Effect of gap width on combustion behavior of two opposing propellant surfaces

Gun propellants packed in high loading densities can exhibit significant changes in the combustion process. More specifically, burn rates and flame spreading processes for a propellant charge are not only functions of pressure and initial temperature, but also a function of the loading density (or gap width between adjacent propellants). An experimental study has been conducted to investigate the influence of gap width on propellant regression rates between dual propellant strands. In the experimental setup, two opposing strands of the gun propellant JA2 were burned in a double-ended windowed strand burner. This device consists of two identical linear drive systems adjoined to a center combustion chamber. To measure the gap distance between the two strands a contrasting image between the propellant strands and the background was used. This image was achieved by employing a diodepumped solid-state laser with a wavelength of 532nm for illumination, and a black-and-white CCD camera coupled with a video capture card to record in real time. To obtain a constant gap width between the opposing propellants, the image was imported into a LabVIEW control feedback loop, which triggered the linear drive system. A series of tests was conducted for a pressure range from 0.79 to 3.55 MPa (100 to 500 psig), and a gap distance range from 1 to 12 mm (0.039 to 0.472 in). Comparing regression rate data of JA2 burned in a single strand burner at the same pressure, an increase in the regression rate of about 15-20% was observed when the gap distance was decreased to 2- 3 mm. This implies that forcing the propellant flame to stay within the narrow gap between the two adjacent propellant strands the propellant-burning rate can be enhanced significantly.