IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-22, NO. 6 , NOVEMBER 1986 1521

SIX MEGAJOULE HAIL GUN TEST FACILITY

Michael M . Holland, G . Mark Wilkinson, A . Peter Krickhuhn and Rolf Dethlefsen

SUMMARY

CHECMATE, a h igh -ene rgy r a i1 .gun u se r t e s t f ac i l i t y , i s cu r ren t ly ope ra t iona l a t Maxwell Labora to r i e s , Inc . i n San Diego, California. Sponsored by t h e S t r a t e g i c Defense I n i t i a t i v e O f f i c e (SDIO) and the Defense Nuclear Agency (DNA), i t u t i l i z e s Maxwell's newly- developed high energy density capacitor technology. I t t h u s r e p r e s e n t s t h e f i r s t r a i l gun f a c i l i t y where the h i g h r e l i a b i l i t y , f a s t t u r n a r o u n d , and v e r s a t i l i t y of capaci tor banks are combined with the multimegajoule energy previously available only with explosive f lux compressors or large homopolar genera tors .

With t h i s system, users may s t u d y r a i l gun component development (such as bore wall ablation and armature des ign ) , smar t p ro j ec t i l e su rv ivab i l i t y i n a high llg't and high B environment, and issues of i n t e r i o r , e x t e r i o r , and t e rmina l baL l i s t i c s a t t e s t ing r a t e s exceeding 1 shot/day. The capacitor-based approach al lows the ra i l gun issues t o be decoupled from problems of power source development.

The r a i l gun ' s bas i c con f igu ra t ion is a 5 meter long, 5.1 em s q u a r e b o r e . P r o j e c t i l e s a r e i n j e c t e d a t 500 t o T O O m/s, using a high pressure helium gas gun. The r a i l s and i n su la to r s a r e f ab r i ca t ed from copper al loy (MCZ), and epoxy/fiberglass composite (G-lO), respec- t ively. Current is f e d t o t h e r a i l s a t t h e b r e e c h from s i x independently-controlled 1 MJ capacitor modules, each employing a -15 VH ser ies inductor . The modules a re t r iggered us ing Maxwell low-inductance r a i l gap- swi tches , which have an i n d i v i d u a l f i r i n g j i t t e r of l e s s t h a n +5 ns . Simultaneous f i r ing of t h e s i x modules produces a peak gun cu r ren t of up t o 2 MA with a r i s e time of 220 ps. Sequen t i a l f i r i ng i s a l s o p o s s i b l e , i f a broader current pulse of more near ly cons t an t acce l e ra t ion is required. Explosively-driven, metal-to-metal crowbar switches, adapted from NRL des igns , [1 1 a r e used ac ross t h e module output terminals to minimize vol tage reversal on the capac- i t o r s , and to force the induct ive ly-s tored energy to c o u p l e e f f i c i e n t l y t o t h e r a i l gun load .

Users may avai l themselves of a f u l l complement of r a i l gun and p ro jec t i l e d i agnos t i c s . Ra i l gun measurements include breech and muzzle voltage, breech current, and rai.1 and arc B measurements. P ro jec t i l e d i agnos t i c t oo l s i nc lu e f l a s h X-ray un' ts , high speed framing cameras (10 frames/s and 1 0 frames/s), break wire and laser ga te ve loc imeters , a b a l l i s t i c pendulum, and a VISAR la se r i n t e r f e romet ry ve loc i ty measurement system. Data are collected on an in t eg ra t ed IBM XT computer/LeCroy digitizer-based system, augmented by a

G t,

This work was sponsored by the Defense Nuclear Agency

The au thors are with Maxwell Labora tor ies , Inc . , San under Contract DNA001-84-C-0013.

Diego, CA 92123.

Nicolet 4-channel digi ta l osci l l .oscope. Forty channels a r e a v a i l a b l e with sampling ra tes of up t o 1 MHz.

