Unsteady combustion of homogeneous energetic solids using the laser-recoil method

The laser-recoil technique was used to study the unsteady burning of a fine oxidizer AP-HTPB composite propellant (APF series) and a catalyzed double-base propellant (N5) at one atmosphere. Steady burning rate and temperature measurements were performed and quasi-steady, homogeneous, one-dimensional (QSHOD) theory implemented in order to interpret the unsteady results. The frequency response of the fine oxidizer AP-HTPB composite propellant exhibited two peaks that were shown to correspond to the condensed phase thermal layer and the condensed-phase reaction zone for the low- and high-frequency peaks, respectively. Several other factors were considered and eliminated as possible causes of the two peaks. For the fine oxidizer AP-HTPB composite propellant, at these conditions, the assumption of a quasi-steady surface reaction zone was clearly violated at frequencies as low as 60 Hz. The effect of mean radiant flux level on the frequency response was also investigated for both APF and N5 propellants. N5 showed a pronounced steady-state burning rate plateau with radiant flux (similar to that for pressure) with corresponding effects exhibited in the frequency response. The results of this work show that detailed information can be obtained using the laser-recoil method that clarifies the structure and dynamics of burning solids. Further, the results suggest that more detailed models that relax the quasi-steady surface reaction zone assumption should be developed.