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Historical Development of Linear Shaped Charge

Dave Novotney 1 and Meryl Mallery.

2

Ensign-Bickford Aerospace and Defense Company, Simsbury, CT, 06070

The use of linear explosive products for launch vehicle, strategic missile, tactical missile,

and aircraft applications is very common in today’s aerospace industry. Ensign-Bickford

pioneered the development of low core load linear explosive products more than 50 years

ago including the development of linear shaped charge products in 1957. This paper

highlights some of the historical challenges in developing linear shaped charge and mild

detonating fuse and provides insight into the early development work that was done.

I. History

s with all technology, the development of the shaped charge is no different. An idea was brought forth by a

forward thinking individual or individuals, the idea was tested and explored, theories were postulated for

why the technology works, and applications for the technology were investigated. As more work on the technology

was performed, interest grew, pulling in more and more theories, ideas, tests and applications.

The idea of a shaped charge was first discovered by Charles E. Munroe of the Naval Torpedo Station, New Port

Rhode Island in 1888. Munroe detonated blocks of explosive in contact with steel plates. The explosive charge had

the initials U.S.N. (United States Navy) inscribed on the charge opposite the point of initiation. The initials were

reproduced in the steel plate. Munroe further observed that when a cavity is formed in a block of explosive,

opposite the point of initiation, the penetration of the crater produced in the target increased. In other words, a

deeper cavity could be formed in a steel block using a smaller mass of explosive. This effect has been termed the

Munroe Effect and is the focusing of the detonation products due to the chevron shape created in a charge.

The idea of a hollow charge was born. Munroe furthered this concept by developing one of the first lined shaped

charges in 1894. He created a device that consisted of a tin can with sticks of dynamite tied around and on top of it,

with the open end of the can pointing downward. The device punched a hole in the top of a safe.

Many theories have been put forth to describe the jet formation and cutting potential obtained from shaped

charges. Early theories claimed that the penetrating jet was due to either a focusing of the detonation gases, the

formation of multiple interacting shock waves, the

spallation of the liner material, or some combination of

these effects, including the theory that jets of gas break

through the metallic liner and carry fragments resulting

from the rupture and erosion of the liner. An interaction

of these jets then causes a strong forward wave that

imparts a high velocity to the liner particles.

Then in 1943, Birchoff (1943, 1947) put forth a

preliminary theory to describe the physics that occurs

during the jet formation process. He claimed that upon

initiation of the shaped charge, a detonation wave begins

to propagate through the explosive core at the detonation

velocity of the explosive. When the detonation wave

reaches the liner, the liner is subjected to the intense

pressure of the front and begins to collapse along its axis

of symmetry. This extruded material is the jet. The

intense pressure far exceeds the yield strength of the liner

material allowing the material to be treated as an in

viscid, incompressible fluid.. This basic theory holds true

1 Business Development Manager, Aerospace Programs, Professional Member.

2 Senior Analyst, Design and Analysis, Professional Member.

A

Explosive Core

Sheath

Apex

Leg

Figure 1. LSC Cross-Section.

43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 8 - 11 July 2007, Cincinnati, OH

AIAA 2007-5141

Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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today. Figure 1 provides a photograph of an inert LSC sample manufactured at EBA&D and includes definitions

related to linear shaped charge (LSC) design.

II. Mild Detonating Fuse and Low Energy Detonating Cord Development

For many years prior to the development of Mild Detonating Fuse (MDF), Ensign-Bickford worked on

developing a low core load Primacord® detonating cord, but determined that core loads of less than 40 grains per

foot were unreliable for propagation. With the failure of developing a reliable product using a wet braiding

technique, efforts were devoted to ‘developing a radically new type of detonating fuse of low explosive energy’.

Ensign-Bickford pioneered the development and production of low energy detonating fuse, now commonly known

as mild detonating fuse (MDF) in 1955. MDF was developed for commercial mining, blasting, and seismic

exploration, with ancillary uses (including military) where low force was desired.

The drawing of filled tubes, extrusion of

thermoplastic explosives, coating of strings

or wires with adhesive pastes, and the filling

and wrapping of foils were candidates for

consideration. Futile attempts of coating a

string with lead azide paste resulted in trying

low core loads wrapped in metal foils, at the

suggestion of DuPont’s Eastern Laboratory.

