MH S M P- 71 -4 4

SPRAYED PETN CHARGES

J . K . Boettner

JUNE 1971 FINAL REmRT

P. 0. F29601-70-7-0007

Test fire Report Prepared

For

Air Force Weapons I aboratory Klitland AFB, New Mexico

P. 0. BOX 647 AMARILLO, TEXAS 79105

806-335-1581

a

b u. 5.

the

MMlSSlON

GOVERNMENT Contract DA-11-173-AMC-487 f

SPRAYED CHARGES

J. K . BouYneh

June 1971 Final Report

P.O. F29601-70-7-0007

DISCLAIMER

This report was prepared as an accOunt of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

T e s t F i r e Report Prepared for

A i r Force Weapons Laboratory Kirtland AFB, New Mexico

SPRAYED rn!

ABSTRACT

a A p ro jec t f o r t h e A i r Force Special Weapons Center is t o develop, tes t , and de l ive r t h i n explosive matrices on aluminum and/or copper f o i l Mylar laminates. This r epor t b r i e f l y discusses problems encountered with the recording of manganin pressure gage da ta .

DISCUSSION

F i r ing S i t e work began on t h i s p ro jec t November 5 , 1970. Early work cons is ted of f ami l i a r i za t ion of t h e manganin gage power supply, gages, and gage mounting techniques by the operating personnel. and 6 as an ea r ly attempt a t recording pressure and impulse from HE loads (1 ) . The tests were mounted i n a v e r t i c a l pos i t ion so a framing camera record could be made f o r each. Problems encountered during t h i s series were noise on t h e manganin gage osci l loscope records induced from t h e capaci tor discharge cu r ren t , and extreme sloping of t h e "baseline" of the pulsed manganin gages. HE tests were discontinued a t t h i s time u n t i l t h e gages and the power suppl ies could be s tudied f o r causes of t h e sloping "baselines. 'I

Four t e s t s were f i r e d on November 5

The pressure recording system was checked by connecting a gage t o a power supply, and a f t e r balancing the gage bridge, t r igger ing the system. An osci l loscope recording was made fo r each of the remaining gages, with only one of t h e remaining gages showing an adversely sloped "baseline." This gage burned out when pulsed a second time fo r v e r i f i c a t i o n .

Testing continued with the capaci tor bank a t t h i s point . mounted i n t h e gage block, and a bare aluminum mesh was f i r e d over t he face of the gage i n an e f f o r t t o determine the extent of t h e noise i n the system. The osci l loscope and gage power supply were i s o l a t e d from e l e c t r i c a l ground and from t h e gage block. The gage block was a l so i s o l a t e d from ground. The power supply w a s not t r iggered when the capacitor bank f i r e d , so t h a t a measure of noise being induced i n t o the system could be determined. random i n frequency and amplitude and l a s t ed f o r approximately the same length of time as the capaci tor discharge current .

A Manganin gage was

The noise w a s

In an e f f o r t t o sh i e ld the s igna l cables f r o m outs ide electromagnetic f i e l d s , aluminum f o i l w a s wrapped around the cables and t i e d t o the same ground as the capaci tor . The osci l loscope and gage power supply remained i so l a t ed from ground. No improvement i n noise was noted.

Connecting t h e gage block t o ground resu l ted i n t h e increase of noise amplitude and dura t ion due t o t h e capaci tor cur ren t short ing t o t h e block upon bur s t of t h e aluminum mesh.

a AFSWC deZivery order F29601-70-7-0007.

(1) "Shaped PETN Charges," G. T . Schmitz, Pantex MHSMP-71-18.

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A t t h i s po in t it was decided t h a t a quarterwave tuning s tub (an open ended coaxial cable) placed a t t h e gage would help el iminate the higher frequency noise t h a t w a s being induced i n t o t h e system a t t h a t point . The power supply and the osci l loscope monitoring t h e gage w e r e i so la ted from ground, and t h e t e s t f i r e consis ted of bare aluminum mesh. Several tests were conducted i n t h i s manner i n which varying lengths of tuning stub w e r e used i n an e f f o r t a t optimizing t h e system. not e l iminate t h e high frequency noise a t t h e time of mesh burs t , it did subs t an t i a l ly reduce the high frequency noise a f t e r t h a t time.

