C OMPARISON or CONSTANT CURRENT ANDCAPACITOR DISCHARGE IGNITON OF NORMALLEAD STYPHNATE

4oward S. Leopo!d

2 MARCH 1971

NoLNAVAL ORDNANCE LABORATORY, WHITE OAK, SILYER SPRING, MAIR LAWD

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NOLTR 70-236

* CCOMPARISON OP CdNTAWT CURRENT AND

SCAPACITOR DISCHARGE _V RXTICN OF NORMAL LEAD STYPILINATE

by:

Howard S. Leopold

ABSTRACT: A comparison was made of const-3nt current an,! capacitordischarge ignition of normal lead styphnate in the time regimetypically employed foz firing initiators by each type of signal.The loading pressure was varied from 2,500 to 60,000 p-i. Forthe regimes sti;died, as the explosive loading pressure was in-creased, constant current activation gave ionger ignition timesbnt capacitor discharge fitinq gave shorter ignition times. Theenergy requirement for ignition increased with loading pressurefor constant current ignition, and was constant for capacitordischarge ignition.

*Explosion Dynamics DivisionExplosions Research DepartmentNaval Ordnance Laboratory

t Silver Spring, Maryland 20910

VP

OF

,K2:°

-NOLTR 70-236 2 March 1971

COMPARISON OF CONSTANT CURRENT AND CAPACITOR DISCHARGE IGNITIONOF NORMAL LEAD STYPHNATE

This report compares the effects of constant current and capacitordisaharge activation signals (used in their typical time regimes)on the igniticn of normal lead styphnate over a range of loadingpressures. The work was performed under task ORD 332 01/UF17 354314 Problem 20i, Explosive Initiation and Safety.

The results should be of interest to persons engaged in initiationrezearch and in the design off electrical initiators and powersupplies therefor.

The identification of commercial materials implies no criticism orendorsement of these pr:oducts by the Naval ordnance Laboratory

GEORGE G. BALL"Captain, USX

Commander

(I'6C. J. 41ONSON

By dirito

NOLTh 70-236

CONTENiTS

page

IMPBDUCIONA . . . . .. . . . . . . . . . . 2

YfRESULTS. . . . . . . 4

DISCUSSION . . . a.................... . . . . . . . . . . a 5

CONCLUSIODNS. . . . . . . . . . . 9

REFEECES... . . . . . . . . . . . 10

TABLES

Table I Effect of Loading Pressure on Conlstant Current(0.3-Ampere) Ignition Characteristics of NormalLead Styphnate 11

Table ZI Energy-Division fro6m Constant Current Pulse 12

FIGURES

Fig. 1 Mlodified initiator Plug 13

Fig. 2& Amplification of Voltage Signal Acrusa Bridgewire 14

Fig. 3 Experinental Oscillograms 15

Fig. 4 Effect of Loading Pres~sure on Ignition TPime ofNormal Lead Styprrnate by Constant Cu~rrent andCapacitor Discharge signals in their TypicalTimue Regiier. 16

Fig. 5 Typical Resistance Changes During ConstantCurr.Pnt Pulse 16

NOLTR 70-236

Contents (continved)

Fig. 6 Relationship of Bridgewire Temperature to CapacitorDischarge Pulse 17

FAg. 7 Distribution of Temperature in a Sphere ofDiameter 100 V Heated at the Surface (Aftergyring, et al1) 17

iv

NOLTr 70-236

INRODUCTON

1.The effective utilization Of explosives and explosive trainsrequires a thorough understanding of the in tiation process andthe growth of explosion. Marty factors aff~cting the initiationof explosives in explosive components have still not been explained.Further studies of these factors, both chemical and physical, isneeded in order to build safe, reliable, and effective fuze trains.

2. There is-increarsing interest in the effect of various olectricalactivation signals upon thermally activatted electro explosive devices(BED's). For example, military specification JPL-1-23659 for- in-

;E sensitive SED's places upper (all fire-5 amperes, 50 milliseconds)and lower (no fire-I ampere: 5 minuteav) limits on the constant

- current sensitivity, makinig the constant current charactericticsand fufictioning times of designer initerest .1 BED's meeting the

Fff_ above requirements must sometimea be fired by capacitor dischargefiring_ circuits, -thbereby also Senerating interest in the pulsefifring chAraterk-stics and functioning times.2-

3. Normsl lead styplhnate (NWS) is probably the most widely usedignition material in ED'S. Although ;onsidefed a ielatively poor

RE initiating material, A~s is easily ignited by a hot wire and hasgood storage properties.

