Medium Caliber Multipurpose Ammunition Technology Study – Uses of Modeling and Simulation A. Atwood Naval Air Warfare Center Weapons Division, China Lake E. Friis, M. Stromgard Nordic Ammunition Company, Raufoss, Norway B. Richards Naval Surface Warfare Center, Crane April 14, 2004 NDIA Gun & Ammunition Conference

Purpose Demonstrate application of modeling and simulation tools developed in the Multipurpose Program

Why Modeling and Simulation? Used to save time and reduce operation costs Decrease the number of live fire tests Reduce development time Examine effects of manufacturing changes Optimum product to the war fighter

MP-Ammunition Technology Program RMATS has validated models for manufacturing, ballistics, trajectory, and target function of the MP ammunition. Impact & Penetration Ignition & Burning Effect within target

Press loaded energetic powders are major constituents of the MP-Ammunition Pyrotechnic charge Powder: Zirconium Powder: Tracer Powder: Pyrotechnic charge Powder: High explosive Powder: Self-destruct device

Key Technology Areas - Manufacture and Launch Development of powder models Evaluation of press loading techniques Quasi-Static Compaction Field testing of nose tips with various fill techniques Cat Scan Compaction Apparatus

Numerical Simulations Material models • metal parts incendiary & HE target Interactions Material geometry Loading forces & initial velocities Numerical parameters OUTPUT press-filling operation launching effects flight effects initial condition (prior to impact) impact & penetration (course of events) ignition stimuli burning fragmentation Numerical code

Model Applications Improved 20 mm MP LD Development of penetrator Evaluation of yaw Effects of manufacturing PGU28/B

Background Out bore unintended ignition of the 20 mm MP LD round

Task – Product Improvement Requirements No ignition when hit by particles Ignition when hitting the target No ignition in drop test Procedure Used knowledge and tools developed in the RMATS program to suggest a new nose cap design Use numerical simulations to study the behavior of different nose cap designs Firing experiments with different (selected) nose cap designs

Original and chosen robust nose cap Original design 0.9mm Robust design 5mm

Task – Penetrator Development Developed an analytical penetration model which unites the Walker-Anderson model and cavity theory Simulation of penetration of tungsten carbide penetrator, to study when and why it penetrates and when and why it brakes up Used the powder model as material model for the penetrator The grid of the target and projectile after 20 microseconds

Task - Yaw Have studied the connection between propellant gas flow by the muzzle and yaw The results from the simulation were used as input into a mathematical model in Mathematica, to calculate the yaw angle.

Task – Effect of Manufacture PGU28/B In-bores/prematures Most probable causes

PGU 28/B, PGU-28A/B and M70LD Design Press fitted nose cap M70LD PGU-28A/B Threaded nose cap

Damage with 20 mm PGU 28/B Cobra

Causes of Prematures? Early in the investigation: It was believed that normal function of the round could not cause the observed damage Possible ignition mechanisms investigated: Plugged bore resulting in 2 rounds firing simultaneously A single MP round exhibiting abnormal behavior Detonation instead of deflagration

Model of barrel damage Establish the material data for the M61 A1 gun barrel and the 20 mm PGU 28/B shell body: Tensile tests Expanding ring tests Study of fragmentation pattern of 20 mm PGU 28/B (outside barrel) Simulation of these experiments to establish/calibrate material data Firing tests in barrel Static Dynamic Simulation of these experiments to be able to study the nature of the prematures

Example of Results: Simulation: Experiment: Simulations where PGU 28/B is set off while the round is moving (dynamic situation). Burning regime was as a normal functioning round, i.e. convective burning. Round functioned in the thin region of the barrel. Barrel damage as a result of the experiment of a dynamic function of the PGU 28/B round in the thin region of the barrel.

Nature of the Observed Prematures: Normal initiation of the round in the barrel will give a barrel rupture as observed for the incidents of investigation A single round is sufficient Initiation of a round passing an area previously damaged This means: The mechanism is deflagration and not detonation Plug bores disregarded Barrel damage from one round may cause the ignition of subsequently fired rounds

Possible Causes for the Observed Prematures One of the energetic materials must be brought to a situation where it meets the ignition criterion to ignite the round Possible causes: Pinching of nose tip incendiary between the nose cap and the shell body Friction between nose tip incendiary and the closure nozzle Nose tip incendiary particles impacting projectile incendiary during set-back Rapid compaction of a low density area in the nose cap specific to the PGU 28/B

Pinching of Loose Incendiary Between the Nose Cap and the Shell Body The tolerance extremes show that the fit of the nose cap may vary significantly A loose fit may result in: Loose incendiary migration between the different parts Possibility for relative movement between nose cap and shell body during launch

Possibile Pinching in PGU 28/B Incendiary pressed into the gap during the assembly process PGU-28/B Incendiary Shell body This is a potential ignition phenomenon specific to a press fitted nose cap design Nose cap Closure nozzle Loose incendiary from assembly process

Loose Incendiary from the PGU28/B Assembly Process Simulations of compaction: Last press increment results in loosely compacted RS41 During assembly process closure nozzle acts like a loading punch with a center hole Loose incendiary Loose powder found in opened rounds

Friction Between Incendiary and Closure Nozzle Powder may be “shed” due to the set-back forces, causing friction as it slides down the closure nozzle: Incendiary Closure nozzle Nose cap Shell body Risk of reaching the ignition temperature due to friction between powder and the closure nozzle is higher for PGU 28/B Steel closure nozzle versus aluminum in other versions

Nose incendiary impacting projectile incendiary during setback v Incenidary 2 incendiary1 K The hot-spot and bulk temperature as a function of time. Hot spot temperature calculated in Mathematica. NIKE2D simulation of nose incendiary hitting projectile incendiary at 240 m/s. - shows the grid after the initial hit This is a probable ignition cause for the 20 mm PGU 28/B with low compaction of last increment and press fitted closure nozzle

Hot-Spots Due to Compaction of Low Density Areas Simulated set-back during launching Performed hot spot calculation This is below the ignition criterion, but it is a significant temperature increase. Rapid compaction of this low density area can not be ruled out as a possible ignition mechanism. Thot-spot = 130 K Low density in the nose tip will NOT cause ignition Thot-spot = 10 K

Conclusions Validated modeling and simulation tools developed in the RMATS program are being used Product improvement and development Improved 20 mm nose tip Penetrator development Calculations of yaw Explain complex phenomena In-bore PGU28/B prematures Most likely that a combination weaknesses unique to the design and manufacture Press fitted nose cap/closure nozzle Low-density area of incendiary in the middle of the nose tip Fine particle size distribution of the nose tip incendiary Weaknesses are not found in other MP ammunition 20 mm PGU 28A/B will “fix” the problem