Laser ignition of solid propellants using energetic nAl-PVDF optical sensitizers

Abstract

Laser ignition of propellants could provide better control of the ignition process and improve safety. However, sustained laser ignition of solid propellants is often challenging in practice due to a lack of optical absorptivity at convenient wavelengths. Common optical additives, i.e., opacifiers, improve the absorption characteristics but are typically inert and fail to contribute to the reactivity of the propellant. In this work, ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB) composite propellants are optically sensitized by adding reactive nano-aluminum (nAl)/polyvinylidene fluoride (PVDF) particles (ranging from 600 to 1000 µm). These propellants are compared with neat (no additive) AP/HTPB, and other AP/HTPB propellants with carbon black or nano-aluminum (nAl) additives. The nAl/PVDF additives enable sustained ignition with relatively low laser energy levels (< 5 J/cm2) at common wavelengths (1064 nm). Only the propellant with added nAl/PVDF exhibited sustained ignition. The ignition delays of the nAl/PVDF propellant ranged from 2 to 5 ms, depending on the energy density of the laser. We propose that the sustained ignition in nAl/PVDF propellants is due to the lower critical energy needed for nAl/PVDF composites (estimated 0.5 J/cm2 compared to 15 J/cm2 needed for AP/HTPB based propellant). The propellant's calculated critical energy is similar to the reported values of nAl/PVDF composites. The nAl/PVDF based propellant has a faster burning rate and increased pressure dependence due to the PVDF's gas generation and rapid burning of aluminum particles near the surface. Potential applications for this propellant, such as flash ignition and subsurface ignition, are also explored.