Studies on Laser Ablation of Polymer Coated Propellant Films

Abstract

In artillery, safety issues make propellants highly preferable as compared to ordinary explosives. However, the same properties that make propellants safer, such as their low shock sensitivities, also make them more difficult to ignite directly. In large bore artillery shells, ignition of the propellant is achieved using a small amount of an ordinary explosive, such as lead azide, as the primer. This sequential ignition process results in irreproducible delays that prevent targeting of fast moving objects. Numerous attempts have been made to develop improved ignition processes. Laser ignition would overcome the ignition delay problem and be practical. Unfortunately, all of the propellants used today cleanly ablate under laser irradiation, and ignition cannot be achieved. Electrothermal Chemical (ETC) ignition has proved capable of overcoming the ignition delay problem, but it is not practical. In this method, a large capacitor is discharged across a small piece of plastic, generating a high-pressure, high-temperature plasma that ignites the propellant after a short and highly reproducible delay. Unfortunately, the capacitor and related power supply to charge it are too large and heavy for practical application of ETC ignition. In this work, a new approach to ignition is developed based on the generation of a high-temperature, high-pressure plasma, directly at the propellant surface, that has a chemical composition similar to that of the ETC igniter, using laser ablation of the plastic coating on the propellant.