Explosive behavior of a simple composite propellant model

COMBUSTION AND FLAME, 17, 32.t.--.~t36(1971)

323

Explosive Behavior of a Simple Composite Propellant Model Donna Price, A. R. Clairmont, Jr., and J. O. E r k m a n United States Naval Ordnance Laboratory, 9.PhiteOak. Silver Spring, Maryland Ammoniumperchlorate(AP}/waxmixtureswere used to model a simplecompositepropellant.Addition of wax to the AP increasedits infinite.diameterdetonation velocityand its shock sensitivity,and decreasedits critical diameterand its detonation reaction time. n'~e maximumof these effectsoccurred at about 2~,~ wax. At this composition,the granular model is an explosivecomparableto TNT.

Introduction A systematic study of ammonium perchlorate (AP) in previous work [1, 2] showed that its explosive behavior differs from that of conventional explosives (e.g., TNT). Because its detonation failure diameter*, de, increases with increasing loading density (Po), its curves of detonation velocity (D) versus Po exhibit a maximum in D. Such explosive behavior is typical of materials in G r o u p 2 of a convenient classification 1"3]that contrasts the AP behavior with that of TNT-like or G r o u p 1 materials; the latter show decreasing d, with increasing Po. The purpose of this paper is to make a comparable systematic investigation of the explosive properties of a simple composite propeliant model, AP/fnel. Wax was chosen as the model fuel, and AP/wax mixtures were prepared at 50 ~ - 9 6 % theoretica~ maximum density (TMD) and at 0~'o-31.5% wi.~. The model propellant showed G r o u p 2 explosive behavior, as did its oxidizer component. The presence of the relatively volatile fuel reduced the failure diameter, * The critical diameter is the diameter of a cylindrical charge at or above which steady-statedetonation can be propagated and belowwhich it fails.

increased the shock sensitivity, and incre,ased the detonation "~elocity. Maximum effects were found at about 2 0 ~ wax. AP/wax is not only a propellant model, but is in fact a propellant, al:thongh one of poor mechanical properties. Its stoichiometric composition is approximately 90/10, AP/wax; this mixture exhibits a burni~g rate of about 0.3 in,/ sec at 1000 psi and a specific impulse, in a small motor firing at 400 psi, o f 230 see (228 see calculated) [4]. The calculated impulse at I000 psi is 252 see. It has been known for some time that either preheating A P or adding a fuel to it will lower its pressure limit for stable burning from about 22 to 1 atm {see, e.g., [5]), and that relatively volatile fuels such as paraffin increase the burning rate [6]. A low-density (1 gm)"cm3) large-diameter charge of 90/10, AP/paraffin, was reported to show greater detonation velocities than that of AP I"7]. More recently it has been shown that preheating AP [8] or adding an organic fuel to it I-9] wi!l decrease its critical diameter for detonation. Inasmuch as the wax used here should be a fuel very similar to paraffin (wax), the results of the present work are consistent with the literature data. Copyright© 1971by TheCombustionlnst~ut¢ Publishedby AmericanElsevierPublishingCompany,Inc.

324

DONNA PRICE, A. R. CLAIRMONT,JR., and J. o. ERKMAN

They present, moreover, a quantitative study of the explosive behavior of a simple composite propellant.

Experimental Materials and l?roeedures Materials All AP used was propellant grade and contained 0 . 1 5 ~ - 0 . 2 5 ~ tricalcium phosphate. The various lots used had weight-median particle sizes ranging from 7.2 to 200 #. AP of 25 p or larger was used to prepare the models because the larger size minimized formation of agglomerates during dry mixing. A refined, powdered, grade I yellow carnauba wax was supplied by Frank B. Ross Company. It had an average particle size of about 125 /~ and an elementary analysis corresponding to Cts.zgHzg.zgO~. o 1.4]. According to the literature, grade I material has a density of 0.999 gm/cm a at 25°C; melting point ca. 83°C; boiling point ca, 320°C: and fire point ca. 330°C. It should, therefore, be a rdatively volatile fuel. Throughout the paper it is referred to as wax except where a distinction must be made between paraffin and carnauba wax. Charge Preparation and Experimental Procedure AP/wax mixtures were prepared by tumbling the dry components; they were stored in watervapor-proof bags until used. Charge preparation and experimental procedures were those reported in the previous work 1.1"] except that waxed mixtures were not heated above 35°C. All charges were either unconfined or supported in 0.08-mm-thick cellulose acetate envelepes. Boosters were of the same diameter as the test charge and 5.08 cm long; they were of 50/50 pentolite (I.56 gm/cm3). Charges were 0.64-7.62 cm in diameter, 20.3 cm long, and were frequently followed by a pentolite witness. {Near the end of this work, booster length was changed to 2,54 cm and charge length to 22.8 cm. This is noted in the appropriate tables.) The shock-induced disturbance was recorded by a 70-ram smear camera at writing speeds of

I--4 mm/psec. Smear camera records were similar to those of previous work 1"1] with a tendency for waxed AP to produce greater luminosity of both the detonation front and the gas products for Po < 1.2 gm/cm 3.

Record Reduction and Data Obtained Record reduction ,was carried out as in the earlier work. I'1]. In 1"2]we introduced two small corrections to the measured detonation velocity. Subsequent work 1.10"1showed that this treatment is not justified for AP data; consequently, it has been omitted here. Fortunately, treatment of the AP data of [2"1 resulted in changes in the D values of only 1 ~ or less. Coefficients of the infinite-diameter velocity equation D~ =A +Bpo are (1.15, 2.59) and (1.147, 2.576) for the untreated and treated data, respectively. Representative data from the present work are tabulated in the Appendix. Eight sets included replications (seven pairs and one set of three). These showed an average precision of 0 . 6 4 ~ with a range of 0 . 1 4 ~ - 2 . 2 5 ~ , How° ever, seven of the eight had a precision <0.78 "/o and the average of these was 0.41 ~o. Results and Discussion Effect of Wax on Detonation Velocity At ConstantDiameter Figure 1 shows the detonation velocity data obtained at constant diameter (d =5.08 cm)

iF-, ~°I

,

J

t tMo

Figure I. Effect of wax on detonation velocityof ammonium perchlorate (25-/* AP trod constant diameter of 5.08 cm).