In i n i t i a l t e s t s ( u p t o December 1985) more than 20 e l e c t r i c a l s h o t s were f i red , wi th p ro jec t i le masses ranging from 75 t o 156 grams. Good agreement was obtained between predicted performance and experimental ve loc i ty da ta . A peak ve loc i ty of over 3.1 k m / s was achieved f n r a 106 gram Lexan polycarbonate p ro jec t i le , enabling a 2.5 em s t e e l w i t n e s s p l a t e t o be pene- t r a t e d . T h i s represented a peak acce le ra t ion of 3.7 x lo6 m/s2. Unlike systems where t h e p r o j e c t i l e is e l e c t r i c a l l y a c c e l e r a t e d from r e s t , t h e h i g h i n j e c t i o n ve loc i ty r e su l t ed i n minimal r a i l and in su la to r damage. A t e s t s e r i e s e x t e n d i n g f o r many t ens of sho t s is possible before ra i l refurbishment becomes necessary.

I . In t roduct ion

In recent years , a number of researchers have reported successfu l opera t ion of r a i l gun a c c e l e r a t o r s , ach iev ing p ro j ec t i l e ve loc i t i e s subs t an t i a l ly i n excess of those achievable with chemical guns. C2-61 In genera l , most of these experiments have been conducted using small bore (1 em or less) r a i l guns and small mass (1 t o 10 g ) p r o j e c t i l e s . These resu l t s ho ld ou t the promise of major advances in gun technology, pro- vided they can be scaled to higher energy, more massive and sophis t icated payloads.

Thus, it is des i r ab le t o have a r e l i a b l e r a i l gun power source ava i lab le in the severa l megajoule range , enabl ing pro jec t i les >5 cm in diameter and >100 grams t o t a l mass t o be accelerated to energies approaching a megajoule. Such a s c a l e would evab le t e s t ing of model l lsmart t t project i les in the high B, h igh acce lera t ion environment of a r a i l gun. Likewise, saboted projec- t i l e s , poss ib ly w i th pene t r a to r s , cou ld be developed f o r l e t h a l i t y t e s t i n g a g a i n s t v a r i o u s armor t a r g e t s . Data gathered on armature arc vol tage and bore wall erosion, would, together w i t h small bore results, enable sca l ing laws to be obta ined for assess ing the f e a s i b i l i t y of s t i l l larger bore devices .

Such a system is CHECMATE, b u i l t by Maxwell under the sponsorship of t he S t r a t eg ic Defense In i t i a t ive Of f i ce (SDIO) and the Defense Nuclear Agency ( D N A ) . An acronym f o r compact, high energy capacitor module advanced technology experiment, CHECMATE cons i s t s of s i x 1 MJ capac i tor modules arranged i n a c i r c u l a r fashion and connected by inductors to the breech of a r a i l gun. Figure 1 shows t h e f a c i l i t y i n o p e r a t i o n , and Fig. 2 , a schematic diagram. The system uses the high energy density capacitors developed by Maxwell under DNA sponsorship. I t . i s t h i s advance i n c a p a c i t o r technology that makes the s ix megajoule system tech- n i c a l l y and economically feasible.

0018-9464/86/1100-1521$01.0001986 IEEE

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GRADE 8 BOLTS

Fig . 1. CHECMA’TE f a c i l i t y i n opera t ion

HELIUM GAS GUN INJECTOR ONE MEGAJOULE

CAPAClTOR MODULE / ,-.n.,.<cr. ,TING 3RK

p I

. /

COIL PLATES , / - ELECTROMAGNETIC

LAUNCHER / v’I INTERMEDIATE STORAGE

INDUCTOR ASSEMBLY PROJECTILE (ONE OF SIX)

Fig . 2 . Schematic of CHECMATE system.

Sec t ion I1 descr ibes the system design and the diag- n o s t i c s a v a i l a b l e t o p r o s p e c t i v e u s e r s . I n Sec t ion 111, we summarize experimental results trom t e s t s conducted in November-December 1985, and compare them w i t h ca l cu la t ions . These r eu l t s l ead t o t he con - c lus ions of Sec t ion I V .

