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A. AtwoodNaval Air Warfare Center Weapons Division, China Lake

E. Friis, M. StromgardNordic Ammunition Company, Raufoss, Norway

B. RichardsNaval Surface Warfare Center, Crane

April 14, 2004NDIA Gun & Ammunition Conference

Medium Caliber Multipurpose Ammunition Technology Study – Uses of Modeling and

Simulation

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Purpose

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

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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

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Press-loadingPress-loading

LaunchingLaunching FlightFlight

Ignition & Burning

MP-Ammunition Technology Program

Impact & Penetration

Effect within target

RMATS has validated models for manufacturing, ballistics, trajectory, and target function of the MP ammunition.

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Press loaded energetic powders are major constituents of the MP-Ammunition

Powder: Pyrotechnic charge

Powder: Pyrotechnic charge

Powder:High explosive

Powder:Zirconium

Powder:Tracer

Powder:Self-destruct device

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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 ScanCompaction Apparatus

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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

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•Model Applications•Improved 20 mm MP LD•Development of penetrator•Evaluation of yaw•Effects of manufacturing

•PGU28/B

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Background

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

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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

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Original and chosen robust nose cap

Original design

Robust design

0.9mm

5mm

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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

10 20 30 40 50 60 70TimeHmicro sec .L200

400600800

velocity HmsLCav .Autod .

T im e ( s )

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0

8 0 0

6 0 0

4 0 0

2 0 0

0

Vel

ocity

(m

/s)

C a v e ty

A u to d y n

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Task - Yaw

• Have studied the connection between propellant gas flow by the muzzle and yaw

t = 120 st = 150 s

t = 250 s

The boundary betw een the propellant gas and the air

A typical flow pattern around the projectile during launching at different m om ent tim e

The results fromthe simulationwere used as inputinto a mathematicalmodel in Mathematica, to calculate theyaw angle.

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•Task – Effect of Manufacture•PGU28/B

•In-bores/prematures•Most probable causes

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PGU 28/B, PGU-28A/B and M70LD Design

PGU-28/B

Press fitted nose cap

M70LD

PGU-28A/B

Threaded nose cap

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Damage with 20 mm PGU 28/B

Cobra

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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

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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

Simulation of these experiments to be able to study the nature of the prematures

• Firing tests in barrel

– Static

– Dynamic

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Example of Results:

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.

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.

Simulation: Experiment:

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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

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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

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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

24Loose incendiary from assembly process

Incendiary

Clo

sure

no

zzle

Nos

e ca

pShe

ll bo

dy

PGU-28/B

Incendiary pressed into the gap during the assembly process

This is a potential ignition phenomenon specific to a press fitted nose cap design

Possibile Pinching in PGU 28/B

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Loose Incendiary from the PGU28/B Assembly Process

Loose incendiarySimulations 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 powder found in opened rounds

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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

Clo

sure

no

zzle

Nos

e ca

p

She

ll b

odyRisk 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

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Nose incendiary impacting projectile incendiary during setback

This is a probable ignition cause for the 20 mm PGU 28/B with low compaction of last increment and press fitted closure nozzle

NIKE2D simulation of nose incendiary hitting projectile incendiary at 240 m/s. - shows the grid after the initial hit

2´ 10- 64´ 10- 66´ 10- 68´ 10- 60.00001TimeHsL400

5006007008009001000

TempHKLInNikeTop

The hot spot temperature

The bulk temperatureThe bulk temperature

The hot-spot and bulk temperature as a function of time. Hot spot temperature calculated in Mathematica.

K

v

Incenidary 2

incendiary1

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• Simulated set-back during launching

• Performed hot spot calculation

Thot-spot = 130 K

Thot-spot = 10 KLow density in the nose tip will NOT cause ignition

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.

Hot-Spots Due to Compaction of Low Density Areas

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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