325'

EXPLOSIVE BEHAVIOR OF A SIMPLE COMPOSITE PROPELLANT MODEL

with 0H, 10~o, and 20% wax added to a 25/~ AP. The D versus percentage of TMD curves for the waxed AP are oi" the same nature as that for the unwaxed; that is, tht'y parallel the base curve (0~o wax) and exhibit a maximum in D. Hence, AP/wax, like AP, shows Group 2 explosive behavior. Although additier~ of the wax has not changed the explosive classification, it has considerably extendied the detonability range at higher percentage of TMD. Presence of the wax has effected increases of up to 60% in D. This is much greater than the increase found in the infinite-diameter detonation velocities (Dr) described in a later section. Comparisons are made here at equal percentages of TMD or equal percentages of porosity ( 1 0 0 - % TMD) to assure equal void space, equal volume of materials being compared, and approximately equal interior surface area. As Fig. 1 shows, on this basis, the curves for the two waxed mixtures show sortie convergence, but do not cross. The inset of Fig. I shows the same data, plotted on an absolute density basis instead of on a porosity basis, in this case the curves do cross because the 80/20 mix exhibits higher velocities than the 90/10 at Po <--1.25 gm/em 3, and lower velocities at Po > 1.25 gin/ ¢m a .

Diameter Effect

AP/wax compositions with 5%, 10%, 20~'~, and 31.5~ wax were used at about 55% and 67% TMD to study the diameter effect on D. The 90/10 mixture gave results representative of all the materials, and is the only one that will be illustrated. Figure 2 shows the D versus d curves for this composition at 52.8% and 67.5% TMD. They are typical of those found for the other three mixtures in their regular, smooth increase of D with increasing d. The analogous curve for a 10-,u AP at 51.3% TMD (I gm/cm 3) [1] is also shown. It is evident that the curves for the 53% TMD mixture and 51 ~,', TMD AP are essentially parallel, although the former extends to much lower d values before it reaches its

s.a f

g 3,0

--- ,.--

i,Ùi

: DIaM[~R d

Icm)

Figure 2. Diameter effect on detonation velocity of ~axed and unwaxed ammonium perehlorate. Brokert llne. l(gu AP/paraffin, 90/10 [7]: solid liae. 10./~ AP [1]: open circles, open squares, 25-it AP/earnauba wax, 90/10.

failure limit. (Interpolated values [1] will also give a D versus d curve at 67.5% TMD fo'r 10-,u AP that parallels the 67:5 % TMD AP/wax curve.) Finally, Fig. 2 contains a curve for 10-,it AP/paraffin, 90/10 at 56.1% TMD [7]. Curves for 95/5 and 68.5/31.5 AP/wax showed an approximate coincidence, like that of Fig. 2, for the two compactions at the lowest d values, but those for 80/20 showed a definite crossing of the two curves at about d = 2.54 cm. None of the four mixtures showed an S-shaped curve like that reported for AP/paraffin [7], although carnauba wax should be comparable to paraffin wax as a fuel. The difference cannot readily be attributed to the difference in particle size or in compaction. Figure 3 displays the D data for the 90/10 mixtures plotted as a function of reciprocal diameter (d-l). In contrast to pure AP [1, 2] all the data cannot be fitted with one straight lille. Of course, pure AP is subcritical for the smaller range of diameters (d < 2.5 cm), but in the few cases where data f~.qloffthe linear curve used to extrapolate to d- t = 0, they fell below rather than above the curve [1]. Data for AP/wax show the opposite behavior in Fig. 3. The form of the curves in Fig. 3 is evidently an effect of the fuel/oxidizer reaction. For

326

DONNAPRICE, A. R. CLAIRMONT, JR., and J. O. ERKMAN extend detonability to such small values of d), its effect in increasing D and D~ is most evident at larger diameters, specifically, at d greater than the break point of the D versus d - t curve. The break point occurs at increasing d with increasing percentage of T M D .

!,o g

Infinite-Diameter Detonation Velocities z.o etClPR0CALOtAM~te 10~.1 rein"11 Figure 3. Extrapolation to ~nfinite-diameter value for 25-/t AP/wax, 90/10.

5 ~ - 2 0 % wax the breaks occurred at about 2.5 and 3.5 cm, respectively, for 55% and 67% TMD; the change was much sharper in the less. porous charges. The curves for the 68.5/31.5, AP/wax, showed no break, but ~his mix was fired only at d > 2.54 era. Supplementary information on the effect of fuel/oxidizer reaction can be found in work on 7.7.p AP/high explosive, 80/20, mixtures at 9 5 % - 9 9 % T M D [II~. The three explosives used were 2,2,2-trinitroethyl 4,4,4-trinitrobutyrate (TNETB), pentaerithritol tetranitrate (PETN), and cyclotetramethylene tetranitramine (HMX); their respective oxygen balances to CO2 are - 4 ~ , - 1 0 % , and - 2 2 % . Over the d range of 2.54-7.62 cm, the mixtures with TNETB and PETN gave linear D versus d - I curves that indicated the same contribution to D of the AP in each case. By contrast, the AP/HMX showed a curve similar to those o f Fig. 3 with the break at d ~ 4.4 cm. Extrapolation of the section of the curve (d < 4.4 cm, d - l > 0.23 cm - l ) gave a D~ value indicating the same con~!ribution of AP in this mixture as in the compositions with poorer fuels. But extrapolation of the section of the D versus d - ~ curve (d > 4.4 era, d - ~ < 0 2 3 c m - t) gave a much higher D~ value, indicating a fuel/ oxidizer reaction. A/though zhe fuel/oxidant reaction must occur to some extent in small-diameter charges (otherwise addition o f wax to AP would not