The CHECMATE f a c i l i t y i s composed of three p a r t s : t h e ra i .1 gun acce le ra to r , t he 6 MJ power supply, and t h e d iagnos t ic and data acquis i t ion system. Each is described below.

Rail Gun

The r a i l gun barrel design incorporates a pre-s t ressed bol ted conf igura t ion . A c ross -sec t ion is shown in Fig . 3 , w i t h dimensions. The ra i l s a r e backed w i t h G - I O i n su la to r s and clamped with 3 i n . t h i c k s t e e l strongbacks using 1 9 2 , 1 .25 i n . diameter grade 8 bo l t s . A 2 in. minimum th ickness of G-10 in su la to r SUrrGUnds t h e bo re , a l l owing fo r t he r e tu rn of magnetic f l u x . An i n i t i a l 0.020 in . c l ea rance be tween t he r a i l backing insulators and t h e G-10 s i d e r a i l i n s u l a t o r s ensures t ha t t h e sides are dr iven aga ins t the s teel s i d e walls du r ing p re s t r e s s . The s ide walls are

w COPPER RAILS ( M C Z ) ~

1

1 r I I I I T I

0 5 10 15 20 25 30 cm

Fig. 3 . Barrel cross-sect ion.

captured in grooves in the strongbacks. Fiigura 4 pro- v ides a muzzle end view of t h e ba r r e l . The turned-up ends of the r a i l s form an a r c shube t,o minimize a r c damage d u r i n g p r o j e c t i l e ex i t . The overa.;l. l.ength of t h e b a r r e l is 5 meters.

F i g . 4. Muzzle view of CHECMATE ’,auncher.

Two-dimensional f i n i t e element modeling of t h e barrel. c ross -sec t ion (Fig. 5 ) , based upon s t ? t i c de fo rma t ion under peak stress, r e s u l t e d i n a conservat ive barrel design. The 350 MPa r a i l and pLasma pressure (corre- sponding t o 2 MA peak c u r r e n t ) r e s u l t s i n some plasma leakage , s ince the p re load in t h e bol t s (90 ,000 lbs each) i:j l imi t ed by allowable com2ression in the G-10 s idewal l s . The calculated preload deformations were used i n pre-dimensioning the ba r re l components so t h a t the pre-loading would squeeze the bore from rectangular to square . This worked ou t qu i t e well i n p rac t i ce ; ho r i zon ta l and ver t ica l d imens ions d i f fe red by l e s s than 1 percent f o r the assembled barrel .

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Fig. 5. Finite-element analysis of r a i l gun bore s t r e s s r e sponse : ( a ) undeformed; ( b ) exaggerated deformation due p r e l o a d , r a i l , and plasma pressures

Lexan p r o j e c t i l e s a r e i n j e c t e d i n t o t h e r a i l gun ba r re l a t 500 t o 700 m / s (depending upon mass) by a pressurized helium gun opera ted a t 3000 p s i . The device i s f i r e d by ruptur ing a s t a i n l e s s s t e e l diaphragm, using a lance . The p r o j e c t i l e ( F i g . 6 ) , i n i t i a l l y rides on t h i n sk i r t s which provide a pressure s e a l a t t h e 5.3 em gas injector dimension. A 50 cm

Fig. 6. Rail gun p r o j e c t i l e .

long , t apered sec t ion makes a t r a n s i t i o n from 5.3 em t o t h e nominal 5.1 om r a i l gun bore. In t r ave r s ing t he t ape red s ec t ion , t he p ro j ec t i l e skirts a r e wiped o f f . Repeated broaching of t h e r a i l gun bore throughout its l i f e t i m e will eventua l ly increase t h e b o r e s i z e t o 5.3 em, r e s u l t i n g i n t h e shor ten ing and gradual e l imina t ion of the t apered t rans i t ion . Th i s condi t ion is not expected to occur for many hundreds of shots . Rai l erosion (Fig. 7 ) has been minor , s ince the p r o j e c t i l e is i n j e c t e d a t f a i r l y h i g h v e l o c i t y .