4. The effects of loading nressure on the hot wire ignition ofnormal lead styphnate by capacitor discharge have been previouslyinvestigated. 3 it was found that the capacitor disclaarge energyrequirement of IlLS remains fairly constant over a wide leadingpressure range and the average ignition tim decreases as the load-ing density in increased. in this report, the effects of loadingdensity on the constant current ignition of IlLS are examined anda comparison is made with the capacitor discharge ignition results.The investigation was mrade with a wire size t1-ypical of current EEDuse., and the results are believed to be applicable to larger diameterwires used in insensitive EED's.

LI

SOLTR 70-236

EXPERDIAWrAL

5. The effect of loading density on the 4constant current ignitiontime and energy requirement of NWS was ihvestigated-at three loadingpressures - 2,500 psi, 10,000 psi, anid 60,000 psi. The time ofignition was determined by two sep'arate techniques - the "voltageinflection technique*4 .,5 and the 'light pipe techniquew6 s-o thata comparison could be made of the two methods. Results were alsocompared tith those obtained previously by capacitor discharge. 3

6. FiigCircuit -A d.c. power supply in conjunction with amerury wttea =mlisecond switch was used to produce the const~intcurren~t pulse. f The circuit contains -a 15.-ohmballast r.2"istor.

-A constant current Level of O-3 ampere was used for ignition becausethis value gives ignition times withim the -control period of theswitch and will not burn out the bridgewite during the time regionof interest. An equivalent registor (w Ithin 0.25 ohm of the bridge-wire resistance to be tested) was ftirst-substituted before eachshot to regulate to the desired current level of 0.3 amapere.

7. Initator"jug --A specially modified initiator plug was usedfor loadi g hexposive on the bridgewire. A hole was drilledaxially through the initiator plug between the-two contact pins,and a light pipe patted in this hole so that radiation from the

- ~ initial reaction of the explosive could be detected and-trartsmittedto a photodetector tube. see Fig. 1. The light pipe method isdescrib)ed fully in SiOUR 69-148. The plug is bridged with a-1-mildiameter nichrome wire having an effective length of 0:050 aftersoldering to the contact pins. This bridgewire has a resistancerange of 2.5 to 4.0 ohns,-

8. Rec-order - A Tektronixc 555 Dual-beam Oscilloscope with twofast-rici7t pe K preamplifiers was used to observe the voltageacross #.he wire and the photodetector tube signal. The voltage

"nlectior. techniquie for deteimining the ignitica time requires nospecital equipment other than a means of monitoring the voltage(.Ia drop) across the bridgewire during the current pulse. InM additionto determining the time of ignition, the observed voltage chang wasemployed to determinie the bridgewire temperature. For ItslTepurpose it is difficult to mak~e precise measurements of the voltagecriange across the wire when the entire signal amplitude is-observed,since there is only a 4-8% increase in resistance- before ignitionoccurs. Figure 2A Shows the voltage signal obtained with a bare

2

NOLTR 70-236

bridgewire usinq a 72 millisecond duration pulse. in-crder to amplifthe voltage change in the signal by using a high oscilloscope sensi-tivity, the beam spot must be set well below the viewing screen,then only the signal portion of interest is observed. See Fig. 2B.A variable bucking voltage can be employed if necessary to lowerthe beam spot if the oscilloscope vertical pcsition dial does notprovide sufficient latitude.

E9-9k

3

NOLTR 70-236

RESULTS

9.Constant current pulses of 0.3-anpere a=zlitude mtaintainled fora longer period than necessary for tiring, were used to fire tenloaded initiator plugs at tacit of the three loading pressrers- TheIgnition times were Onsenve'; by the -otg netnand the 'Ii;ht pipe techniques. The two %*b~ewre- found tocorroboralte each other3 Indica-ting the SamZC. ignition time witbinthe resoluition of tbe dillieconad s-weep sneeds.. Sec Ficg 3 ;--. Teaverage tiiAte of ignition was foumnd to incrtease 3mrkedWy and thecalculated tear.,*rat-ure* of the brdge--Ire at the time of ignitionito drop with increasinc bla~ingq pressure. See Table T..

