52 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 1, JANUARY 2003

Why “C” Armatures Work (and Why They Don’t!)David C. Haugh and Grant M. G. Hainsworth

Abstract—While plasma armatures perform well at high veloc-ities, they have severe effects on railguns at the shot start position.Solid armatures overcome this factor but perform less well as ve-locities increase. Many types of solid armature have been tried, in-cluding metal fibers and magnetic obturator types, but the classic“C” type remains the armature of choice for high-velocity projec-tile launches. The recovery of fired armatures at the Kirkcudbrightrange has allowed many of these ideas to be formulated. This paperlooks at what happens to “C” types, explains the basic processesoccurring as the armature gains velocity, and identifies the lim-itations of this design. It offers suggestions for improving perfor-mance, including materials and design options, with outline resultsin some areas, and offers some design characteristics to be consid-ered with this classic armature shape. Comparison with alternativearmature concepts is briefly made so as to present an overall view.

Index Terms—Contacts, EM propulsion, railguns.

I. BACKGROUND

I N THE LAST 15 years or so, a large number of electromag-netic (EM) gun shots have been fired world-wide, using a va-

riety of armature types. Many early shots used metallic plasmaarmatures, which allow the attainment of very high velocities(beyond 7 kms ) but have to be preinjected into the gun to re-duce the amount of erosion at shot start. Additionally, the largevoltage drop across plasma armatures results in poor efficiency,particularly at large calibers. Overall system complexity, mass,and volume can be reduced by the use of solid armatures. Theycan be fired from a standing start and lend themselves to loadingin a conventional manner. It was these considerations whichconvinced the U.K. to work only on this type of armature. Workhas continued elsewhere on fiber brush armatures [1]–[4], butthe U.K.’s efforts have focused on solid metallic types.

The reasoning behind our suggested armature behavior isbased upon physical and metallurgical examination of dozensof recovered 90-mm armatures and of the correspondingcondition of the launcher bore. Previous papers from the teamhave given partial information [5]–[8] and described the facilityand techniques in some detail [9].

An armature only has to operate for some 5 or 6 ms but in thattime, it sees huge forces, high temperatures, and high velocities.This paper considers base push designs, though the general prin-ciples apply to any “C” type variant.

EM propulsion is a simple fundamental physics phenom-enon, but understanding the detailed mechanisms involved is

Manuscript received January 14, 2002. This work was supported by the U.K.Ministry of Defence as part of the Corporate Research Programme TechnicalGroup 01.

D. C. Haugh is with the Defence Science and Technology Laboratory, WEWDepartment, Kent TN14 7BP, U.K. (e-mail: dchaugh@dstl.gov.uk).

G. M. G. Hainsworth is with QinetiQ, FST Division, Kent TN14 7BP, U.K.(e-mail: ghainsworth@qietiq.com).

Digital Object Identifier 10.1109/TMAG.2002.805910

extremely complicated and tasks the most acute minds andmodels. Examination of the papers published at Electromag-netic Launch (EML) Symposia in the last decade reveals amultitude of approaches and possible analyses of the problemsof solid armature-rail interactions. Work is still going on totry and produce finite-element models which can reproducethe passage of an armature down a launcher. Modeling thecomplexity of the sliding interface is difficult, and most modelswhich have a velocity capability fail to give answers whichmatch real firings [10]–[12]. The changes in temperature andphase-state and the very dynamic nature of the processes arethe principal factors which make modeling difficult.

The behavior of solid armatures along a gun bore can be di-vided into four discrete regions.

II. REGION ONE: START TO 1000 ms

When loaded into a gun, a “C” type armature makes contactbecause of a premachined mechanical interference. The empir-ical “gram per amp” rule for contact force seems to work, thoughthere appears to be a degree of latitude on this point. Increasinginitial interference produces no benefits other than raising theinsertion force. There seems to have been little work on identi-fying the lower bounds of this interference force—experimentalcaution outweighing scientific curiosity! Once the current startsto flow through the armature, this force is dwarfed by that pro-duced electrically.

Determining the exact time at which an armature begins tomove is not a trivial task. In-bore Doppler radar trials need areasonable bore size in order to produce meaningful answers.At calibers under 50 mm, it is doubtful whether sensible an-swers can be obtained. However, at 90 mm, results were goodenough to estimate that 600 kA were flowing through the circuitwhen movement began. The exact figure is affected by armaturedesign and interference, rail and armature deformation, surfaceconditions and friction, to name but a few.

When an armature starts to move, the current is still risingrapidly toward its peak value, which at 90-mm caliber can be ofthe order of 3 MA. The contact force pressing the armature legsonto the rail surface also increases, and if the armature leg mate-rial is too weak, then the accelerating forces on the armature canbe great enough to break it and leave the legs behind near shotstart. This gives almost instantaneous transition to a plasma ar-mature and copious quantities of rearward-ejected plasma fromthe gun. As in most designs, a compromise is needed betweenmechanical and electrical strengths and efficiencies. Using ahigh-strength material solves the mechanical issues but at theexpense of electrical conductivity, efficiency, and greater resis-tive heating.