It then became apparent t h a t some lower frequency in te r fe rence needed t o be eliminated. A quarterwave tuning s tub w a s appl ied t o the s ignal l i n e a t the input t o t h e gage power supply. Again several t e s t s were made t o optimize t h e system. Addition of t h i s s tub necessi ta ted a change i n t h e previous one, u n t i l r e l a t i v e l y noise-free records could be obtained with a 12-foot s tub a t t h e gage and an 83-foot s tub a t t h e gage power supply. However, no ise s t i l l pe r s i s t ed f o r 1 vsec a f t e r mesh bu r s t time.

Although a 9-inch tuning s tub d i d

A negative s igna l of approximately 0.5 v o l t w a s recorded a t approxipately 1.5 psec a f t e r mesh burs t with t h i s system, and t h e introduct ion of HE t o the mesh resu l ted i n a negative s igna l of approximately 0.8 v o l t a t t he same r e l a t i v e time. The po la r i ty of t h i s "s ignal" w a s opposite t o t h a t expected. fu r the r t e s t i n g revealed it t o be an induced s igna l and not one r e su l t i ng from t h e pressure pulse. Fig. 1 is a typ ica l recording of the gage output. I t should be noted t h a t , f o r some unexplained reason, a s ignal l i k e t h a t of Fig. 1, even with the same test condi t ions, d id not appear again. Fig. 2 is a bas ic diagram of t h e instrumentation setup.

Using another power supply r e su l t ed i n a pos i t i ve s igna l , but

Further t e s t i n g revealed t h a t grounding t h e osci l loscope made l i t t l e d i f fe rence i n t h e basic noise l eve l and durat ion.

A t t h i s po in t it was decided t o determine what e f f e c t the gage or ienta- t i o n with respect t o t h e d i r ec t ion of capaci tor cur ren t discharge had upon the amount of noise induced a t t h e gage. The oscil loscope records indicated t h a t although the re was some reduction i n noise amplitude when t h e gages w e r e o r ien ted i n such a manner so as t o minimize magnetic pick- up from t h e capaci tor discharge cur ren t , t he bulk of t he noise s t i l l remained i n t h e system.

Upon consul ta t ion with J. S u t t l e and L t . M. H i l l , A i r Force Special Weapons Center, Kirt land AFB, it w a s decided t o enclose the gage i n a Faraday sh ie ld . This w a s done by covering t h e "face" of t he gage, which w a s mounted i n a copper s leeve, with a piece of 1 m i l brass shim. The brass shim and the copper s leeve were then connected t o t h e coaxial sh ie ld of t h e gage s igna l cable with a shor t piece of copper ribbon. The gage osci l loscope was grounded and cons t i tu ted t h e only ground point i n t h e instrumentational system. The gage osci l loscope t r a c e w a s "blown" off-screen within 1 usec a f t e r i n i t i a t i o n of cur ren t t o t h e mesh, and remained of f - screen f o r a period of time greather than 50 psec. f o r t h i s is t h a t some of the cur ren t from t h e capac i tor bank shorted t o t h e Faraday Shield and flowed t o ground through the gage s ignal cable sh ie ld , thus causing a vol tage drop a t the osci l loscope input su f f i c i en t t o d r ive the t r ace off-screen.

A possible explanation

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The next test was e s sen t i a l ly the same a s t h a t j u s t ou t l ined , except that the s ing le grounding point was a w i r e a t tached t o t h e Faraday Shield (see Fig. 3 ) , s t i l l attached t o the coaxial sh i e ld of t h e s igna l cable, and t o a metal s take d r ive r i n t o the ea r th a t a nearby point . The gage osci l loscope and the gage power supply were i so l a t ed from ground. Although t h e osci l loscope t r ace d id not go off-screen a s quickly with t h i s arrangement, l i t t l e difference w a s noted between the two t e s t s .

