426 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 1, JANUARY 2007

Electric Armor Against Shaped Charges:Analysis of Jet Distortion With Respect

to Jet Dynamics and Current FlowMatthias Wickert

Fraunhofer-Institute for High-Speed Dynamics, Ernst-Mach-Institut (EMI), 79104 Freiburg, Germany

Shaped charges are a warhead technology often applied to rocket propelled grenades and represent a dangerous threat for armoredvehicles in combat as well as in peace-keeping operations. Their armor piercing performance rests upon an explosively induced collapseof a metallic liner to a stretching jet with very high particle velocities. A copper jet produced by a shaped charge can be distorted by highelectric currents injected into the jet by means of spaced electrode plates connected to a high voltage capacitor. In tests carried out atFraunhofer EMI, a shaped charge with a well characterized jet was used for the experiments in order to examine the current flow throughthe jet and its effect on the jet evolution. The measured current flow is related to the jet dynamics and the distortion pattern observed bymultiple flash X-ray images. As expected, the current flow starts when the jet tip reaches the back electrode plate. No significant changeof the current flow is observed at the characteristic jet break-up time. The current flow continues after the tail of the copper jet has leftthe electrodes and resembles a damped sinusoidal. A distortion of the jet is observed where the jet particles are not aligned along the jetaxis. Instead the particles are stretched orthogonally to the jet axis with increased separation along the jet axis. The tip part of the jetis hardly affected. The jet distortion is analyzed with respect to jet dynamics and current flow which allows formulating criteria for thedesign of electric armor systems. The current injection effective for jet distortion is limited to a time slot of a magnitude of 60 s for the44-mm caliber-shaped charge used in the experiments. To a first approximation, the current flow can be modeled by an electric arc. Anelectric circuit model can describe the current flow behavior with respect to the electric impedance and allows designing an electricalcircuit adequate for the time slot. By the analogy of a wire explosion the necessary current magnitude for an effective jet disruption withrespect to the interaction time slot can be estimated to begin at 300 kA. For the tip portion of the shaped charge jet, the time of effectivecurrent injection is very short. When the current starts to set in, the jet tip is already passing the back electrode plate. For this reason,an effective distortion of the jet tip represents a challenge that has to be mastered.

Index Terms—Electric armor, electromagnetic armor, shaped charge jet disruption.

I. INTRODUCTION

THE IDEA to utilize electrical energy to distort a shapedcharge jet was described by E. H. Walker in 1973 [1]. The

experimental proof of the potential of an electrical current todistort and disrupt a shaped charge was given by X-ray images,e.g., from C. E. Pollock in 1994 [2] or from G. A. Shvetsovin 1999 [3]. The aim of the following test series was to performexperiments with shaped charge jet distortion in order to analyzethe interaction of a current injected through a front and a backelectrode plate with respect to the jet dynamics, as indicated inFig. 1. For the test series the electrical equipment available inthe laboratory stock was used, thereby tests with moderate jetdistortion were realized and the analysis of these tests allowsformulating design criteria required for an electric armor systemwith effective jet disruption.

II. EXPERIMENT

For the experiments, a well-characterized type of shapedcharge with a copper liner of 44 mm in diameter is used. Tipposition and tail position of the shaped charge jet can

Digital Object Identifier 10.1109/TMAG.2006.887650

be described with respect to the time of the ignition pulse forthe shaped charge by

mm s mm (1)

mm s mm (2)

where s denotes the working time of the igniter.The particularization of the shaped charge is to be expected

at s s after ignition.The experimental setup is sketched in Fig. 2. The shaped

charge jet is positioned at the left side and points at the spacedplate electrode module. Two metal plates made of 10-mmcopper are separated by a 50-mm plastic spacer. In this testarrangement, the plate normal is tilted by 60 relative to the jetaxis resulting in an effective gap of 100 mm along the jet axis.

High voltage capacitors are connected in parallel to forma total capacity F. The capacitors are protectedfrom the shaped charge by a metal plate wall with a cablefeed-through for the electrical connection. This arrangementresults in a minimum cable length which attributes to the totalcircuit inductivity. The capacitors are loaded with a voltage

of 16 kV resulting in an available electrical energy of100 kJ according to

(3)

0018-9464/$20.00 © 2006 IEEE

WICKERT: ELECTRIC ARMOR AGAINST SHAPED CHARGES 427

Fig. 1. Shaped charge jet dynamics with respect to the stationary position offront (left black line) and back electrode plate (right black line). Principle sketchfor an undisturbed jet.

Fig. 2. Shaped charge jet location as a function of time and corresponding elec-tric current flow measurement.

The focus of the diagnostic is on the evolution of the jet dis-tortion with respect to time after passing the loaded electrodemodule. So, behind the electrodes, a large space is left to takeX-ray images of the shaped charge jet. At a distance of 600 mmbehind the electrode module, RHA witness plates are placedin order to measure residual penetration. In every experiment,three X-ray flashes are triggered with a time delay of 20 s.