Pure AP presents difficulties in extrapolation o f the D versus d -~ curve to the infinitediameter value (D~) at greater compaction than 5 1 % T M D for the relatively small charges ( d < 7 . 6 cm) to which our firing range is restricted [2]. The similarity of the AP/wax curves to those for AP (Figs. 1 and 2) suggests that the same difficulty exists for the mixtures. Since all of the AP/wax compositions seem to exhibit continuous D versus d curves, the slopes of which also appear to be continuous functions of d [Fig. 2], we would expect similar behavior in the D versus d - t curves. Moreover, if D approaches D~ asymptotically with increasing d, a portion of the D versus d - 1 curve should approximate a straight line. O¢ course, a smooth curve of moderate eurvat'.. re can be approximated by a series of straight i. es. Hence the segment we have used to do thi~ and have extrapolated to D~ at d-~ = 0 ,aai~;ht be replaced by a different one if measurements were available on charges of larger size. In this connection, it should be noted that the curves o f Fig. 2 at 5 1 % - 5 3 % T M D have flattened out at the larger diameters much more than that at 68 % T M D . Thus extrapolation o f the former should produce' the better results. The D~ values obtained by ~he foregoing extrapolation procedure are given in Table 1 and plotted in Fig. 4 where the values for each composition at two compactions are used to obtain the assumed linear curves in the D~versus percentage of T M D plane. The curve for AP [2] is also given for comparison. Except for that of AP/wax, 68.5/31.5, all the curves have the same slope (to 1.5%). If D i is in error because o f the extrapoJation procedure, it is prob-

327

EXPLOSIVE BEHAVIOR OF A SIMPLE ,COMPOSITEPROPELLANT MODEL Table 1. Apparent D I Values for AP/Wax Mixtures

AP/Wax Wax

gin/era ~

Percentage of TMD

DI mm/#sec

5 10 20 31.5

1,025 0,940 0,901 0.826

55.1 52.8 54.9 55.1

4.34 4.54 4.86 4.50

5 10 20 31.5

1.250 1.200 1.100 1.006

67.2 67.5 67.1 67.1

4.96 5.27 5.46 4.97

Po

ably too large and the error would increase with increasing percentage of TMD. Hence the correct values might produce less steep slopes. Values were taken from the D~ versus percentage of T M D curves to construct the curves D~ versus percentage of wax shown at the top of Fig. 4. Note that the percentage increase in D i is about the same as that found in D on going from 10-p AP to 25-# AP/wax (Fig. 2). (It was for this reason that the 10-# rather than the 25-# AP was used for the base curve of

Fig. 2.) The curves show D~ increasing with percentage of wax up to a broad maximum at about 20 ~o wax and decreasing thereafter. The increase is greatest at low wax concentrations; in fact, the slope of the D i v+ersus percentage of wax curve is approximately halved in the successive intervals 0--5~0, 5~o-10~o, 1 0 ~ - 2 0 ~ wax, Both curves may lie too high (in Di value) because o f extrapolation error, as discussed earlier, but the same trend, effect of wax content on D~, will be obtained even with large extrapolation errors. A very similar curve has also been reported for the effect of polystyrene on the detonation velocities of 5 0 ~ TMD mixtures o f AP/polystyrene at their failure limits [9]. Computations of D l at 5 0 ~ and 8 0 ~ TMD for AP and AP/wax at I 0 ~ and 2 0 ~ wax were carried out with the Ruby code [12]. F o r this purpose wax was approximat6d as (CHz), with a standard heat of formation of 5.5n kcal. The results are shown in Fig. 5. The absolute values of the computed D~ are too high by about 0.7-0.9 mm/#sec, but the maximum increase in D i is about the same percentage as that measured experimentally. The Ruby computations indicate that the 90/10 mixture is very close to stoich~ometric for detonation, and computations on a propellant evaluation code 1"4] show maximum impulse 7.0

^P~WAX

+ -..

moto

+

+= ~- ~.o

8.0

~

~

..~ Iron ......

d "~' I

Figure 4. Effect of wax on inllnite-diameter detonation velocity of ammonium perehlorate lAP/wax : open triangles, 95/5; open circles. 90/10; open squares. 80/20: open diamonds, 68.5/31.5).

.+.+.+'+'~" I

I

Figure 5. Computed effect of wax on infinite.diameter ~e(onation velocity of ammonium perehlorate (Ruby code).