For a t h i r d t e s t , t he system ground w a s returned t o t h e oscil loscopes and the quarterwave tuning stubs w e r e removed from the s igna l cable. Furthermore, t h e gage block and the gage power suppl ies were i s o l a t e d from ground. The gage block w a s covered with a shee t of 10 m i l Mylar i n order t o i n h i b i t any arcing of the high voltage m e s h f i r i n g system t o the gage block and Faraday Shield surrounding the Manganin gages. Although noise w a s s t i l l present a t t he time of mesh burs t , t he osci l loscope trace returned t o "baseline" approxi- mately 3 vsec a f t e r mesh burst . remainder of t he sweep.

The t r a c e w a s r e l a t i v e l y noise f r e e for t he

Because time w a s growing shor t , it w a s decided t o accept t h i s system f o r the attempt a t recording the pressure generated by the proposed HE systems. Consequently, a PETN loaded (0.022-inch average thickness) 7-1/2 x 7-1/2 inch aluminized Mylar mesh test piece was f i r e d and monitored with two Manganin pressure gages. The osci l loscope recordings a r e shown i n Fig. 4 . From these records it is immediately obvious t h a t t he re is a s i g n i f i c a n t d i f fe rence (% 31 psec) i n the a r r i v a l t i m e of the pressure pulse t o the two gages. The Manganin gages indicated pressures of 3.9 kbars and 6.5 kbars respect ively. Impulse measurements were taken by a compressible copper b a l l placed i n an aluminum housing beneath an aluminum slug which is driven down by t h e test explosive. The impulse measurements are obtained by measuring the amount of deformation of t he copper b a l l caused by t he impact of the aluminum slug. Measurements obtained i n t h i s manner revealed an explosive impulse of 24 t o 26 ktaps (1 t a p = 1 dyne-sec/cm2 = bar-wec) . Since there w a s a s ign i f i can t difference i n t he a r r i v a l t i m e s and because the measured pressure w a s low, it w a s surmised t h a t a "sweeping" detonation had occurred across the face of the t e s t piece. Consequently, a Beckman & Whitley Model 189A high speed framing camera was used with a s imi la r ly loaded t e s t p iece t o ve r i fy t h i s supposition. The f i r s t ind ica t ion of detonation appeared 4 vsec a f t e r t he bu r s t of t he mesh. Furthermore, only a small amount of the HE had been consumed 9 vsec l a t e r . Thus, it s e e m s qu i t e l i k e l y t h a t a sweeping detonation had occurred on the previous instrumented test. Fig. 5 shows the framing camera record of t h i s t e s t . A second t e s t w a s performed i n which only an aluminized Mylar mesh was f i r ed . The framing camera record indicated t h a t the mesh w a s consumed within 1 psec, and therefore could not account f o r the long delay t i m e associated w i t h the HE of the previous test.

Testing PETN loaded aluminized Mylar mesh was discontinued a t this time. Instead, e f f o r t s w e r e focused toward the t e s t i n g of PETN loaded copper mesh. I n order t o accommodate the capaci tor bank capaci ty , the HE loaded copper mesh was reduced to 4 inches wide and 8 inches long. Subsequently, a copper mesh w a s loaded with an average thickness of 0.024 inch of PETN and f i r e d . The pressure gage recordings (Fig. 6) i nd ica t e pressures of 8.0 and 9.4 kbars. occurred.

They a l so ind ica t e t h a t a sweeping detonation had again

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Consequently, a framing camera was employed to help determine t h e qu f t h e detonation. 0.0123 inch w a s f ired. A s ign i f i can t amount of PETN powder found i n immediate v i c i n i t y a f t e r t h e shot indicated a "no-go." The framing camera record (Fig. 7) revealed some very slow react ion i n the HE. Another shot w a s f i r e d with t h e average HE thickness of 0.0311 inch. N o res idue w a s found from t h i s test , and the framing camera record (Fig. 8) ind I'spotty" i n i t i a t i o n of the HE. Further analysis of the framing re indicates a detonation veloci ty range of 5.5 t o 7.3 mm/usec across and width of the PETN surface.

