digital signal processing for burning-rate measurement. The data are corrected for compression of the propellant by the chamber pressure. The effects of a changing thermal profile on the measurement are also discussed. Results of the experiments compare favorably to data from two other response function measurement techniques.; In the modeling section of this study, two transient heterogeneous propellant combustion models, applicable to fine oxidizer composite propellants, are examined. The “surface accumulation model” supposes that components accumulate in a layer at the surface. Each component reaches an equilibrium concentration inversely proportional to its burning rate. The “double reaction layer model” supposes that a molten binder layer covers the propellant. The oxidizer gasifies underneath the layer, while the binder gasifies at the surface. The double reaction layer model qualitatively produces features observed in experimental laser-recoil response function data: a sharp resonance peak accompanied by a shift from negative to positive phase. The surface accumulation model does not produce these features. The presence of time delay terms—the time lag is caused by heat conduction through the binder layer—in the double reaction layer model accounts for the sharp resonance peak. The frequency of the peak produced by the model is lower than what is observed in the data; this discrepancy is attributed to uncertainties in the properties of the binder layer.