The six capacitor modules, described below, are con- nected t o a pa i r of copper cur ren t co l lec tor p la tes . These plates connect to the upper and lower rails through copper bars which pene t r a t e t he G-10 backing p l a t e s . The en t i re l lb reechl l reg ion , where cur ren t con- vergence occurs, is secure ly clamped by 1.25 i n . d iameter s teel bol ts through 3 i n . s t e e l body p l a t e s and 2 i n . G - I O i n su la to r p l a t e s .

The f i r i n g sequence is i n i t i a t e d when t h e p r o j e c t i l e ' s f o i l backing (Fig. 6 ) p rov ides e l ec t r i ca l con tac t between the upper and lower ra i ls . The l a t t e r a r e main ta ined a t a nominal 2.5 kV po ten t i a l d i f f e rence , which, when shorted, provides an unequivocal tr igger s igna l .

Fig. 7. Observed r a i l e ros ion i n t he r eg ion of peak cu r ren t .

-- Pulsed Power

The capac i t i ve power s u p p l y f o r CHECMATE is based upon t h e 1 MJ, 1033 pF, 44 kV capac i tor module described previously. E71 Br ie f ly , t he module cons i s t s of twenty, 50 k J Scyl lac-s tyle cans, s tacked two ac ross and ten h i g h , s e t i n a welded s t e e l frame and connected t o a low-inductance, Mylar-insulated aluminum output bus (see F igs . 1 and,2) . Switching is ca r r i ed ou t w i t h fou r pa ra l l e l r a i l gaps . Each module contains a 4.5 mn s e r i e s damping r e s i s t o r t o limit t h e c u r r e n t i n t h e event of a f a u l t . The design was based on Maxwell experience in building, the 9 MJ SHIVA bank a t AFWL, b u t i s considerably more compact, w i th f ive t imes the energy/can of SHIVA. Originally conceived as a d r ive r for inductive energy storage experiments, CHECMATE'S s i x modules a re capable o f energ iz ing a 40 nH s to rage inductance to a cu r ren t of over 1 2 MA i n 20 us.

In o r d e r t o d r i v e a r a i l gun load , l a rge i nduc to r s (nominally 15 UH each) are placed i n s e r i e s with t h e capacitor modules. The inductors are twelve- turn so lenoids , wound from e i g h t p a r a l l e l t u r n s of copper s t r a p i n a conf igura t ion s imi la r to L i tz wi re . Th i s provides nearly complete use of the copper cross- s ec t ion fo r ca r ry ing cu r ren t . The c o i l s a r e immobilized by f iberg lass b locking and filament-wound i n a fiberglass-epoxy cocoon. The completed c o i l i s housed i n a 24 i n . d i ame te r coax ia l cu r ren t r e tu rn , providing magnetic flux containment and allowing use of SF6 insu la t ion . The coi l inductance limits the c u r r e n t h o d u l e t o a maximum of 330 kA, r e s u l t i n g i n a maximum bar re l cur ren t of 2 MA. The r i s e t i m e of the cur ren t is 220 us.

In order to couple more e f f i c i e n t l y t o t h e r a i l gun load (and to p revent vo l tage reversa l on the capac i tor bank), a crowbar switch is p laced before the se r ies i n d u c t o r , a t the output of each capacitor module. The crowbar switch is a s ix-s i te , de tona tor -dr iven c los ing switch, which provides a metal-to-metal connection a f te r punctur ing a polye thylene insu la tor . The c losure time may be c o n t r o l l e d t o w i t h i n +2 us, and t h e s i x channels have carried i n excess of 400 coulombs w i t h no damage t o the device. The switch was or iginal ly devel- oped by R . Ford of NRL [ l ] , and modified by Maxwell t o f i t t h e CHECMATE geometry.