In this early part of armature movement, up to about 1 kms,copper alloy rails acquire a coating of aluminum which appears

0018-9464/03$17.00 © 2003 BRITISH CROWN COPYRIGHT

HAUGH AND HAINSWORTH: WHY “C” ARMATURES WORK (AND WHY THEY DON’T!) 53

Fig. 1. Sectioned armature showing heat flow across width.

to have been mechanically wiped onto the surface. There is littlesign of melting, and the amount of aluminum covering the railsgradually increases with shot travel. There must be some local-ized melting on the armature contact surfaces, but the amount issmall enough to be instantly solidified on rail contact.

III. REGION TWO: 1000 ms TO TRANSITION

(It is possible to get transition below 1000 ms, but suchbehavior would be considered indicative of a poor armature de-sign). As the contact pressure increases, the degree of surfacemelting increases too, until at around 1 kms, the material de-posited upon the rail changes appearance to that of a resolidifiedlayer. At peak current, the contact forces between armature andrail are enormous. As the armature surface erodes away onto therails, the leg separation force continues to squeeze the legs out-wards, maintaining the contact pressure, distorting the armaturephysically to do this. Recovered armatures have a greater in-ternal root radius than when manufactured, due not to inner sur-face erosion but to outward mechanical displacement of the legs.

At shot start, it is probable that much of the current flowsthrough the rear of the armature legs, and MEGA finite-element(FE) modeling predicts this. Once the armature moves, the clas-sical velocity skin effect (VSE) predicts that much of the currentstays at the rear of the armature legs and flows around the in-side of the armature. According to this theory, the tips of thelegs should become very hot and ultimately melt. In practice,this effect is virtually never seen. Recovered armatures clearlyshow (Fig. 1) that current flows into the armature ahead of thetips of the legs, which remain relatively unscathed by the events.If the VSE is operating, then it cannot be the dominant factor inarmature behavior.

Static FE modeling shows that the current path moves fromthe rear of the legs to the front contact surface. This probably oc-curs because it is the path of least resistance. Copper alloy railshave a much lower resistance than an aluminum alloy armatureand so present a clear “low” resistance path to the current. Thecurrent appears to flow directly across the armature in a straightline from rail to rail, minimizing the distance traveled within thearmature.

Fig. 2. Graph of time versus muzzle volts, current, and velocity for a typical1600 ms firing of a 90-mm C-shaped armature.

IV. REGION THREE: TRANSITION

The corollary to this shift in current flow is that the arma-ture legs see less contact force pushing them against the rails.Once the current begins to fall, then at some point on the downslope, the legs lose contact with the rails. This region is theoft-observed transition from notional solid–solid contact to in-clude a plasma element. A repeatable and smooth transitionfrom metal–metal contact to plasma would be desirable but isnot seen. As the armature legs lose contact with the rails, dueto loss of leg material and/or reducing contact force, the muzzlevoltage records show that good metallic contact is lost (Fig. 2).During this period, the armature ballots from side to side until aplasma layer on both sides allows the armature to “float” downthe remainder of the bore. Lateral accelerations can be manythousands of “gees,” somewhat greater than is seen in conven-tional guns. It is in this part of the launcher that mechanicaldamage occurs to the rails, with gouging a near certainty withsoft rails and high velocities.

V. REGION FOUR: PLASMA CONTACT TO MUZZLE

Once a plasma has been established in the bore, minor surfacemelting is visible upon the insulators but is virtually immeasur-able. However, plasma rapidly attacks the armature surfaces anderodes them at a significant rate, reducing the size (and mass) ofthe armature. If transition occurs late in the launch cycle, thenplasma damage is minimal to the bore (and armature), and verylittle plasma is ejected rearward. However, the plasma presencedoes accelerate the loss of material from the armature surface,flowing in the gaps between the armature and rail, and thenfilling the space at the rear of the armature. Measured muzzlevoltage rises from that equating to metal–metal contact, i.e., upto about 40 V for a large-caliber armature, to several hundredvolts—indicative of a partial bore plasma. Full plasma armaturevoltages (for large bore guns) approach 1000 V, but this valuewould only ever be seen in the case of total armature failurein-bore.

54 IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 1, JANUARY 2003

Fig. 3. Recovered 90-mm armatures.