10. Several of the wire tenperature excursions were cbserve tobe quite irrarcular uh-en subjected to -a constant cLrent Pualse.See Fiq 33. Prevulstng of all the tLast initiator plus bfre-lzs ding was used to -3inmirnize the 'irregular cbtanzek. Prapdalttof the bridce-ire With a 0.5-micro! arad capato a ae t ~volts before loading of the exab mosive tentO to prothice a o-Xre lnearresistance chance- -but does not entirely r=0v aei~euaiisIrregularities were found to' be more Prevalent at the 2,500-_ti&oa-ding pressure and drop exff as the loadia -pressure is increased.No irrecul1ar resistance changes were observced at the 60,000-nsi!loading pressure. FrC-, this observatk~n, onec miigbt snnaise that

the irre-cularitbsaelaeydet stress relief frozm -.irerceetdurinc t,'he hrcati~r and/or cbarnaes in th61ermal ccntact with2a

the exclusive._

*Calculated fron Wire resx*_tanc ,, aSS!2z no Linear :;ncrease ofresist-ance with temperature inobserved region -

h4 Te .Se"sti4jity of elecr z1.aeawcss ~~ia1beem dftextL b ca-maclfom die-2s or Sat ~ ~

'S. _e-a---:

raeo e~Jin~at is reae. 72is is acc--pisbe Laaiymknt)he at diudaxv ume L~ rain~a~c a taO tie 0

tb _iiewmre. iczsa11V tt-e mc t:-; LS j- i-p z icosecamd resim-.Cwsta.,t CLurrent PI1ses axe, wManl act A1~c lutze 2Crn iic-.seer,--. xreqcr becanse of tUbe djfiCeu1; Of Siiaira A~ e2t

zire eals. be~ tba ewaed si~ -~~t 3:alisem*, pu~s i&c~ cait e vamle 0.1tl zoIr zIrg ZO~ u 3

Of lbsimbsitiwe ims aie erz tr1 t ica oftL* t~e tbr the caacto d habiige and t t wa-

3DI 1** ira2I t--I ef fedt OfF a-~ -emitx UPO tbe l ira-C UUMG time Of iLs, tb icniticm tlxa- waNS foud to Uwersie 14VIt

thie ie. In Frwiva a*r it Vag -f= tba --ijc tine~eri~Sviuo demeui~ -y 'asims a ca itor dii

-~-to lea thinrie. See V!S 4- it Can be see-i tbe that the 10aditq desitv-igmdtl is effect is Sb ujye t~Of

d1*E~31firing 419mai- JL aiscmssibm~ ftIHosm cc th tv~ofelecticmll ctiva tim sisas-

U3. onstanat care -- I&---*cs er the b~or~~iiitct t*~~eaed y acc~tart crealt paxsex tbe iidAcewixe at

rat4. beatS up Xapi-dlr aad thera aST--w=tic1 , ap-ramacbmes a 1i=-t "metemper~tra, lbim~~ii a sa a rtcaed Se-- V1.7 3

-The pm~wr ima (?I, t= the Vwire as tbe ccmst cm-rretpaIse viii1 rise x-cmxd~ms

and T temp-rat- zise. Th cmxea 31-evels -!!kmzli eap15inedfo~~- --cmt t=e-t -- Lng9"1..iia-- r-Ax

c~itI= t- usI1 ie~dtie u.- zdtat is v,-- eii.evun- reportedm -ee

NOLTR 70-236

14. When the explosive is poorly coupled (low .ensity) to thenichrome bridgewire, keeping the radial loss of heat from the wireat % low rate, the resiscance rise due to heating effects willoccur at a relatively rapid rate. See Fig. 5. Therefore, thepower input to wires in contact with low density explosive willincrease more rapidly than to wires in good thermal contact withthe explosive. The low density explosive will conduct heat awayfrorm the bridgewire at a low rate due to its joor thermal contactto both the wire and with itself. One might then anticipate thatthe fast bridgewire heating observed with low density explosive,coupled with the poor heat transmission through the low densityexplosive will cause ignition of a critical volume of NLS much morerapidly than a wire in good contact with the explosive. This isobserved to occur even though a higher ignition temperature existsfor the explosive due to the shorter period of heat exposure.