While this worked with small samples, it was

not successful when producing long lengths

of detonating fuse. In July of 1955, Ensign-

Bickford determined that by filling and

reducing a lead tube loaded with PETN or

RDX it was possible to manufacture

detonating fuse under 2 grains per foot that

had satisfactory initiation and propagation

characteristics.

Three methods for reducing the loaded

tubes were developed as part of the

development of MDF. Drawing, swaging,

and rolling were the methods developed and

are still used today in the manufacture of

MDF and other metal clad linear products.

Testing was performed on the safety of using

these methods, and all were determined to be

safe methods of producing MDF.

Several types of lead materials were

investigated, including chemical lead with

fractional percentages of copper, silver, and bismuth. This material had a low tensile strength, so various alloys

were investigated, including 50/50, 40/60, and 30/70 tin-lead solders. These worked out well for drawing and

swaging, but were deemed too expensive to use for commercial mining applications. A 6% antimonial lead was the

best alloy to use based on the best balance of cost and tensile strength.

A number of energetic materials were tested with the elongated lead tube, but many had poor propagation with

low core loads. These included TNT and Nitromannite. Successful trials were done using PETN and RDX, and

PETN was base lined due to Ensign-Bickford’s history and experience with PETN.

A number of different methods were investigated to protect the fragile lead core of the low energy detonating

cord. The final assembly consisted of multiple layers of jute, asphalt, rayon, and polyethylene to a finished diameter

of 0.210 inch. ‘These low energy detonating cords had many interesting properties including low noise, the ability to

convey detonation without initiating explosives alongside the cord …’

Further development was done in 1959 to develop cord that completely confined the detonation by-products

under a contract to the Frankfort Arsenal for Cartridge Actuated Devices. The work was limited to the investigation

of one and two grain per foot, lead sheathed, PETN based Low Energy Detonating Cord (LEDC). The resultant

work was a one grain per foot LEDC assembly that is shown in Figure 2. This assembly was constructed with an

Figure 2. Low Energy Detonating Cord.

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explosive core with multiple layers of Rayon, Polyethylene, and Fiberglass. This became the basis for many

aerospace products, including Confined Detonating Fuse (CDF) Assemblies and Flexible Confined Detonating Cord

(FCDC) Assemblies.

III. Linear Shaped Charge Development

With manufacturing and processing techniques developed for MDF, Ensign-Bickford was well positioned to

develop Linear Shaped Charge products. In 1957, the Martin Company issued a purchase order to Ensign-Bickford

to develop and deliver 1400 feet of cutting charge capable of cutting through 0.125 inch of 6061 aluminum. Cutting

sheets of metal with Primacord® detonating cord was known to be inefficient; a 100 grain per foot core load of

Plastic Primacord® was required to reliably cut through the target material. Ensign-Bickford started to investigate

different methods of cutting metal. To enhance the cutting action, three types of charges were considered:

1) Charges to give greater contact between the explosive and metal sheet.

2) Charges with binders to eliminate the external jacketing material.

3) Charges to take advantage of the Munroe effect.

The first attempt was to load lead tubes with PETN and roll the tube into a ‘D’ shape. This resulted in greater

contact with the target material, and resulted in the ability to cut 0.125 inch of 6061 aluminum with a 30 grain per

foot charge. The fabrication of charges with binders was abandoned because the charge could not be pushed or

pulled in a tunnel section, which became a requirement of the charge after the program was kicked off. By

modifying the ‘D’ shaped charge and producing a ‘U’ shaped channel on the charge, a 20 grain per foot linear

shaped charge was able to cut through the 0.125 inch aluminum target material. The original design of the ‘U’

shaped charge is shown in Figure 3.

Figure 3. ‘U’ Shaped LSC Cross-Section

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A secondary requirement was levied to not damage a

second target plate that was 1.19 inches above the cutting

target. The ‘U’ charge did significantly less damage to the

secondary plate than the ‘D’ charge and the Primacord®.

Trials were run to investigate the shape of the ‘U’ cavity

and the effect of stand-off from the target material. A

picture of the severed target with the undamaged

secondary structure is shown in Figure 4.

