Experimental and numerical approach to the study of the frequency response of solid propellants

The experimental study of the frequency response of burning solid propellants has been done using, as external forcing energy source, a CO$_2$ laser (60 W, 10.6 $\mu$m). The laser radiant flux intensity was sinusoidally modulated and the response of the burning propellant was detected by measuring the recoil force generated by the gases coming out from the burning surface using a strain-gauge load cell which can operate inside the combustion chamber at the operating pressure. The tests were performed in the subatmospheric pressure range, and a composite propellant (AP.HTPB/86.14) was used. The combustion chamber was filled by inert gas (N$_2$), and for each working pressure several tests were carried out at different radiant flux frequency modulations in the range from 5 to 50 Hz. The results evince that the recoil force amplitude depends on the forcing laser frequency with a maximum for every working pressure. This experimental data set was then used to compare the nonlinear frequency response curves obtained by numerical integration of the combustion model equations. Comparisons between experimental and numerical results at 0.3 and 0.5 atm are shown, and the general trend, obtained by numerical simulations, of the propellant frequency response vs pressure in a broader range is presented and discussed.