15. An important controlling factor appears to be heat conductionthrough the explosive away from the granules adjacent to the wire.See Table II, The table indicates that under the non-adiabaticconditions of the constant current pulse, the heat loss is animportant factor in determining the probability and time of ignition.Of interest to the present study is a paper by Austing and Weberwho in the examination of the constant current ignition of metal-metal oxide mixtures noticed that increases in density increasedthe time to ignition.9 Even though the thermal conductivity of NLSis approximately three magnitudes lower than the metal-metal oxidemixtures, the heat transmission is sufficiently affected by theloading density to exhibit the same type of effect.

16. In constant current initiation the longer the required periodof current flow, the greater the energy requirement. Therefore,the denser the NLS, the greater the ignition energy requirement inthe non-adiabatic time region.

17. Capacitor Discharge - During a capacitor discharge, for theexperimental parameters used, the wire heats rapidly, reaching themaximum temperature near the end of the discharge. It then under-goes a comparatively long cooling cycle. See Fig. 6. The NLScan ignite either during the course of the temperature rise orduring the cooling period of the wires In the present constant currenttests however, ignition must occur during the temperature rise sincethe pulse length was made intentionally longer than the ignition time.

18. Capacitor discharge parameters commonly used to effect hotwire initiation usually give initiations in the microsecond timeregion. I3yring, et al, in studying heat conduction into a grainof 100 u diameter have shown the grain surface requires 1 to 10microseconds to reach the temperature of the hating bath. 0 The 10center of the grain can remain cool for times up to 100 microseconds.See Fig. 7. The diagrams are for a fully immersed grain. For acomparison with wire heating, one can assume heat entry from only

6

NOLTR 70-236

one surface with a grain thickness of 5 - 10 u. Only the firstfew layers of explosive adjacent to the wire will be of importancefor discharge times in the order of microseconds.

19. Even for the short times involved in a capacitor discharge,the temperature excursion of the bridgewire is dependent upon thedegree of coupling to the explosive. A wire poorly coupled to theexplosive (low density) will reach a higher temperature and coolat a slower rate than one in good thermal contact with the explo-sive.3 This indicates a slower transfer of heat from the wire tothe explosive. In general, the poorer the therwal cvntact, thelater the time of ignition due to the slower heat transfer. Evenwith fairly good coupling, the temperature of the explosive wouldbe expected to lag that of the wire as indicated by the results ofEyring, et al. If ignition does not occur on the temperature riseof the wire, and the wire temperature remains above the ignitiontemperature of the explosive, heat transfer will continue andignition can occur during the wire cooling phase. Ignition ispossible until the wire temperature drops below the explosive igni-tion temperature.

20. The energy requirement for the ignition of NLS remains constantover a wide density range even though the heat capacity of thebridgewire system increases with loading pressure. The exactexplanation is unknown at this time but it appears to have someconnection with the rate of heat transfer. Greater heat lossesto the inert parts appear to occur at the lower densities becauseof the slower heat transfer to the explosive, thereby equalizingthe energy requirement over a wide density range.

21. implications - It can be surmised from the above discussionsthat for fast rates of energy input the thermal contact resistancebetween the wire and the explosive is of prime importance whilefor slow rates of energy input, the energy input rate in comparison-;o the cooling time is of prime importance. It is well known incapacitor discharge work that as the capacitance is increased andthe firing voltage decreased, more energy is required for firinGsince the initiation process becomes non-adiabatic. Conversely,as the constant current level is increased, the initiation processapproaches an adiabatic condition and less energy is necessary.The phenomena observed therefore, cannot be attributed to the typeof waveform used, but to the time regime in which each waveformis typically employed. One would therefore expect that as theinput times from the two types of electrical signals approach eachother at some intermediate time region, density effects upon theignition time would gradually lessen because of the opposingtendencies with perhaps the cooling effect predominating.*

*Measured cooling time constants for the experiments reported hereare 2600 microseconds at 2.5-K psi loading pressure and 1400 micro-seconds at 60-K psi loading pressure. Thermal lag effects appearto be important up to 100 microseconds.

7

NOLTR 70-236

22. The results are of interest in the design of insensitive EED's.They show that the insensitivity of NLS (and most likely otherstyphnates) to non-adiabatic constant current ignition can be in-creased* by increasing the loading pressure and yet not affect theenergy sensitivity to capacitor discharge. The occurrence of fasterignition times with capacitor discharge initiation as the loadingpressure is increased would in most cases be considered a favorablecondition.