The lead jacket was made of antimonial lead due to its

superior tensile strength than chemical lead. Specific

testing was performed on top and end initiation of the LSC

(65 tests), velocity of detonation (16 tests), tensile strength

(40 tests), and a number of tests related to piggybacking

charges, branching charges, and butting charges together.

The ‘U’ charge was considered the best solution, given

the ability to meet the cutting and collateral damage

requirement and the quick timeframe required for delivery.

IV. Present Day Linear Shaped Charge Design

The understanding of linear shaped charge performance has come a long way since the 1940’s when a great deal

of active research was undertaken in support of World War II where shaped charges were used to penetrate tank

armor, bunkers and fuel storage containers. Since then, shaped charges have been developed for many varied

applications including canopy breaching for aircraft crew escape, launch vehicle stage separation, door breaching in

army applications as well as many more.

With the rapidly increasing computation capabilities of desktop computers, the modeling of detonation events,

including shaped charges, is becoming a tractable and cost effective problem to model. This emerging analysis

capability is helping to drive shaped charge designs to become more efficient and cost effective. Understanding the

effect of variations in critical design variables is allowing more efficient designs to be developed, i.e. deeper

penetrations can be obtained with less explosive. This understanding also aids in appropriate controls to be placed

on the shaped charge manufacturing process to control critical parameters, and aids in the understanding of

anomalies in the penetration plates that do not meet customer requirements.

At Ensign-Bickford Aerospace and Defense, detonation modeling is becoming a core competency that is used to

drive design development and understanding. Modeling is also used to assist in defining manufacturing controls and

to evaluate test anomalies. The primary software package that is used at EBA&D is a shock physics simulation

software code developed by Sandia National Laboratories, called CTH. CTH is a flexible software system designed

to treat a wide range of shock wave propagation and material motion phenomenon in one, two or three dimensions.

The finite difference analogs of the Lagrangian equations of momentum and energy conservation are employed with

continuous rezoning to construct Eulerian differencing.

To illustrate the capabilities that have been brought forth in the last 40 years, a ‘U’ shaped channel was analyzed

as well as an advanced shaped charge of similar core load. The advanced charge is then modified to demonstrate

how analysis can predict variations in penetration due to changes in critical design variables. The predicted shaped

charge Shaped channel. Figure 6 illustrates the jet formation in a 30 grain/ft advanced charge.

Figure 4. ‘U’ Shaped LSC Cutting Test.

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1 us

Figure 5. Jet Formation of ‘U’ Charge.

2 us

3 us

Figure 6. Jet Formation of Chevron Charge.

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As you can see from comparing Figures 5 and 6, the shaped charge jet formation process has been greatly improved

due to an increased understanding of the underlying physics that governs shaped charge jet formation. What was

designed by trial and error in the 1950’s is now done more efficiently through the combination of analytical models

and test. The following paragraphs describe the variables that affect LSC performance and illustrate how analysis

has been used in recent years to aid in the understanding of LSC performance.

There are several design variables that are now known to affect LSC penetration performance. These variables are

measured during the manufacturing process and during acceptance tests for each LSC tube at EBA&D. Explosive

effectiveness is ensured through VOD testing, core load measurements and moisture analysis. Geometric

measurements are taken to ensure compliance to drawing requirements. These measurements include apex height,

apex angle, apex thickness, apex radius, charge height, charge width, and charge outer angle. Standoff height and

target plate material are also critical, and are established during acceptance testing of the product.

Shaped charges manufactured within the specification limits sometimes fall outside the required penetration limits

for the product line. The question asked was: “Why” and “What” is causing the shift in performance? Figure 6

illustrates an idealized jet that is formed in a perfectly symmetric charge. In manufactured product, however, the

shaped charge is not always perfectly symmetric. The goal of this study was to determine how the shaped charge

performs due to variances induced during the manufacturing process that aren’t directly measured. The variables

analyzed and described in this paper for illustration are explosive core lateral offset and explosive core longitudinal

offset (apex thickness variations). Performance variations due to manufacturing induced variances were assessed by

comparing shaped charge penetration performance into one inch thick AL 7075-T7351 test plates with a constant

charge standoff.