Current from each inductor module is c a r r i e d t o t h e breech of t h e gun via multiply-interleaved copper b u s bars . Th i s arrangement provides a means of reducing

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the magnetic forces on t h e b a r s t o a manageable l e v e l . Each inductor output is connected via a 10 kV spark gap t o a 35 mn e l e c t r o l y t i c dump r e s i s t o r . I n the event of a system p r e f i r e (wi th no p r o j e c t i l e i n the bore) t h i s provides a means of harmlessly d i s s i - pating the system energy. Figure 8 shows t h e equ iva len t c i r cu i t of the pulsed power system. We assume t h a t t h e s i x modules a r e f i r e d simultaneously so tha t they may be modeled a s a s i n g l e u n i t . Table 1 summarizes the system parameters.

Fig. 8. Equiva len t c i rcu i t of CHECMATE ra i l gun d r i v e r . (S, r ep resen t s t h e r a i l gap switches which t r igger the capac i tor bank , S2 represents the crowbar switches, and S the spark gaps of t h e f a u l t - protector circult . Other components are described i n Table 1 ) .

3

E’RAMETER

CB

*B

L B

LS

LC

RS

dL/dx

RD

D_ESCRIPTION VALUE

Bank Capacitance 6161 uF ( a )

Bank Resistance 0.75 ma ( a )

Bank Inductance 8 nH (a)

I_

System Storage Inductance 2.5 uH ( a )

Connection Inductance ( a t i n i t i a l p r o j e c t i l e locat ion) 0 .7 uH ( a )

Series Resistance 0.32 ma ( b )

Inductance Gradient 0.42 pH/m ( a )

Faul t Pro tec tor Resistance 6 ma (a )

f l o w i n g t o t h e r a i l gun breech, and to detect any abnormal i t ies i n ind iv idua l module operation. Figure shows a typical inductor current waveform.

9

I 400 pslDIV

Fig. 9. Typical inductor current waveform.

Magnetic probes embedded i n t h e i n s u l a t i n g s i d e r a i l s e n a b l e t h e r a i l a n d armature d I / d t t o be determined as a func t ion of time. Typical probe data are shown i n Fig. 10. Such s ignals enable one to measure the t ime a t which the p ro jec t i le passes a given probe location and to e s t ima te t h e length of the plasma armature.

I 7 I . . , .: .,,.,...,,.

( a ) Measured values ( b ) Calculated values.

The CHECMATE f a c i l i t y employs a number of d iagnos t ic probes for monitoring the raj.1 gun and capacitor bank performance. Additional diagnostics continue to be brought on l i n e . T h i s sec t ion descr ibes some of the more important ones presently in use.

Current f ed t o t h e r a i l gun breech is monitored with a Rogowski c o i l on each of the s i x inductor modules. In addi t ion, each capaci tor module has a Rogowski c o i l , Located i n the output bos before the crowbar switch. These two probes provide redundant information u n t i l t he crowbar switch is closed. A t iming s ignal , der ived from the detonator current pulse, monitors the t ime of crowbar switch closure. From these d iagnos t ic channels, i t is poss ib l e t o de t e rmine t he t o t a l cu r ren t

0.002 s/DIV

Fig. I O . Rai l current d I / d t :jignals. Probe positions a r e : ( a ) 1 1 7 em, ( 5 ) 218 em, ( e ) 320 em, and (d ) .480 ern (muzzle Location). Data are from Shot 37.

A voltage probe Located a t t h e r a i l gun muzzle, measures t h e arc-voltage drop as a func t ion of time. The probe consis ts of a 10 ka resis tor connected across the ou tput ra i l s th rough a 1 2 gF capac i to r . Ground loop problems are e l imina ted by sensing t h e cu r ren t drawn by t h e probe, using a current t ransformer. A t yp ica l waveform is shown in F ig . 11 d i sp lay ing severa l f ea tu re s of the gun opera t ion . Most no tab le a re the a r c i n i t i a t i o n , t h e a r c v o l t a g e , a n d t h e e x i t t i m e of t h e p r o j e c t i l e from the muzzle.

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I PROJECTILE BRIDGES RAILS i I j , PROJEC ;TILE I

-s I 'I

0.001 SlDIV

Fig. 1 1 . Muzzle voltage probe signal.