23. Analysis of hot wire initiation is sometimes limited to a studyof the bridgewire temperature history. It can be seen from theconstant current results that the relationship between -the bridge-wire temperature and the probability of initiation can be somewhattenuous over an experimental parameter range (i.e., density) whereignition temperatures varying from 339 to 4500-C were derived fromthe wire resistance change. The relationship between the bridge-wire temperature and the probability of initiation depends uponsuch diverse factors as thermal contact resistance, variation ofexplosive initiation temperature with time of exposure to heatsource, radial loss of heat, and particle size of the explosive.The bridgewire temperature has been successfully used for mathema-tical predictions based upon a specific set of conditions, but careshould be taken when there is any parameter variance.

*A Five to one constant current energy increase was effected underthe experimental conditions. See Appendix A for supplemental testswith an insensitive bridge element.

8

ZNOLTR 70-236

CONCLUS IONS

240 The effect of loading density on the ignition of XLS dependsupon the type of electrical activation signal. For very shortactivation times (miicrosecond regjion) the thermal contact resistancebetween the bridgewire and the explosive appears to be of primeimportance. For longer input times-(millisecond riegion), heat lossthrough the explosive appears to be the prime consideration.

25. The time of ignition and energy requirement of kILS will in-crease with increasing density when employing a constant-currentpulse in its typical time regime (non-adiabatic -region). Thistechnique can be employed to assist NLS to neet the 1-amp/i-wattno-fire requirenient of kM-X-23659.

26. At high constant current inputs (ca. 5 amperes) where adiabaticconditions are being-approached, changes in the density of NLS havea negligible effect upon the time-of ignition and-energy req;uire-

27. Studies relating the bridgewire temperature to the probability- -~ of initiation should be carefully examined when any parameter is

varied.

9

NOLTR 70-236

R~EFERENCES

1."Initiators, Electric, Design and Evaluation of", MIL Spec.MIL-1-23659, June 1966.

2. S. G. Nesbitt, "Functioning time of a 1-amp/I-watt Detonator%,Proc. 6th Syrap. on Electroexplosive Devices.. Franklin Inst.,Jul 1969.

3. H. Leopold, "Effect of Loading Density on the-Hot Wire initiationof *q ormal Lead Styphnate and Barium Styphnate", NOLTR 70-96,in process.

4. V. M. Korty, "A Differential Resistance Method for Measuringignition Times and Threshold Firing Energies of ElectricPrimrs *, NOLM- 8636, Jul 1946.

5.- H. D. Mallory and F. A. Gos, "A New Method for the Masurementof-Initiation Times in Hot Wire Ignitions". Proc. of theGilbert B. L. Smith Memorizl ConE. on ~poieSniiiyNAVORD Rpt. 5746, Jun 1958, conf.

6. H. Leopold, "A New Technique for Detecting the Initial Reactionof Primary Explosives Initiated by Hot Aire". NOLTR 69-148,Nov 1969.

7. L. A. Rosenthal, "Millisecond Switch for ElectroexplosiveTesting", IEEE Trans. on Instrumentation and measurement, Vol. 1,M.-18s Mar 1969.

S. "Ordnance Explosive Train Designers'* Handbook"4, NOLR 1111,Apr 1952.

9. J. Austing and 3. P. Weber, "Constant Current ignition of Metal-Metal oxide Mixtures", Proc. 5th Symp. oh ElectroexplosiveDevices, Jun 1967.

10. H. Eyring, R. E. Powell, G. H. Duffey, and R. B. Parlin, Chemn.Rev., 45, 69 (1949).

10

NOLTR 70-2 36

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/SOLDER MOUND

LIGHT PIPE

BRIOGEWIRE (2.5-4 OHMS)

PLUG FACE BEFORE LOADING

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EXPLO51VE

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PHENOLIC PLUG

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FIG. I MODIF.ED INITIATOR PLUG

13

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NOLTR 70-236

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NOLTR 70-236

APPENDIX A

FA.l. The effect of loading pressure on the sensitivity of NLS with

the Kk 101 type 1-amp/i-watt ribbon bridge2 was briefly examined todetermine if the investigation results were applicable to actualhardware. The plug element oniy was employed in these tests. Itconsists of the glass/kovar plug, the evanohn bridge element, plasticinsulator, and aluminum charge holder. See Table A-1.