LSC Model

A two dimensional model of the LSC was developed utilizing CTH based on a manufactured tube that was cross-

sectioned, potted in “Quickmount” self setting resin, and photographed. The photograph was digitized and imported

into OneSpace Designer where dimensional measurements were taken to create the CTH model. Figure 7 provides a

comparison of the potted cross-section, and the final CTH model cross-section.

Figure 7. LSC Cross Section and CTH Model.

The CTH model consists of the PBXN-5 LSC shown in Figure 6 along with a nominal standoff from a 1 inch thick

7075-T7351 aluminum test plate. Although 3D analysis is possible in CTH, a 2D model was deemed appropriate

due the nature of the studies being conducted and the computation time required for a full 3D model. Off-axis jet

effects are therefore not included in the model.

The model consists of three main elements: the LSC explosive core, the LSC aluminum sheath and the test plate.

The PBXN-5 core is modeled with the JWL equation of state for LX-10 which consists of 94.5% HMX and 5.5%

Viton and closely represents the composition of PBXN-5. The PBXN-5 detonation is modeled with a programmed

burn rate model which initiates the entire LSC cross-section simultaneously. The Aluminum 7075 test plate and

aluminum shaped charge sheath were each modeled with the Mie-Gruneisen equation of state. The strength of the

materials are then analyzed with the Steinburg-Guinan-Lund Model. The baseline symmetric model refined the grid

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in the region of the shaped charge to provide a minimum of 3 cells through the thickness of the sheath in the vicinity

of the charge apex. The CTH results for this model were compared to actual test data to validate the model results.

The model was then modified, as described in each subsection, for the studies enclosed.

Model Validation

To assess the validity of the CTH model, the predicted penetration performance for a symmetric charge was

compared against the actual penetration performance history of the LSC under study. Figure 8 provides the average

LSC penetration performance per test plate (11 penetration measurements taken every inch) vs. test for 600

acceptance tests of the product. As can be seen, there is good agreement between the test data and model

predictions. The model predicts deeper penetration than test due to the assumption of perfect symmetry and the

neglect of 3D effects that result in a jet that is not perpendicular to the plate. Two model points are given in Figure

6: one to the base of the cut with all slag removed and one with slag included. Slag is the residual metal from the

shaped charge detonation process that is deposited in the cut as the jet penetrates the test plate. It is sometimes

difficult for the test engineer to remove the slag at the base of the cut for penetration measurements. In the

symmetric case modeled, the slag fills approximately 18% of the cut.

Average Penetration vs Test #

0 100 200 300 400 500 600 700

Test #

Pen

etr

ati

on

(in

ch

es)

Min Req.

CTH Base of Cut

CTH Top of Slug

Figure 8. LSC Penetration Performance History

Figure 9 provides a comparison of the model results to the penetration within a test plate. The model closely

matches the cut geometry as well as the penetration.

Figure 9. Model and Test Plate Comparisons.

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Lateral Core Offset Study

The first study under taken was to evaluate the effect of lateral explosive core offset. The lateral core offset is

illustrated in Figure 10. In this case, the explosive core was shifted, off center, to the right by 0.0045 inches.

Figure 10. Lateral Core Offset Definition.

When the explosive core is offset within the sheath, the leg thickness on either side of the apex becomes unequal.

When this occurs, the shock front collapses the liner legs at different velocities causing a distorted jet to form. This

phenomenon is illustrated in Figure 11 at 2 us after initiation with a 0.0045 inch core offset.

Figure 11. Lateral Core Offset Jet Formation.

Studies were conducted to determine the penetration performance when a distorted jet is formed under varying

degrees of lateral core offset. To understand the magnitude of lateral core offsets produced, a LSC sample was

sliced every 2 inches along a 28 inch length and the lateral explosive core offset was measured. The results of this

study are provided in Figure 12. Offsets of up to 3.5 mil were measured on the dissected tube.

Core Shift

Leg Thickness

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Figure 12. Sample Results for LSC Lateral Core Offset.

Figure 13 provides a comparison between actual test samples and simulation results for a lateral core offset of

0.0025 inches. The CTH simulation predicts the cut depth as well as the geometry of the cut. The hook shown in

these results is characteristic of all the simulations run with lateral core offsets. The amount of hook varied with the

amount of lateral core offset. The thinner of the two legs, in this case, the left leg, is launched at a greater velocity

than the thicker leg, thereby pushing the jet toward the right of center.

