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Predicting the spin history of munitions
Michael Minnicino
U.S. ARL Weapons and Materials Research Directorate
Materials Division
9 April 2009
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Motivation
Guided munitions spin much less than conventional munitions.
Guided munitions fired from rifled cannons need to be despun.
The guided munition is uncoupled from the cannon rifling by a two-piece band (aka split-band).
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Split-Band Theory and Design
x
centerline Projectile Nose
Projectile Base
r
Base Pressure
Tube Pressure
subprojectile
bandseat
slipband
axial surface area
radial surface area
Guided Projectile
Split-Band
A A
Section A-A
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VAPP Case Study
Very Affordable Precision Projectile (VAPP) Program• Weight ~ 42 lbs• Axial MOI ~ 108 lb-in2
• Muzzle spin rate ~15 Hz• Gun System = M119 105 mm Howitzer (M20 Tube)
The M20 tube has a “gain-twist” rifling profile• initial twist is 1:35 at the origin of rifling• final twist is 1:18 near the muzzle
Goal: Develop a split-band that results in a reduced spun projectile at all firing zones
Twist = the number of calibers for a fully spun munition to make one revolution
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Test Projectile Shot Results
Muzzle velocity is linear with WRT charge weight
Scatter evident in the spin rate data
Largest spin rate is ~24 Hz
Muzzle Velocity
Spin Rate
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“Bandslip” Prediction Software
Originally developed by SNL for constant twist gun tubes
Rewritten to enable the simulation of gain-twist gun tubes
Input to software includes•projectile inertial properties•Split-Band geometry•Split-Band material properties•gun tube characteristics•base pressure-time history•estimate of the “tube pressure” and slipband-bandseat friction
The slipband angular quanities are kinematically constrained to the projectile’s linear quantities through the rifling profile
The bandseat/projectile’s angular velocity and position are calculated from first differences
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VAPP Zone 3 Simulation
Zone 3 Pressure-Time Data Resulting linear displacement and velocity histories
Input Output
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Slipband Angular Acceleration
The slipband’s angular acceleration remains large due to the gain-twist rifling profile
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Slipband Angular Velocity
The slipband’s angular spin rate f = v/(D*twist)
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Projectile Spin Rate Prediction
The projectile spin history depends significantly on the initial twist rate
Z3 Shot Data
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Summary
• Split-Band theory and design features are understood
• A numerical predictive capability is demonstrated
• A projectile spin rate of ~19 Hz (21 Hz actual) was predicted for the VAPP shot at Zone 3
• 24 Hz was the largest spin rate recorded for the VAPP
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Questions
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Backup Slides
More slides
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Split-Band
• Geometry and thermal material properties are used in determining slipband behavior
• Base pressure and “Tube pressure are used to determine slipband “lift” time, i.e., when the base pressure is greater than the effective Tube pressure
• Constant twist rate is used to determine required torque needed to spin subprojectile at same rate as slipband
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Linear (Axial) Velocity and Travel
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Summary