DONNAPRICE, A. R. CLAIRMONT,JR., and J. O. ERKMAN

328

a t IQ.6~ wax; that is, that the stoichiometrie amount of wax for burning is a little less than

10.6%. It seems evident from present experimental results that some o f the wax reacted with AP in time to contribute to the detonation velocity but that its reaction rate was such that about twice the stoichiometrie amount of .wax was required in the original formulation to obtain the maximum effect on D. It is quite possible that the reaction rate is dependent on factors other than initial concentration (e.g., wax concentration in the vapor phase). The Ruby code with its present parameters underestimates detonation temperature while it overestimates D. However, both it and the propellant code agree in predicting an appreciable increase in reaction temperature as a result of any oxidizer/fuel reaction. Table 2 shows a few of the values for the 90/!0 mixture. The effect of wax addition on D shows that an appreciable amount of oxidizer/fuel reaction did occur and the computations indicate that it must have resulted in an appreciable increase in reaction temperature. Reaction Zone Length and Reaction Time

By use of the curved front theory and simplifying assumptions of previous work [1], the reaction zone length (a), reaction time (~), and detonation velocity o f two explosives are related by al/a 2 = (Dil/ Di2)('cit/Ti2) (I) Values of the reaction zone length calculated from the slope of the D versus d - t curves for

AP/wax (the segment extrapolated to obtain Di for the I0~o-32~o wax mixtures) were the same (to about 1 0 ~ ) as those for unwaxed 10-g AP at 55 ~ T M D ; at 67 ~o ' r M D , a for the waxed A P was about 8 0 ~ o f that for the 10-p AP. This last result is attributed to the too great slope o f the D versus d - 1 curve for AP/wax rather than to any real difference. if we restrict ourselves to the lower compaction, where extrapolation procedures seem better justified, a t ~ a 2 and ~tt/~i2 ~ O!z/Ott

(2)

This equation gives a reduction in the reaction time o f the waxed AP o f 1 0 ~ , 16~o, 20~o, and 1 5 ~ o f z (10-,u AP) fer 5%, 10%, 20%, and 3 1 . 5 ~ wax, respectively, if 25-/2 AP is used as a base instead of 10-g AP, the maximum reduction o f z is, by Eq. (1), about 70,% at 20~0 wax and 55 ~ T M D . The increase in temperature as a result o f the oxidizer/fuel reaction is evidently sufficient not only to compensate for the necessary diffusion process (intrinsically slower than the unimolecular decomposition of AP) but also to increase the overall reaction rate. Effect of Wax on Detonability and Shock Sensitivity Detonability

Addition o f wax to A P extends its detonability to smaller diameters and higher compactions. as Figs. 1 and 2 have already shown. In Fig, 6 all the failure limit data have been plotted d= versus percentage o!r T M D together with the comparable curves for A P (10 g), AP {10-80 ~t) ['8], and A P (25 g) [1], a n d T N T (70-200 p)

[14]. Table 2. ComputedEffect of Wax on Reaction Temperature Temperatures,°K Material AP AP/~ax. 90/10

Detonation at 50~, TM D [12]

Flal:le at 1000ps~ [13]

ld03

1417

3174

29970

o Computedfor the stoichiometri¢compo.~ition.

The curves for AP show the usual decrease in dc with decrease in particle size [ 1 , 8 ] and approximate coincidence at high compaction [1]. These data are at densities Po > 1 gin/era 3 and show the G r o u p 2 behavior of increasing dc with increasing compaction. Jt should be mentioned, however, that a suggested reversal of that trend at higher porosities [1] has been confirmed with a 7.7-,u AP. Data in the Ap-

EXPLOSIVEBEHAVIOROF A SE~PLECOMPOSITEpROPELLANTMODEL ~110 60

o

P

Iooto ~1,

10~

N g -- ~ ~ ....

~

...... ,~

~ ~'o

uo.~,

~

,~o

~. TMO

Figure 6. Detonabilitycurves for waxedand unwaxedammonium perchlorate and for TNT. Detonation shown by solid symbols: failure, by open ones• Circles,90/10. 25-/1 AP/wax: diamonds. 90/10. 200.p AP/wax: squares. 80/20. 25-1t AP/wax. Reference curves: lOp .and 25-i1 AP [1]; 10-80-/t AP [8"1;70-200-# TNT [14]. pendix show the G r o u p 1 trend in the range 0.5-0.7 gm/cm a (25.6 ~o-35.9 ~ TMD). The failure.curve from [.8] falls between those for 10- and 25-/~ AP. The AP used in that work was obtained from a sieve cut and a microscopically observed particle size range of 10-80/~, but the particle size distribution and median were not determined. The weight median particle size would of course he smaller than the mean of the projected area diameter of microscopic observations. Addition of 1 0 ~ or 20~o wax to 25-/1 AP decreases its d, by a factor of 5 or more. In fact, the detonability curves for AP/wax are below • that for 10-/t AP and very close to that "for T N T at eompactions - < 7 0 ~ TMD. The limit curves for the two mixtures are indistinguishable up to 70~o T M D , above which the 80/20 mixture shows greater detonability (smaller de). Addition of 109/oowax to 200-p AP lowers its d¢ from > 76 mm to about 40 mm or that of the 25-/~ AP at 60% T M D , but does not comparably lower the d, value at 67% TMD. The AP particle size effect on the 90/10 mixtures is qualitatively that found for pure AP; that is, the failure curve shifts toward higher diameters and lower percentages of T M D for increasing particle size. Thus for the 200-p AP, we find the steep portion of the 90/10 detonability curve at

329

6 0 ~ o - 6 7 ~ TMD, .whereas for the 25-1t AP it occurs at 7 6 ~ - - 8 2 ~ T M D . . Both the 90/10 and the 80/20 mixtures with 25-p AP have d, < that o f T N T (70-200/~) at a percentage of T M D -< 64. At lower porosities their dc is larger than that of the TNT. G o r ' k o v and Kurbangalina [8"1have reported an interesting parallel study of the detonability of AP. They showed the effect of particle size, water content, temperature, and stool! amounts of fuel on the limit de. In particular, working at 1.1 gin/ore a ( 5 6 ~ TMD), a particle size range o f 10-80 p, and glass confinement, they found that raising the initial temperature from 25°C to 200°C lowered d c from 23 to 12 nun. Moreover, adding either 0 . 9 ~ carbon black or 2.4~o R D X lowered the d c at 25°C from 23.to 14-15 ram. At 200°C these mixtures had a d c of 11-12 ram. They concluded that the addition of fuel increased the reaction temperature and thereby decreased reaction time and de. This seems a reasonable conclusion, as far as it goes, and is equally applicable to our results~ In a more recent investigation, Belyaev et al. ['9] showed that for 5 0 ~ T M D mixtures o f A P " with polymethylmethacrylate (PMMA) and polystyrene the maximum rate of decrease in dc with increase in fuel concentration occurred at 2 ~ fuel or less. Presumably the same large effect would be found with small additions of wax, but such low concentrations were not investigated in the present work. The largest effect with the first "fuel added is in accord with the greatest rate of increase in D at small w ~ e o n centrations (Fig. 4). Oxidizer/fuel reaction is considered the cause in both cases. Shock Sensitivity

