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

October 1, 2024

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.

A time-marked sequence of paired visual and thermographic images of an impact-initiated reactive material combustion. (Image: Texas Tech University, Connor Woodruff & Michelle Pantoya)

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 quasistatic 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.

After the ignition event, gun chamber pressure is generated by the propellant combustion creating heat and gaseous products. Pressure is directly linked to force and stress in the gun chamber and barrel. Safe designs must restrain this pressure without yield and fatigue of the metal components to keep soldiers safe. Pressure over time also directly correlates to acceleration and velocity of the projectile. As next-generation weapon systems aim to push traditional performance boundaries, an aggregate macroscopic view of the interior ballistic cycle is no longer adequate. In addition, advances in manufacturing technology have opened the design space for control of geometry and spatial distribution of chemical species, even within a single propellant grain, for optimal performance. Therefore, further improvements to projectile velocity and range, without exceeding a gun chamber’s material allowable stress level require detailed knowledge and fine-tuning of several parameters such as grain geometry, chemistry/burn rate and ignition propagation through the porous propellant bed. However, it all starts with the ignition event and flame propagation throughout the propellant bed. Without a more thorough understanding of the early time initiation mechanisms, weapon system performance improvements from modification of other system parameters will be limited.

The initial study highlighted in this article will occur at the Texas Tech University Combustion Lab. Pictured here is the lab’s High Velocity Impact Ignition Testing System (HITS). (Image: Texas Tech University)

A team of scientists and engineers led by Texas Tech University, Department of Mechanical Engineering Combustion Lab with support from Element U.S. Space and Defense and the U.S. Army Combat Capabilities Development Command – Armaments Center (DEVCOMAC) at Picatinny Arsenal are in initial phases of an experimental program to measure and better understand these gun propellant ignition phenomena.

Typically, the ignition event starts with either a mechanical device such as a spring-accelerated pin striking a percussion primer, or an electric current passing through the primer. This stimulates the first chemical reaction in the ignition train, with a small primary explosive generating heat and products that further ignite the main primer charge. Finally, the thermal energy, pressure and products from the primer charge vent into the main propellant chamber as a two-phase flow. The main propellant chamber is loaded with propellant grains which can be characterized as a porous bed of solid particles. The two-phase primer exhaust flows in and around the grain voids, enveloping some grains head-on with a stagnation point and boundary layer flow and then brushing by other grains generating a more convective flow. The individual grains are not homogeneous but have various layers of graphite or inhibitors that influence heat transfer. Grains close to the primer vent(s) may be exposed to hot solid particle impacts and grains one or two layers deep are only exposed to hot gases. The goal is to transfer heat energy until all propellant grains reach ignition and start their own exothermic reaction. The dynamics then become notably more complex in that some grains fully ignite; some partially ignite and others may not burn. The ignited grains are quickly adding heat and hot gases to the chamber that then expose and ignite the remaining grains at an exponential rate.

The energy conversion processes throughout this ignition train are not well understood and are plagued with assumptions. Current gun propellant ignition models are empirical and based on an aggregate initial state of the system. To make matters worse, temperature data is often highly fluctuating and based on measurements of the condensed phase that may not be a good indication of gas phase heating. To improve the safety, reliability and performance of our gun systems, scientists need to understand the heat transfer and energy conversion processes controlling the ignition event on a granular level or even smaller, on a molecular level.

The study at Texas Tech will explore the step prior to the main combustion event to focus on the fluid dynamics and heat transfer events that ignite the propellant. Understanding how and when the propellant ignites will give modelers more detailed information to optimize current gun systems and design better future gun systems. Researchers at Texas Tech and Element U.S. Space and Defense will use a new diagnostic tool known as highspeed thermography to analyze the coupled flow and thermal events of the igniter exhaust as it interacts with propellant grains. This tool uses specialized lenses and software to convert the RGB color data of a modern digital high-speed camera to high-resolution and high-speed thermal mapping. This technique is similar to two-color pyrometry but rather than a single point, temperature is measurable at all pixels within the high-speed image.

One goal of the research is to measure the heat transfer time scales of individual grains in actual and simulated primer exhaust flow fields. Engineers will isolate and vary flow conditions such as gas temperature, gas velocity, solid particulate size and solid particulate concentration within the gaseous flow field. A second goal is to measure thermal conditions at various radial and axial positions within the gun chamber. The measurements will provide important information about the uniformity of heating and root causes for ignition delays and pressure waves throughout the porous propellant bed.

Experiments have not started yet, but the tests will provide the potential to reveal fundamental mechanisms leading up to the ignition event, such as the temperature gradient of the igniter exhaust as it flows through the choked channels of the porous propellant bed. It is hypothesized that grains more proximal to the igniter vent are exposed to higher temperatures and higher convective heat transfer therefore reaching ignition thresholds early. In contrast, grains that are further from the igniter vent are exposed to reduced heat flux and ignite later. This non-uniform ignition is on the order of tenths of microseconds but affects early pressurization rates.

The goal of the primer is to provide rapid, smooth and symmetric ignition of the propellant bed. In some cases, primers can create pressure spikes that crush grains and cause unpredictable surface area and dangerous pressure spikes. Experiments using thermographic imaging can identify the causes of these dangerous events and help scientists design features to mitigate these risks.

Detailed thermal maps and time histories of the ignition event will be helpful for modelers to more accurately and precisely predict propellant burn start conditions. The modeling benefits are likely more significant in large gun chambers with large propellant grains because of their more sizable temperature gradients and longer timescales. There are already plans to apply this thermography diagnostic directly to large-caliber gun chambers.

Rounds feed into the chamber of an M2HB .50-caliber machine gun on the fantail aboard Nimitz-class aircraft carrier USS Carl Vinson (CVN 70) during a live fire exercise as a part of Annual Exercise (ANNUALEX) 2023. (Image: U.S. Navy)

Once scientists develop a deeper understanding of the physics controlling the ignition event, innovative ideas will advance the technologies needed to make ignition more consistent, homogeneous and predictable. This research will transform the reliability and safety of gun systems while simultaneously improving projectile velocity and range. It is plausible that the new data will also inform improved designs for primers or ignitors. Currently, propellants are manufactured with varying geometries to tailor surface area for an ignition response and burn rate. The thermographic flow data will provide more insight into an optimized geometric condition to improve ignition and provide superior ballistic performance. Fine-tuning of the ignition process, in combination with an evolution in propellant manufacturing technologies can also extend the weapon system lifetime by optimizing the temperature and pressure conditions in the breech and barrel that contribute to erosion and deteriorated performance.

Internal ballistics and gun propellant combustion have been studied for centuries and modern guns have experienced great strides in performance. The percussion primer was a huge technology leap from flint-lock powder trays or burning fuses on canons. Observations and data from this research may lead to great innovations in propellant ignition and overall ballistic efficiency, enabling the next leap in gun system safety and performance with both extended range and improved weapon system longevity.

This article was written by John Granier, Ph.D., Chief Engineer, Munitions & Energetics at Element U.S. Space & Defense (Belcamp, MD). For more information, visit here .