Paul J. Conroy

MULTIPHASE CFD SIMULATIONS OF SOLID PROPELLANT COMBUSTION IN GUN SYSTEMS

The Army Research Laboratory has developed a scaleable, 3D, multiphase, computational fluid dynamics (CFD) code with application to gun propulsion (interior ballistics) modeling. The NGEN3 code, which incorporates general continuum equations along with auxiliary relations into a modular code structure, is readily transportable between computer architectures and is applicable to a wide variety of gun propulsion systems. Two such systems are the Armys Modular Artillery Charge System (MACS) and the Future Combat System (FCS). The MACS is being developed for indirect fire cannon on both current and developing (e.g., Crusader) systems. The efficiency of the MACS charge is dependent on proper flamespreading through the propellant modules; a process that has been repeatedly demonstrated in gun firings, successfully photographed using the ARL ballistics simulator, and numerically modeled using the NGEN3 code. The FCS requires weapons systems exhibiting increased range and accuracy. One of the technologies under investigation to achieve these goals is the electrothermal-chemical (ETC) propulsion concept, in which electrically generated plasma is injected into the gun chamber igniting the high-loadingdensity (HLD) solid propellant charge. NGEN3 code development and application to the MACS and FCS is currently a DoD HPC Challenge Project (No. 112) and is being greatly advanced by access to the DoD high performance computers (HPCs). Associate Fellow AIAA. Propulsion Physics Team Leader, Ballistics and Weapons Concepts Division, Weapons and Materials Research Directorate. Mechanical Engineer, Propulsion Physics Team, Ballistics and Weapons Concepts Division, Weapons and Materials Research Directorate. This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. INTRODUCTION A solid propellant gun system consists of a reaction chamber connected to a gun tube through which a projectile is guided once propelled by pressurization of the chamber. Chamber pressurization is accomplished by placing a solid propellant (SP) charge in the chamber and igniting it by various means. Current SP charges are generally complex structures consisting of hundreds or even thousands of distinct regularly formed (e.g., spherical, cylindrical) grains, which may be loaded in either regular or random arrangements. In addition to small-scale voidage between grains (i.e., porosity) many charges also contain large-scale voidage (i.e., ullage), which surrounds the entire charge (such as when the charge does not fill the entire chamber volume) or separates distinct subcharges (i.e., increments or modules) that together comprise the whole charge. The addition of energy to the chamber, usually near the gun breech, or rearmost end of the chamber, and in some cases through a tube extending along the centerline of the chamber, ignites the SP. In general, all of the grains are not ignited simultaneously, but an ignition flame spreads from the breech to the projectile base. The burning of the SP transforms chemical energy into heat as hot gases evolve from the surface of each grain of propellant. Initially the projectile resists movement allowing the pressure in the chamber to climb rapidly. Since the burn rate of the propellant is proportional to the pressure, hot gases are produced at an accelerated rate until peak pressure is reached in the chamber. Movement of the projectile down the gun tube, usually slight before peak pressure and much more significant afterwards, causes the chamber volume to increase, and generates rarefaction waves, which lower the pressure and thus the burn rate of the propellant. Upon ignition and burning, the gas dynamic flowfield in the gun chamber takes on a highly complex structure that includes the dynamics of propellant motion and combustion and various gas dynamic flow phenomena such as turbulent mixing, highly transient pressure waves, steep gradients in porosity and temperature, nonideal thermodynamics, and gas generation.

Small Arms Gun Tube Nondestructive Evaluation Characterization

Over the course of the past four years, advances have been made in the in-bore characterization of small arms to include 5.56 mm and 7.62 mm systems at the Army Research Laboratory (ARL). This involves the application of in-bore laser measurements of the gun tube surface to very high level of resolution and accuracy which was previously unobtainable using existing conventional systems. The first 5.56 mm in-bore laser measuring system was manufactured for the ARL by Laser Techniques Co. (LTC) of Redmond, WA. The 5.56 mm system has been applied to the following weapons: M4, M16A1, M249 as well as other 5.56 mm weapons. It produces a three-dimensional (3-D) map of the bore surface to a depth resolution of 10 microns and axial resolution of 40 microns and circumferential resolution of 0.6 degrees. A color pallet is created and applied to the radial displacement from a baseline, and a picture is produced showing details of the bore surface. These characterization efforts have been augmented by visual scopes as well. One of the first applications of these systems was to analyze a series of firings in three each of M4, M16A2, and M249 weapons. This produced scans that were subsequently analyzed using Matlab to determine a history of wear life. This data is useful in enhancing predictive wear rates for barrels as well as for comparisons against recorded wear rates of fielded barrels.

Thermal and Ballistic Dynamic Analyses for Gun Ceramic Nozzles

The primary concern for the design and test of the ceramic gun nozzle is the combination of the thermal stress and ballistic dynamic stress. On the thermal analysis basis, a transient sequentially coupled finite element model (FEM) is performed to investigate the thermal stresses due to large temperature gradient and coefficient mismatch of thermal expansion between the ceramic nozzle and the steel holder. The fully coupled thermal stress analysis is conducted for verification. A DYNA3D model is used for interior ballistic analysis for the ceramic nozzle. Three candidate ceramic materials, SN47, STK4, and ZRO2, are investigated and compared with the 4340 steel nozzle. All the stress components from both thermal and dynamic loads are determined. These predictions are significant to the selected ceramic materials for the gun nozzle design.