A calibrated gap test [15 ] was used toinvestigate the shock-to-detonation sensitivity of AP ~nd AP/wax. This is a 60-NO~O test; its criterion of a positive result is the effect of the shockinduced reaction on a witness. The witness is usually a steel plate (in which a hole is punched by a detonation), but occasionally (e.g., lowdensity AP) is another explosive. The quantity

330

DONNAPRICE, A. R. CLAIRMONT,JR., and J. o. ERK~AN

measured is the thickness of a gap of PMMA necessary to attenuate the shock from a standard boo~:er to the strength that produces a ~o in 50 % of the trials, The measured 50 % gap is then converted, by the current calibration data, into the corresponding shock amplitude at the end of the gal~; this value is called the 5 0 ~ pressure P~. Data for two particle size APs and the 90/10 and 80/20 AP/wax mixtures are given in the Appendix and plotted in Fig. 7; analogous data for pressed TNT [3] are als9 shown for comparison. The general fornas of these P9 versus p~,rcentage of T M D curves is that to be expected of any explosive exhibiting the Group 2 behavior of increasing de with increasing compaction (see Fig. 6). As Fig. 7 shows, the common trend of decreasi~lg shock sensitivity with increasing percentage of TMD holds until the material approaches its critical density in the gap test. With this approach, the slope of the sensitivity curve increases aud the curve itself is terminated at the "dead-press" density, that compaction above which the explosive is subcritical in the gap test. Each curve of Fig. 7 has been made vertical at that percentage of the TMD which fits the gap test data and is also in accord with observed subdetonation reactitans at compactions above the critical or failure

~~~ 0

J

r 65

i

it IM fl ! I I

i ~k_-.-.gv..~~ 75 85

t 9~

TMD

Figure 7. Shock sensitivity~:urvesfor waxedand unwaxed ammoniumperchlorateand for TNT. Verticallinesat top of graph mark lowestpercentageof TMD at which a subdetonatior reaction was observed.

value. Thus the dead-press point, where the curve is vertical, lies slightly to the left or at a slightly lower percentage of TMD than the last shock-induced, subdetonation reaction; the solid vertical lines show the lowest compaction at which such reactions were observed. The effect of particle size on critical diameter, and hence on 'the dead-press limit, is relatively large, but in the high-porosity, supercritical region its effect on Po is relatively small. It is most evident for pure AP; the 7.2-g material is definitely, though not greatly, less shock sensitive than the 25-# AP. On the other hand, the sensitivity of the 200-~ AP/wax, 90/10, differs insignificantly from that of the 25-~ AP/wax, 90/10, despite the large difference in their respective failure diameters. It is evident that the addition of wax sensitizes AP to shock ; it lowers the sensitivity curve and extends it toward higher percentages of T M D because it increases detonability. Wax as a fuel also sharpens the transition from .the detonable to the dead-press region. This is particularly evident in the sensitivity curve for AP/wax, 80/20. As in the case of detonability, 90/10 and 80/20 mixtures of the 25-/~ AP are indistinguishable at lower percentages ofTMD ; at higher percentages of TMD the 80/20 mix remains detonable up to a higher percentage than does the 90/10. Finally, the waxed mixtures show a sensitivity curve very close to that of TNT; the 80/21) mix is comparably sensitive up to 90% TMD, where it diverges because of its approach to the dead-pressed condition. The large critical diameter and dead-pressed condition of AP/wax at voidles.,; density are typical of simple composite pr,apellants; in other words, the model reproduces this propellant behavior. But at greater porosities, say 7 0 % - 8 0 % TMD, both detonability and shock sensitivity of AP/wax, 80/20, are equal those of TNT. (This composition further resembles TNT in having about the same voidless density and, at 55% and 67% TMD, the same D t and the same computed chemical energy release. In the same range, ii exhibits a greatez volume of gas

EXPLOSIVEBEHAVIOROF A SIMPLECOMPOSITEPROPELLANTMODEL production and longer reaction time.) This similarity to TNT demonstrates the potential explosive behavior of simple composite propellants under appropriate conditions (e.g., broken up and at 8 0 ~ TMD or possibly at high initial temperatures).

Effect of Particle Size on Results The AP/wax mixtures were prepared with 25-,u AP and 125-,u wax. The infinite-diameter values of D are independent of particle, size. Hence the curve of Ot versus percentage o f wax should be the same as that found with much smaller particle sizes of both AP and wax provided our experimental range in charge diameter was adequate. Even if extrapolation errors have resulted in apparent Dt values higher than true D~ values, the trend of D~ versus percentage of wax and location of its maximum should be correct. Assumed errors of 1 0 ~ in D~ or 25% in the slope of D~ versus percentage of TMD do not change the trend. Moreover, the critical detonation velocity versus percentage of fuel for AP/polystyrene (90/10 is also the approximate stoichiometrie mix, as it is for AP/wax) showed the same trend for mixtures in which the fuel