When the p ro j ec t i l e l eaves t he ba r r e l , t he very br ight muzzle Plash provides more than enough i l l umina t ion t o allow fast cinematography using a 10,000 frame/s HYCAM I1 camera. The individual exposures are of 1 p s duration, enabling sharp images of t h e b u l l e t t o be obtained (Fig. 1 2 ) . T h i s provides an accurate measure- ment of t h e p ro jec t i l e speed and a l so enab le s one t o observe any fragmentation or tumbling of t he p r o j e c t i l e .

F ig . 1 2 . Photographs of p r o j e c t i l e i n f l i g h t (100 p s i n t e r f r ame) .

Excellent agreement ( to within a few percent ) is obtained between the HYCAM I1 da ta and three o ther measurements of p r o j e c t i l e v e l o c i t y . ? h e f i r s t is pro- vided by t h e f i n a l two magnetic probes w i t h i n t h e bar re l (F ig . 101, which give the t ime of t r a n s i t between two po in t s a known d is tance apar t . The second is a pai r of break wires located beyond the muzzle ?idst shi.eld and i n f r o n t of t he t a rge t box. The t h i r d is provided by a p a i r of l a s e r beams which a r e i n t e r - rilpted by t h e p r o j e c t i l e i n f l i g h t . These four diag- n o s t i c s e s t a b l i s h t h e p r o j e c t i l e v e l o c i t y t o a very high confidence level.

Final ly , one may obtain semi-quantitative information on p r o j e c t i l e momentum and energy from the target i t s e l f . The l a t t e r is configured as a b a l l i s t i c pendulum, and contains a s t e e l witness p la t e , t yp ica l ly 1 i n . i n th ickness . From the observed mass and na tu ra l frequency of the pendulum, the momentum t r a n s f e r may be

measured from i t s observed maximum swing upon impact. The witness plate may a l s o be examined f o r c r a t e r i n g , backspall and other evidence of hyper-velocity penetra- t ion (F ig . 13) .

Fig. 13. One inch thi .ck witness plate , penetrated by Lexan p r o j e c t i l e ( l e f t : f r o n t view, r ight : rear view; s c a l e is i n inches) .

Add i t iona l d i agnos t i c s ava i l ab le t o f ac i l i t y u se r s upon request are: very high s p ed photography (for target pene t ra t ion studies) a t 10 f r ames / s and h igher , two f l a s h X-ray u n i t s , and a VISAR system (for measuring t a rge t backspa l l ve loc i ty ) .

E

111. Test Resul t s and Analysis

Operation of the CHECMATE launcher was i n i t i a t e d on August 30, 1985. Until December 13, 1985, t h e da te of public demonstration, 2 1 sho t s were f i r ed w i th increas ing power l e v e l s , T h i s i n i t i a l t e s t i n g i n v o l v e d a learning experience, where weak poin ts were strengthened and diagnostics improved.

Test data chosen from unequivocal diagnostics are shown i n Fig. 1 4 . The t h r e e p r o j e c t i l e masses shown were of i den t i ca l des ign , b u t d i f fe ren t mater ia l s . In o rder of increasing mass, they were: unreinforced Lexan, 1 0 percent glass-reinforced, and 40 percent glass- re inforced Lexan. The re inforced mater ia l s were t r i e d i n an attempt to el iminate fragmentation of the pro- j e c t i l e a s i t ex i t s t he ba r r e l (F ig . 12). However, t he f ragmentat ion cont inues to be observed for the re in- fo rced p ro j ec t i l e s . I t is be l ieved tha t t h i s is caused by explosion of t h e f o i l backing when the a r c is i n i t i a t e d . Improvements to the a rmature and p r o j e c t i l e design are planned in an e f f o r t t o s o l v e t h i s problem.

3,000

2,000

1,000

MEASURED 6106 g 0126 g "156 g

I I WJ)-

1 2 3 4 5 6 , I ,

Fig. 14. Measured and ca l cu la t ed p ro j ec t i l e ve loc i ty of CHECMATE r a i l gun as func t ion of energy stored in the capac i tor bank.