The X-ray channels are aligned on a vertical axis with thesame distance. So, three images of the target structure and theshaped charge jet appear on the same X-ray film. The X-rays aretaken at 100, 120, and 140 s after the ignition of the shapedcharge and allow to observe the evolution of the jet distortionwith respect to time.

Two tests with no voltage applied and three tests with a highvoltage of 16 kV are performed. The tests show little spread. For

the analysis, the X-ray pictures of one exemplary test withoutand with high voltage are presented.

Without any high voltage applied to the electrodes, the jetdoes not show any significant distortion in Fig. 3. At 100 s theparticularization of the jet is observed. At 120 s the jet particlesare separated. The elongated particles are oriented along the jetaxis. At 140 s the particles appear a bit less regular due tothe effect of long traveling time. The central portion of the jetat the different times is highlighted. The residual penetrationmeasurement yielded twice 116-mm RHA.

With a high voltage of 16 kV applied, a significant distortionof the shaped charge jet is observed, compare Fig. 4. At 100 sthe status of the jet particularization is more enhanced due to thecurrent effect. In the central portion of the jet, the length of theparticles is reduced and their separation has progressed faster.The later images reveal that jet distortion grows and that theparticles tend to become stretched orthogonally to the jet axis.At 140 s the distortion effect is the most obvious with verylarge gaps between irregular cloud-like shaped particles. In theX-ray images available, the distortion effect is strongest in thecentral portion of the jet. For better comparison of particles withsimilar velocities, the corresponding central portion of the jet ishighlighted again. The back part of the jet is still passing throughthe electrodes in the X-ray images available and, thus, distortionmay still evolve there. On the other side, the tip portion of the jethas passed the electrode plates already. Nevertheless the tip ishardly affected by the loaded electrode module. The applicationof the high voltage is correlated by a decrease of penetrationdepth in the three tests to 68-, 62-, and 60-mm RHA. At longstand off, a large spread of penetration depth for shaped chargesis common, but the positive correlation indicates a performancereduction due to the electrical distortion effect.

III. ANALYSIS AND DISCUSSION

For the interaction analysis, with respect to the jet dynamicsand the current flow, the position of the jet tip and the jet tailaccording to (1) and (2) are plotted in Fig. 5 as a function ofthe time elapsed since the ignition of the shaped charge. Thus, avertical line between tip and tail position visualizes the jet’s lo-cation and momentary length with respect to time. At a distanceof 175 and 275 mm, the position of the front electrode and theback electrode plate are indicated. The onset of the current flowis to be expected when the jet tip reaches the back electrode.

The jet location is also visualized in the diagram for the char-acteristic particularization time—and for the time when the jettail leaves the front electrode. Taking tail exit as the end pointof effective current flow for jet distortion, the magnitude for thetime slot for effective current injection into the jet is of the orderof 60 s for the 44-mm shaped charge. Now, focus on the currentgraph plotted into the diagram. As to be expected, the currentstarts to flow at the moment when the jet reaches the back elec-trode. The current starts to rise in a sinusoidal manner. When thejet breaks up at the characteristic particularization time no sig-nificant effect on the current flow is observed. The current risesfurther and it still rises, after the expected time for the jet tailleaving the front electrode at 115 s. The maximum current of210 kA is reached at 125 s. The current follows the sine curveand falls to zero at 215 s—although the jet is expected to have

428 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 1, JANUARY 2007

Fig. 3. Time evolution of an undistorted shaped charge jet after passing of an unloaded electrode plate arrangement at 100, 120, and 140 �s after ignition of theshaped charge. The local velocity of the jet particles is indicated at the limits of the highlighted regions and for the jet tip.

Fig. 4. Time evolution of jet distortion after passing of a loaded electrode plate arrangement at 100, 120, and 140 �s after ignition of the shaped charge. The localvelocity of the jet particles is indicated at the limits of the highlighted regions and for the jet tip.

Fig. 5. Shaped charge jet location as a function of time and corresponding elec-tric current flow measurement.

also passed through the back electrodes at 160 s. And evenafter the time depicted in Fig. 5, the current continues to falland rise and follows a damped sinusoidal. The observed timebehavior of the current flow indicates that the current flow istriggered by the shaped charge jet but that the time evolution ofthe current flow is not restricted to jet particles traveling throughthe gap between the electrodes. After the current is triggered, itevolves similar to the current of an electric arc that is capableof sustaining its flow as long as the local conditions support thecreation of carriers of electric charges. The enlarging separationdistances between the jet particles after the characteristic partic-ularization time do not seem to effect the current flow substan-tially—neither does any gap between the jet and the crater holeedge in the electrode plates.

Fig. 6. Circuit model for current flow initiated by a shaped charge jet.

As a consequence, an electric armor design against shapedcharge jets has also to avoid parasitic current paths, where thecurrent does not flow through the jet particles and, thus, doesnot contribute to the jet disruption.