and oxidizer particle sizes were 6 g and 15 #, respectively [9]. Although 25-# AP was used to prepare the AP/wax mixtures, detonation velocity data ob; tained on 10-g AP have been used as a base for the effect of wax on D measured at various diameters: This was done because the AP/wax mixtures exhibited velocities greater than that of the comparable compacts of 10-# AP by percentages close to those found for the increase in analogous D~values. It seems probable that the higher reaction temperature of the mixture minimizes the effects of particle size that can be observed in pure AP. Finer mixtures will undoubtedly shift the detonability curves by large amounts, but in directions established by the present results. They will probably also shift the shock sensitivity curves by a smaller amount.

331

Parallelism between Combustionand Detonation AP and its mixtures have beez~studied for many years as propellants. Recently several exhaustive reviews [13, 16-18] of such work have appeared. They exhibit striking Similarities be.tween the results of deflagration studies and those of detonation studies, such as ours on AP/wax. Successful propagation (and its failure) in burning are governed chiefly b y transport processes, whereas detonation and ~its ,failure limits are determined chiefly by hydrodynamic phenomena; we "can therefore interpret the failure diameter in deflagration as due to a heat loss by conduction, convection, or both, and the failure diameter in detonation as due to a heat loss by reaction quenching, caused by lateral rarefaction waves. In both cases, the dominant factor is the relative energy loss rather than the mechanism whereby it occurs. It seems possible that the chemical reactions could be the same in both burning and detonation, although the products and rates will reflect the different pressure and temperature ranges. Factors affecting the burning rate of AP are charge diameter, density, particle size, confinement, initial temperature, and initial pressure. In general, these variables affect the burning rate and the detonatiou rate in the same way. There a r e a few situations for which there is no parallel i).l the two fields (e.g., initial pressure limit for dellagration). However~many trends are the same, and deflagratibn, like detonation, is a multidimensional effect. Relatively few failure limit studies are available in either field. AP is a somewhat exceptional prope!lan, t as well as an unusual high explosive. Its linear burning rate decreases'with decreasing density, whereas that of the common organic explosives and of mixtures (AP/fuel) increases [17]. It is now generally accepted [16] that the initial step in its themlal decomposition ig the sublimation-dissociation reaction

DONNA PRICE, A. R. CLAIRMONT,JR., and J. o. ERKMAN

332 NH4CIO4'~-' NH3 + HCIO4'

(3)

AH = 58 kcal/mole (this is a multistep reaction and the ratedetermining step is not clear) with an activation energy of about 32 kcal/mole. The reaction sustaining steady burning (and-producing the flame) is then the gas phase oxidation of NH 3 by HCIO4. DTA studies reveal an endotherm at 240°C (crystal transition from orthorhombic to cubic) and two exotherms at about 300°C and 440°C corresponding to "'low-temperature" and "high-~emperature" decomposition. AP wilt not exhibit steady burning at 2025°C and 1 atm. However, if it is preheated or if a small amount of volatile fuel is present, it will burn, and then, under comparable conditions, its burning rate is higher than for pure AP, However. the amount of fuel is critical; very small amounts can decrease the rate or increase the critical diameter for burning [17]. There are numerous theoretical models for AP/fuel mixtures. Many suggest an oxidizer flame supplying heat to vaporize the fuel and thus provide a diffusion flame (e.g., H C I O j fuel). Since addition of fuel increases the barning rate, some energy from the diffusion flame must be fed back to the solid mix, although it may serve only to incrJ:ase the rate of the reaction of Eq. (3). The reactions controlling the burning rate are completed near the solid surface (e.g., within 200 St), but chemical reaction may go on several millimeters beyond the surface. This leads to a concept of "zone of influence," which includes only the portion of the total reaction that can affect the burning rate. Thus the final flame temperature (measured by thermocouples) may be higher than and some distance downstream from the temperature at the beginning of the reaction zone. The necessary preheating for steady-state burning at 1 atm has been reported as initial temperatures of 200°C [17] and 280°C [16]. The latter, for a specific heat of 0.309 cal/(gm "C) amounts to 87.5 cal/gm energy supplied to the

