A New Look at Ignition: Refining Our Understanding of Combustion Dynamics in Gun Chambers

Every time a soldier pulls the trigger on a 7.62 rifle or pulls the wire of a 155 Howitzer, a complex chain reaction ensues over the next millisecond that we refer to as the ignition event. The ignition event involves a highly dynamic interaction with heat and mass transfer between multiple reacting chemicals across a varied spatial domain to achieve rapid and uniform burning of the entire granular propellant bed. After the ignition event, standard interior ballistics apply: Propellant is burnt, pressure increases and the projectile accelerates down the barrel until leaving the muzzle. To date, the details and controlling mechanisms of the ignition event and propagation into granular propellant beds have not been well understood or characterized.

Weapon designers often simplify the ignition and combustion process by assuming it behaves in a quasi-static manner, and therefore the thermodynamic state across the entire combustion chamber at any point in time is modeled by single, uniform state variables such as temperature or pressure. Unfortunately, experience has shown that this assumption is not always accurate, sometimes with dramatic safety and performance consequences. In many cases, deviations from ideal quasi-static conditions in the combustion chamber can be attributed to variations in the ignition of the propellant bed and are therefore a direct consequence of inadequate ignition train design and a fundamental lack of understanding of the ignition mechanisms.