VI. DISCUSSION

As they leave the muzzle, armatures are still conducting cur-rent and so experiencing body forces. The material is quite warmand soft by this time, and it is common to recover armatureswith the legs splayed outwards like a bell (Fig. 3). Clearly, thiscould not have occurred in-bore. Despite their nonaerodynamicshape, armatures fly straight and are usually found within 30 mof the firing line. Impact damage caused by hitting the soft (wet)Scottish turf is minimal, and most are recovered in the conditionobserved in muzzle X-rays.

All recovered armatures have a smaller diameter than at shotstart. The entire contact faces show signs of melting and ero-sion [6] from front to back. As the firing velocity increases, sothe contact face erosion moves forward—a feature aided by theacceleration of the plasma forward along these surfaces. Withhigher velocities, there is more time for this plasma to act uponthe armature faces and mass loss can be severe.

From the point forward from that at which the launch packageattains a velocity of about 1000 ms, the bore receives a thincoating of aluminum removed from the armature and depositedon the exposed surfaces. This phenomenon occurs on every shot,but the layer does not build up in thickness. No evidence for borechoking has been observed, even after 50 shots or more on therails.

At the highest velocities, the armature legs can get so hot thatthey melt off in-bore. The survivability of solid armatures is di-rectly related to the total action that they have to carry, sincethe amount of resistance heating rises with muzzle energy. De-signs that work at small caliber and short in-bore times will notnecessarily work well (or at all) at high velocities and energies.This thermal limit is the ultimate one on loaded armature perfor-mance. All the clever design tweaks possible eventually becomesuperseded by this factor.

VII. L IMITATIONS

C-shaped armatures depend upon their initial mechanical in-terference to allow the current to flow through them from railto rail. However, once this current is flowing, the forces gener-ated within the armature body are huge and usually lead bothto physical distortion and also surface losses, due in part to sur-face melting. Maintaining a high current through an armaturecan delay or even obviate any transition by maintaining contact

forces between armature and rail, but only for lightly loaded sit-uations and relatively short times. The presence of a large cur-rent at the launcher muzzle is an obvious drawback too. Witha normal sinusoidal type pulse shape, C-shaped armatures in-variably fail at some point on the downward slope, dependingupon the exact firing conditions. It is the reduction in current(and hence forces) which leads to contact failure. C-shaped ar-matures work well for short times, before the body of the ar-mature has had time to get hot enough to lose its strength. Thus,there are many reported examples of excellent performance withshort guns and low loadings [13] which cannot be reproducedwith higher energies and practical weapon conditions.

VIII. POSSIBLEIMPROVEMENTS

A number of material options have been assessed to try to im-prove armature performance. Attempts have been made to retainthe current in the armature legs by using a thick layer of resistivematerial at the front of the contact face. Apart from the manufac-turing difficulties associated with this, the actual performancehas been disappointing. The current still seeks the lowest resis-tance path, concentrating at the interface between materials andcausing localized undercutting erosion and plasma generation.Tests were also conducted with high-conductivity material inthe armature regions known to carry most of the current. Thepresence of a discrete boundary between materials of differentconductivities resulted in failure at that position due to local-ized melting. This suggests that graded composition materials(made by powder metallurgy methods) will be better contendersfor armatures.

There are limited possibilities for design changes to the basic“C” shape. Most of them are bypassed because the current pathmoves forward rapidly from the rear of the legs to minimize thedistance traveled through the armature. Other workers have triedinertially loaded reverse wedge designs [14], but none of thesehas performed as well as expected. A novel magnetic obturatortype [15] was tested but has lost favor. Multiple contacts havebeen tried, with these being both far-spaced as well as adjacent.Most work at moderate velocities and loads but fail to persuadethe current to change path as the armature duty increases.

Armatures comprised of fine metallic fibers in an insulatingcage, usually G10 plastic and carbon fiber, have performed quitewell [1]–[4], but it is difficult to see how they could accept themechanical loads that a realistic projectile mass would provide.The requirement for considerable mechanical strength in addi-tion to good electrical properties is often overlooked by elec-trical designers.

IX. CONCLUSION

This paper has been an attempt to qualitatively explain theprocesses that occur as a large-caliber armature passes down agun bore. Quantifying such behavior will be considerably moredifficult, since the full physics of the process must include dif-ferent phase states in nonsteady-state conditions. C-shaped ar-matures have a reasonable pedigree of development and test, andremain the primary design shape for hypervelocity EM firings.They can be integrated into midriding designs (a subject for fu-ture papers), where the design principles read directly across

HAUGH AND HAINSWORTH: WHY “C” ARMATURES WORK (AND WHY THEY DON’T!) 55

from base push concepts. The form of the C-depends upon theexact duty requirements and the physical space available in theprojectile package. However, successful armature performanceis a prerequisite for any tactical projectile firing and must beachieved before serious effort is applied to the design of thelatter.

ACKNOWLEDGMENT

The authors acknowledge the many contributions from all themembers of the U.K. EM gun team who have played their partin undertaking this work.

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