Figure 14 provides a graph of the resulting percent penetration shift due to varying lateral core offsets. An offset of

only 2.5 mils along the length of the LSC is predicted to produce a penetration shift of 20%. The % shifts were

calculated by comparing the shifts to a nominal charge penetration result with no lateral core offset (a perfectly

symmetric charge).

Tube 31 Penetration Test Results

CTH Results

Figure 13. CTH Simulation vs. Test

% Penetration Shift vs Core Offset30 gr/ft LSC

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

0.000 0.001 0.002 0.003 0.004 0.005 0.006

Core Offset (inches)

PenetrationShift,%

Figure 14. Percent Penetration Shift vs. Lateral Core Offset.

Core Width Centrality vs Distance Along the Tube

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0.0045

0.005

0 5 10 15 20 25 30

Distance Along Sample (inches)

Co

re C

en

tra

lity

(in

ch

es

)

Tube 34 - LE - Passed

Tube 34 - FE - Failed

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Longitudinal Core Offset (Apex Thickness Variability) Study

The next parameter that was analyzed was the longitudinal core offset within the sheath. This movement affects the

apex and leg thicknesses within the charge, as shown in Figure 15. Unlike a lateral core offset, however, this offset

does not produce an offset jet. Variations in apex thickness and leg thickness, however, affect the jet formation

process, the jet velocity and jet mass.

Apex thickness is a parameter that is measured on LSC designs manufactured at EBA&D. Figure 16 shows the apex

thickness variability measured during the manufacturing process. It shows variability from the minimum required of

0.0065 inches to a maximum of 0.00115 inches. Using these measurements as a guide, parametric studies were

conducted with CTH. The variation in predicted penetration depths with apex thickness is presented in Figure 17.

The longitudinal offset variability seen during manufacturing produces a maximum penetration shift of 10%. The

design minimum apex thickness is also away from the cliff in the curve which occurs when the apex thins out to the

point of breaking during the jet formation process. As a comparison, a lateral core shift of 0.005 inches resulted in a

40% degradation in penetration performance compared to a 10% degradation caused by a 0.005 inch longitudinal

offset. The nominal configuration used an apex thickness of 0.0065 inches which is the minimum allowable.

Figure 18 provides a comparison of the predicted cut depth and cut shape for three of the different apex thicknesses

studied. The thin apex (0.0025 inches) that is starting to exhibit failure shows a much shallower cut than the other

two cases analyzed. Although just two different design variables have been illustrated many more have been

investigated.

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Apex Thickness vs Tube #

0.005

0.006

0.007

0.008

0.009

0.01

0.011

0.012

0.013

0.014

0 10 20 30 40 50

Tube #

Ap

ex

Th

ick

ne

ss

(in

)

% Penetration Shift vs Apex Thickness

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

0 0.002 0.004 0.006 0.008 0.01 0.012

Apex Thickness (inches)

Pe

ne

tra

tio

n S

hif

t ,

%

Figure 15. Longitudinal Core Offset.

Figure 16. Apex Thickness Variability.

Figure 17. % Penetration Shift vs Apex Thickness.

Apex

Thickness

Longitudinal Core Offset ore

Figure 18. Apex Thickness Cut Comparisons.

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V. Conclusion

Over the past 50 years the shaped charge jet formation process has been greatly improved due to an increased

understanding of the underlying physics that governs shaped charge jet formation. What was designed by trial and

error in the 1950’s is now done more efficiently through the combination of analytical models and test.

With the rapidly increasing computation capabilities of desktop computers, the modeling of detonation events,

including shaped charges, is becoming a tractable and cost effective problem to model. This emerging analysis

capability is helping to drive shaped charge designs to become more efficient and cost effective.

A conclusion section is not required, though it is preferred. Although a conclusion may review the main points of

the paper, do not replicate the abstract as the conclusion. A conclusion might elaborate on the importance of the

work or suggest applications and extensions. Note that the conclusion section is the last section of the paper that

should be numbered. The appendix (if present), acknowledgment, and references should be listed without numbers.

References

Proceedings 1. Mallery, M. Performance Analysis of Linear Shaped Charge for Aerospace Applications. AIAA-2005-3839.