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Curves drawn in F ig . 14 show t h e r e s u l t s of model. ca l - c u l s t i o n s which i n t e g r a t e t h e e l e c t r i c a l and mechanical different ia l equat ions descr ibing the e lectromagnet ic launcher. The model i nco rpora t e s t he e l ec t r i ca l da t a of Table I, together w i t h the experimentally-determined plasma armature voltage (360 V ) . Projecti1.e accelera- t i o n is ca l cu la t ed sub jec t t o d r iv ing fo rce , wa i l f r i c - t i o n , and p a r a s i t i c mass acquired from t h e a i r i n f ron t of i t . I n add i t ion , an es t imate is made of' t h e e f f e c t of increased mass due to a rc -ab la ted wal l mater ia l , using data supplied by J . Parker i81 of LANL. The wall f r i c t i o n is adjusted for matching calculated veloci ty t o t h e measured value i n the barrel (from magnetic probe data). A reasonable match is obtained with 30 t o 40 percent of the d r iv ing force be ing los t to wal l f r i c t i o n , once the driving pressure exceeds the projec- t i l e y i e ld s t r eng th . F r i c t ion is se t t o ze ro fo r l ower dr iv ing pressure .

Approximately 5 percent of the stored energy remains i n the capac i tor banks due to d i ss ipa t ion in the bank series r e s i s t a n c e . Air drag and energy dissipation i n the a rmature a rc a re apprec iab le loss f a c t o r s . The l a t t e r l o s s e s may be removed by evacuat ing the bar re l and employing a metal l ic armature . Figure 15 shows the calculated improvements at tainable for t h e case of a 156 gram p r o j e c t i l e . The metal l ic armature i s S i m U - l a t e d by reducing the armature voltage from 360 t o 20 v.

51000 I f

1,000 t (MJ) - I I I I 1 I I 1 2 3 4 5 6

Fig. 15. Calculated veloci ty of a 156 gram CHECMATE p ro jec t i l e a s func t ion of energy stored i n t he capac i tor bank.

I V . Conclusion

A r e l i a b l e , l a r g e b o r e r a i l gun f a c i l i t y is now ava i l - able for user tes t ing. Payloads of 5 em square cross- sec t ion , ave been s u b j e c t e d t o peak acce le ra t ions of 3.6 x 10 m/s2 and have a t t a ined f i na l ve loc i t i e s of more than 3.1 k m / s . Problems w i t h p r o j e c t i l e fragmen- t a t i o n have been observed. These a r e c o n j e c t u r e d t o be caused by explosive shock due t o a r c i n i t i a t i o n from a backing f o i l . Experiments a r e planned to decrease the shock energy i n an e f fo r t t o p reven t p ro j ec t i l e b reak - up. Data ana lys i s , in conjunct ion w i t h a mathematical model, leads to the conclusion that major sources of energy loss a r e t h e 360 V arc drop and t h e need t o push atmospheric air ahead of t he p ro j ec t i l e . These lo s ses may be eliminated through the use of metal l ic armatures and an evacuated barrel , respect ively. Calculat ions i n d i c a t e t h a t , w i t h t hese mod i f i ca t ions , p ro j ec t i l e kinetic energy could be increased by 50 percent over p re sen t ly a t t a ined va lues , r e su l t i ng i n an ove ra l l system e f f i c i ency ( capac i t i ve t o kinet ic energy) exceeding 20 percent.

t

ACKNOWLEDGEMENTS

The authors w i s h t o acknowledge the t echnica l cont r i - hutions or A . R . Mi l l e r , John Shannon, Floyd Graham and Joseph Sallay, which were of c r i t i ca l impor tance to the successful inauguration of the CHECMATE f a c i l i t y . The support obtained i r o n management, engineering, manufac- t u r i n g , and t echn ica l s e rv i ces (pa r t i cu la r ly , t he e n t i r e CHECMATE crew) has been outstanding. Special acknowledgement is given t o the support of Jonathan FarSer of DNA, without whi.ch t h i s f a c i l i t y would not have been poss ib le . T h i s work was supported by the Defense Nuclear Agency.

i l l [ 23

L31

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C51

161

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