Studies into laser ignition of unconfined propellants

Abstract

Prior to laser ignition tests, spectral absorption properties of three different solid motor propellants were analysed. The extruded double base (EDB) propellant exhibited >95% absorption over the 250-550nm wavelength band whereas, the cast double base (CDB) showed similar absorption over a wider band extending between 375-800nm. The composite sample (CP) showed a uniform spectral absorption at about 90 % over 250-800 nm band. Ignition tests using an average of 500 nm output from an Ar-ion laser showed that the double base propellants undergo deflagration prior to ignition due to the presence of carbon black material. Within the laser power density range of 24-125W·cm-2, the threshold laser energy densities for deflagration and ignition in the double base propellant were found to be between 2-2.5 J·cm-2, and 40-215 J·cm-2, respectively. No deflagration was observed for the composite propellant, and the threshold ignition energy was found to be within the range, 11-18 J·cm-2 for the same range of laser power densities. From the ignition map for this propellant, the threshold energy for ignition at this wavelength was found to be approximately 18 J·cm-2 and was practically independent of laser power density. In the near infrared wavelength (780nm) the EDB propellant was not readily ignitable due to its comparatively much higher reflectance at this wavelength. The ignition threshold values were found to be between 19-23 J·cm-2 for a similar power density level. The results indicate that the ignitability of propellants is enhanced through the promotion of deflagration.