A.2. The results in Table A-I show that the insensitivity of NLScan be increased at the lower current levels (non-adiabatic region)by increasing the loading pressure. At the 5-ampere "all firelevel", the increase in insensitivity is negligible (as one would

E expect when approaching adiabatic conditions) and the ignition timewill be well within the 50 millisecond functioning requirement ofKIL-I-23659.

IIA-

NOLTR 70-236

TABIE A-I

CONSTANT CURRENT TEST RESULTS FOR NLS ON

I-AMPERE/i -WATT RIBBON BRIDGE

Current Loading Pressure Resistance Time of Results(amps) (K psi) (ohms) Current

Flow

1.2 10,000 1.24 5.0 mins No fire

1.2 10,000 1.26 0.82 " Fired

1.2 10,000 1.27 1.48 " Fired

1.2 10,000 1.28 0.55 Fired

1.2 10,000 1.28 2.87 " Fired

1.2 60,000 1.23 5.0 mins No fire

1.2 60,000 1.27 5.0 " No fire

1.2 60,000 1.28 5.0 " No fire

1.2 60,000 1.28 5.0 " No fire

1.2 60,000 1.28 5.0 " No fire

5.0 10,000 1.27 4.3 msec Fired

5.0 10,000 1.29 2.6 " Fired

5.0 10,000 1.31 2.6 " Fired

5.0 60,000 1.27 2.9 ,nsec Fired

5.0 60,000 1.28 2.7 Fired

5.0 60,000 1.29 6.2 Fired

A-2

UNCLASSIFIEDSecurt " Chi;sificatlon

DOCUMENT CONTROL DATA R & DScrit:a f c-s sihcaf.aun of gunk. bodV . absuract atid ndexall ,tfnomanfir, nft t ber ninterd w.hpre !I 'ncn al repr' is cfl. suld)

OfCtINA TING AC TIV; Yr (Corporaie aithor) Za. R F

PORT SC

CURT Y

CLASSIFICATION

Naval Ordnace Laboratory UNCLASSIFIEDWhite Oak, Silver Spring, Maryland 20910 2b. CRO.JP

3 REPORT TITLE

Comparison of Constant Current and Capacitor Discharge Ignitionof Normal Lead Styphnate

4 OESCRIPTIVE NOTES (Tpe of report uh'd Inclusive datej)

I,- AU TmOR; (Fital name, middte initial, lat name)

Noward : Leopold

0. REPORT OATE ?.. TOTAL NO- OF PAGES 'b. NO. OF REFS

2 March 1971 iv + 19 1I0fa, CONTRACY OR GRA04T NO j90. 70236O' RPRTN;fkRS

b. PRO.;ECT NO- 1

ORD 332 001/UF17 354 314 p201 _TR_70236

C. r OTHER REPORT NOMS (Any other numn-b.,, iliemaybe.. mslgedH4. Ibie reporf)

I0 OISTRISUTION STATEMENT

This document has been approved for public release and sale, its

distribution is unlimited.

If. SUPPLEUE54TARY NOTES 2- SPONSORING MikfTARV ACTIVEITY

I Naval Ordnance Systems CcmmandWasbington, D.C.

13- *OSTRACT

A comparison was made of constant current and capacitordischarge ignition of normal lead styphnate in the time regimetypically employed for firing initiators by each type of signaleThe loading pressure was varied from 2,500 to 60,000 psi. Forthe regimes studied, as the explosive loading pressure was iA-creased, constant current activation gave longer ignition timesbut capacitor discharge firing gave shorter ignition times. Theenergy requirement for ignition increased with loading pressurefor constant current ignition, and was constant for capacitordischarge ignition.

FORMA (PAGE 1)DD, v1473 UNCLASSIFIEDS/N 0101.807-6801 Security Classification

UNCLASS IFIlEDSecurity classitication

14 1 LSHK A tN1K6 L 4 H4 C

K y * n &n o t. % W R O L C W T R O L E * T -

Mot wire

Normal Lead styphnate

ignition

Xnitiation

ORKI47 (BACK) UNCLASSIFIED(PAGFE 2) Security Classiication