AP. (The minimum preheating by radiation gives the value 95 cal/gm [19].) The minimum fuel to effect the same result seems to be 3.85 % paraformaldehyde or 2.6~'o methaldehyde, equivalent to 107 cal/gm extra heat [19]. In this case, the extra energy must not only initiate the reaction of Eq. (3) but also vaporize the fuel. (The minimum flame temperatures observed in steady-state burning at I atm were 970°C (preheated AP) and 1000°C (AP/fuel).) It seems clear in the case of AP/wax mixtures that the AP can decompose exothermally at about 300°C, which is below the fire point of the wax. For a given thermal input for initiation of detonation, the vapor phase will probably be richer in oxidizer than in fuel. This difference in concentration of fuel in solid and vapor phases could account for the maximum effect at a fuel concentration of approximately twice the stoichiometric in the solid mixture. The requirement of excess fuel (presumed necessary to keep composition of the gas phase in the zone of influence constant and corresponding to maximum rate of reaction) is commonly observed in burning rate studies. For volatile organic fuels, the maximum effect on burning rate generally lies between 209~ and 30~,,, excess fuel over the stoichiometric, But greater excesses have been reported, and in particular, a homogeneous premixed flame of HCIOdCH, exhibits two distinct flame fronts at atmospheric pressure and at concentrations on the fuel-rich side of stoichiometric [20"1; the second flame front is 100-150°C hotter than the first and seems to be essentially a CO flame. The maximum burning velocity for this gas mixture occurs at a concentration Considerably on the fuel-rich side of the stoichiometric. This is apparently associated with the lag of the oxidation of CO to CO2 behind the other reactions of burning; a similar lag in detonation reactions may well .occur in addition to a lag in volatilizing the fuel and a delay in diffusion between the fuel and the oxidizer. There is thus a parallel to the zone of influence in combustion because the detonation is obviously sup-

EXPLOSIVEBEHAVIOROF A SIMPLECOMPOSITEPROPELLANTMODEL ported by incomplete reaction of the original concentrations of wax and AP. There is also a similarity between the equivalence of preheating and adding a small amount of fuel. Preheating has already been mentioned as an alternate to addition of wax as a means of reducing the critical diameter of AP. The small amount of added fuel need not be volatile, since cargon black is effective [8]. Variables affecting the burning rate nf A P / fuel mixtures are, of course, the same as those determining the burning rate of pure AP with the addition o f the concentration as a variable for the mixture. Adams et al. [21] studied burning rates as a function of pressure, mixture composition, and particle size. They concluded that the dependence of the rate on any one of these variables was affected by the value of the other two; that there was a complex dependency and not simple, separable effects. These conclusions seem to be generally accepted [16, 17]. In conclusion, it seems well established that burning of a simple composite propellant (AP/ fuel) is a multidimensional, multistep, multistage process, dependent on both kinetic and diffusion factors. We know that detonation of a simple composite explosive is a multidimensional process, as all detonations are, and must depend on diffusion processes as well as kinetic, since adding a volatile fuel increases the D~ value above that of pure AP. There is every probability that detonation is also a multistage process and quite as complicated as deflagration.

Summary and Conclusiom; Addition of wax to AP results in a large increase of D~, and a D~ versus percentage of wax curve with a broad maximum at about 20 % wax. The maximum increase [ D i ( A P / w a x j DI(AP)] is about that computed with the Ruby code, although the absolute values obtained with the code are about 0.7-0.9 mm/,usee too large. The computations show a maximum at

333

a stoiehiometric composition of about 10% wax, as compared to the experimental result of a maximum at 20% wax. The effect of wax on the D~ of A P is attributed to the reaction between the volatile fuel and the detonation products of the AP. The observation of maximum effect at •a concentration of wax approximately twice the stoichiometric is believed caused by the kinetics o f the vaporization a n d diffusion processes necessary for the oxidation-reduction reaction. Although determination of the effect of wax on D i and reaction time was at 55 % and 67 % T M D , detonability curves and shock sensitivity measurements were carried to higher compactions. Present results can be illustrated by data for the 25-It AP/wax, 80/20, mixture, which showed the largest effect of wax on each explosive characteristic studied. At 55% TMD the wax reduced the detonation reaction time and increased the critical compaction for detonation of a 5-cm diameter charge from 71% T M D to 88 % T M D ; these determinations were on unconfined charges. In the confinement of the gap test, wax shifted the point of dead pressing from about 82% T M D to about 92% T M D ; it also increased sensitivity (lowered Pg) over the range of detonability. The AP/wax, 80/20, mix is most similar to pressed TNT. It has about the same voidless density, the same D i at the same percentage of TMD, approximately the same d~ at percentages o f T M D < 70, and approximately the same shock z~;n~itivity curve at percentages of T M D < 90. If differs from T N T chiefly in exhibiting greater detonation reaction time and lower detonability at high compaction. These results show very clearly that a simple composite (AP/organic matrix) is potentially detonable. This was expected since a composite (AP/matrix/Al, in which the matrix contained no oxidizing groups or explosive substances) has been detonated at essentially voidless density and d = 72 in. [22]. The present data offer detailed information on how the detonability limit is approached and what explosive be-