Further t e s t i n g of i n i t i a t i n g an explosive sheet by aluminum o r copper mesh has been suspended.

A copper mesh loaded with PETN a t an average thickn

FUTURE WORK: COMMENTS; CONCLUSIONS

A considerable amount of e l e c t r i c a l l y induced noise has been encountered while t ry ing t o record, by a Manganin gage, t he pressure generated from a mesh i n i t i a t e d explosive. This noise is present during the discharge of the main capacitor bank and in t e r f e re s with the pressure data being acquired a t t h a t time.

Future work should include the development of a capacitor bank t h a t could be quenched immediately a f t e r mesh burst so t h a t t he noise would not be present during the t i m e of shock a r r i v a l a t t he Manganin gage element. An a l t e rna te solut ion would be t o place an at tenuator of su f f i c i en t thickness between the explosive f ace and the gage element so t h a t the pressure pulse would be delayed t o a time a f t e r t h e noise has subsided. Hugoniot calculat ions could then be applied t o t h e da ta t o determine the output pressure a t t he HE surface.

Future work should also include the development of a charge and/or an i n i t i a t i n g system t h a t w i l l r e s u l t i n a mre simultaneous output than the systems tes ted t o date.

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( a ) . Induced noise t o the Manganin gage from the f i r i n g of an aluminum m e s h .

Upper sweep: 0.5 v/cm Lower sweep: 5.0 v/cm Sweep speed: 2.0 psec/cm

(b). Capacitor cur ren t and dI /dt record f r o m aluminum mesh f i r i n g .

Upper sweep: dI /dt a t 200 v/cm Lower sweep: I a t 2 . 0 v/cm (71.4 kamp/cml Sweep speed: 1.0 usec/cm

F i g . 1

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-1 L

0 f 2

c

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Manganin Gage

C a s t i n C-7 Epoxy

Fig. 3 . Manganin Gage/Faraday Cage Arrangement

( a ) . Upper Sweep: 5 v/cm and 2 psec/cm. Lower Sweep: 5 v/cm and 5 ysec/cm. Indicated Pressure: 6.485 kbars.

(b). Upper Sweep: I v/cm and 1 psec/cm. Lower Sweep: 2 v/cm and 2 psec/cm. Indicated P r e s s u r e : 3.923 kbars.

F i g . 4 . Manganin Pressure Gage Recordings fo r a PETN Loaded A l u m i n u m Xylar Nesh.

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3.80 psec

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. .

*L. *

6.65 psec

Fig. 5. Framing camera record showing t h e i n i t i a t i o n or‘ 0.022 inch of PETN by an aluminum mesh. The elapsed time from the f irst f i lm indica t ion of mesh bu r s t i s shown below each photograph.

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I

( a ) . Upper Sweep: 1 v/m and 1 psec/cm. Lower Sweep: 2 v / m and 2 pec/cm. Indicated P r e s s u r e : 9.412 kbars.

(b). Upper Sweep: 1 v / a and 1 psec/cm. Lower Sweep: 5 v/cm and 2 vsec/cm. Indicated Pressure: 8 . 0 3 0 kbars.

F i g . 6 . Flanganin Pressure Gage Recordings f o r a PETN Loaded Mesh.

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3.80 psec

Fig. 7. Framing camera record showing the i n i t i a t i o n of 0.012 inch of PETN by a copper mesh. The elapsed time from the f i r s t f i l m ind ica t ion of mesh bu r s t is shown below each photo- graph.

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1.96 Hsec

Fig. 8. Framing camera record showing the i n i t i a t i o n 0.031 inch of PETN by a copper mesh. the f i rs t f i lm indica t ion of mesh bu r s t i s shown below each photograph.

The elapsed time from

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