An electric circuit model of a damped sinusoidal discharge,like in Fig. 6, is capable of describing the observed current flowafter the triggering event by very few electric impedance pa-rameters like the capacity , the inductivity , and the ohmicresistance . An electric arc can be modeled with a conduc-tivity being dependent on time. But, since in a simplisticview, an electric arc can be regarded as a high voltage switch, itmight even be compared with a short circuit switch. Fig. 7 showsgood agreement of the circuit model prediction with the mea-sured current flow. In this diagram, the time scale is extended to1000 s and so the damped sinusoidal form of the current canbe recognized better.

Since there is still plenty of electric energy within the cir-cuit after the shaped charge jet has left the electrode plates, the

WICKERT: ELECTRIC ARMOR AGAINST SHAPED CHARGES 429

Fig. 7. Comparison of measured current flow I and capacitor voltage U (top)with circuit model prediction (bottom).

available electrical energy was utilized for jet disruption only toa partial extent.

For effective energy utilization an overdamped oscillationwould be more preferable—with a half period time aboutthe effective time slot for current injection. The half time width

can be approximated by

(4)

Under the assumption of an identical capacity and the sameloading voltage , for corresponding to the effectivetime slot the circuit model requires a reduction of the inductivityby a factor of more than 7. Since the cable inductivity is propor-tional to the cable length, an alternative low inductivity design isrequired for the experimental setup. Since the electrical energystored in the capacitor is proportional to (3) and does notaffect according to (4), a high electrical voltage is favor-able for increasing the available electrical energy. In the testsperformed, only a modest jet disruption was observed with anenhanced particularization that may be attributed to the pinch ef-fect on an electrical conductor at high currents. Although someradial stretching of the jet particles was observed, for the de-feat of a directed energy weapon like a shaped charge jet it isessential to disperse the impacting particles from the jet axis ef-fectively. Thus, a strong radial disruption mechanism is neededin order to spread the jet particles over a wide area.

A model mechanism for such a type of strong dispersionmechanism might be the wire explosion [4]. For a solid copperwire, a wire explosion is expected, according to [5], when theproduct of action time with the square of current densityreaches

(5)

If this mechanism can be applied to a copper jet of a shapedcharge with an assumed diameter of 2 mm, whose tip passes the100-mm electrode gap with a speed of 7 km/s in s, the

onset of wire explosion like effects can be expected for a con-stant current 300 kA. This value may be used as an indicatorfor the required current magnitude for effective shaped chargejet disruption.

Although the jet tip portion might be smaller than the av-erage jet diameter, the jet tip will remain hard to disrupt effec-tively. This is not only because the tip part of the jet needs theshortest time to pass the electrode module, but the current setsin as late as the tip reaches the back electrode—but then thetip is already leaving the electrode plate again. So even whenthe current would not rise in a sinusoidal manner, but instead, aconstant current would instantaneously set in, the current flowthrough the tip portion is limited to a very short time. In contrastto the rest of the jet, the tip traveling time through the electrodeplates is not used for effective current injection. The effectivedisruption of the jet tip portion will remain a challenge.

IV. CONCLUSION

The interaction analysis with respect to the jet dynamics hasled to design criteria for an electrical armor arrangement. Forthe specific jet with a caliber of 44 mm, the time slot for effec-tive current injection is limited to a magnitude of 60 s. In goodapproximation, the current flow can be modeled by an electricarc. An electric circuit model can describe the current flow be-havior with respect to the electric impedance parameters.

By the analogy of a wire explosion mechanism, the magni-tude of the necessary current for the onset of a strong jet disrup-tion with respect to time slot for effective current injection canbe estimated to 300 kA.

The current injection time through the tip portion of theshaped charge is limited, since when the current flow starts, thetip already leaves the back electrode plate again. The effectivedisruption of the jet tip portion represents a challenge to bemastered.

The analysis results in criteria for an electric armor designagainst a specific shaped charge. But the procedure is generaland also allows evaluating electric armor designs against a va-riety of shaped charge threats with different jet dynamics.

REFERENCES

[1] E. H. Walker, “Defeat of shaped charge devices by active armour,” U.S.Army Ballistic Res. Lab., Tech. Rep. BRL-MR-2305, 1973.

[2] C. E. Pollock, “Electromagnetic effects on the natural hydrodynamicinstability of stretching, high velocity, metallic jets,” in MegagaussMagnetic Field Generation and Pulsed Power Applications. Com-mack, NY: Nova, 1994.

[3] G. A. Shvetsov, A. D. Matrosov, A. V. Babkin, S. V. Ladov, and S.V. Fedorov, “Disruption of shaped-charge jets due to axial current,” inProc. 18th Int. Symp. Ballistics, 1999, pp. 581–587.

[4] “New age electric armour tough enough to face modern threats,” De-fense Sci. Technol. Lab., Jul. 1, 2002 [Online]. Available: www.dstl.gov.uk

[5] J. M. Löffler, Hochleistungspulstechnik - Skript zur Vorlesung Fach-hochschule Gelsenkirchen, 1999.

Manuscript received December 16, 2005 (e-mail: wickert@emi.fraunhofer.de).