DONIqA PRICE, A. R. CLAIRMONT, JR., a n d J. o. ERKMAN

334 havior

to e x p e c t

their compaction

of granular

mixes prior

to

to voidless density.

The writers thank their many colleagues who helped in charge preparation and firings. The assistance of R. R. Bernecker and D. J. Edwards was particularly appreciated,

Appendix The letter symbols used throughout Tables A1-A7 are h, hand-packed; H, hydraulically pressed; i, isostatically pressed; F, failure. Table AI. Detonation Vellmity versus Density for AP/Wax a Po gm/cm ~

Percentage of TMD

d cm

Po gin/eraa

D mm//*sec

5.08 6.35 7.62

1.025h 1.025h 1.025h

3A19 3.602 3.705

2.54 3.495 5.08 6,35 7.62

1.235i 1.262i 1.236i 1.274i 1.250i

2.735 h 3,169 b 3.689b 3.948 b 4.157 b

O (I,250)*

2.738 3,166 3.692 3.943 4.t57

Data are for 28-/t AP: p., = 1,86 gm/em a. Shots were made with boosters 2.54 em long (instead of 5.08 era) and aceeptors 22.8 em long (instead of 20.3 era), * Small correction to Po = 1.25 gm/cmL

D fmm/,asec) Table A3. Diameter Effect in AP/Wax, 90/t0"

AP/Wax. I00/0 (9~ = 1.95 gm/emaJb; AP/Wax. 90/10 (p,,= 1.78 gm/cm a ) 0.941h 52.9 3.784 0,958h 53.8 3.973 1.10111 61.8 4.09! 1.200H 67,4 4.179 1,201b 67.5 4.139 1.281i 1.408i 1.447i 1,516i

72.0 79. I 81.3 85.2

4.118 3,846 3,694 F

AP/Wax. 80/20 (p., = 1£4 gm/cm ~) 0.901h 54.9 4,176 0.90211 55.0 4.231 0.903h 55.1 4.273 i.001h 61.0 4,381 1.101h 67.1 4,447 1.101h 1.201H 1,294i 1.379i 1,384i 1.424i

67.1 73.2 78.9 84.1 84,4 86.8

4,459 4.340 3.904 3.605 3.613 3.18,'

" Data for 25-.u AP, d = 5.08 cm, " See[l].

d cm

Pu gm/cm 3

D mm/#sec

1,905 2.5.4 3.495

1.025h 1.026h 1.025h

1.871 2,688 3.089

D (1.250)'

D mm/.asec

0.635 0.953 1.27 1.905 2.54 5.08 5.08 6.35 7.62

I. 52.8 % T M D 0.901h F 0.907h 2.254 0.90Ih 2.556 0.901 h 3.050 0.918h 3.201 0.941 h 3.784 0.958h 3.973 0.937h 4.002 0,937h 4.066

0.635 0.953 1.27 1.905 2.54 5.08 5.08 6.35 7.62

I1. 67.5% T M D 1.212h F 1.202h 2.149 1.204h 2.586 1.202h 3,127 1.204H 3,457 1.201h 4.139 1.200H 4.179 1.201 H 4.369 1.202H 4.538

3.49 5.08

IlL 60.3% T M D 1.071 h F 1.075h 3.906

7,62

IV. 67.4% T M D 1.201h F

Table A2. Diameter Effect in AP/Wax~ 95/5 ° d cm

Po gin/era ~

D(0.940)"

2,242 2,560 3.060 3.220 3,959

Data in parts I and II are for 25-# Ap, p,, = 1,78 gm/em3: data in parts 111 and IV are for 200-y AP. d~Small correction to D assuming linear variation with density.

EXPLOSIVE BEHAVIOR OF A SIMPLE COMPOSITE PROPELLANT MODEL Table A4. Diameter Effect in AP/Wax, 80/20* d em

Po gm/em a

335

Table A6. Failure Limits for Low-Density 7,7-p AP d, mm

D mm/psee

O(0.901 )b

Po

D

gm/em a 0.635 0.953 1.27 1.905 2.54 2.54 3.495 5.08 5.08 5.08 6,35 6.35 7.62 7.62

54.9 % TM D 0,916h F 0,934h 2.359 0.916h 2.843 0.901h 3.381 0.901h 3.716 0.901h 3,758 0,902h 3.949 0,902h 4.231 0,903h 4.273 0.901 h 4.176 0.906h 4,384 0.90811 4.390 0,901h 4.510 0.905h 4.480

0.953 1.27 1.905 2.54 2.54 5.08 5.08 6.35 7.62

67,1% TMD 1.105h F 1A03h 2,542 I. 102h 3.288 1,101h 3.665 1.101b 3.676 1,101h 4.447 I. 101h 4,459 1.101h 4,651 1.094h 4,791

-2,408 2.867

0,501h 0.601h 0.701h 0,801h 1.002h

+ 2.54 2.22 1.59 1.92 1.59

mm/#sec

2.70 2.54 1.92 1.92 1.92

1.54 1.94 2.18 2.47~ 2.78°

Boost~er 5.08 em long; others were 2.54 em. 4.271

Table A 7 . S h o c k Sensitivity Data for AP and AP/Wax

4.377 4.381 4,473

50% Poitll Po gm/cm a

Percentage of TMD

Gap in ./10"

0,80h 1.073H 1.074H 1.270i 1.270i 1.272~ 1.4571 1.465i 1.567i 1.562i 1.565i 1,579i 1.602i

41.0 55.0 55.0 65.1 65.1 65.2 74,7 75. I 80.3 80.2 80.3 81,0 82.1

1.1 lh 1.25H 1.25H 1.49i 1,52i 1.58i 1.58i 1.65i

56.9 64.1 64. I 76.5 77.7 81.1 81.1 84.7

1.29 ?

66.1

1,08h 1.32H 1.52i 1.60i 1.63i

60.7 74.3 85.4 89.8 91,8

1.42H 1.53H

200-# AP/Wax, 90/10 79.4 199 86.3 175

P0 kbar

7,2-/l AP

4.802 h' "

Data are for 25-# AP, p~ = 1.64 grn/em a. Small correction to D assuming linear variation with density, " For O(l.101).

202 180 182 167 167 162 120 I 13 71 79.5 76 Detonation ~ b. e

17u 22" 21~ 25 25° 27~ 42 44~ 63 59~ 60~ ---

25-# AP

Table A5. Diameter Effect in A P / W a x . ° 6 8 . 5 / 3 1 . 5 6 d em

Po gm/cm 3

D mm//zsec

2.54 3.495 5.08 6.35 7.62

0.825h 0.826h 0.826h 0.826h 0.826h

3,187 3.542 3.802 3.936 4,078

2.54 3.495 5.08 7.62

1,006h 1.007h 1.006h 1,006h

3,137 3.629 4,036 4.362

All shots made with booster 2.54 cm long. b Data are for 28-# AP. p,, = 1,50 gm/cm a.

198 178 178 143 131 93 98 b.,

18 22 22 34 38 .52 50 __

~' ¢

--

200-# AP

25-# AP/Wax, 90/10 240 225 186 110-125

12 14 20 40-46

18 23

DONNA PRICE, A. R. CLAIRMONT, JR., a n d J. o. F.~IO~AN

336 50% Point P0 gm/cm a

Percentage of TMD

1.00h 1.23H 1.39H IA6i 1.50i 1.52i

25-/t AP/Wax. 61.0 75.0 84.6 89.0 91.5 92.7

Gap in./10:

P~ kbar

80/20 236 236 213 194 178

13 13 15 18" 22~

Comp B explosive vdtness, all others had sled plate as witnesff. Test at zero gap. * Reaction. hut no detonation. a Pour density.

References I. PRICE, D.. CLAIRMONT,A. R.. Jr.. and JAFFE. I.. Cotnbastion & Flame, i l . 415 (1967). 2, PRICE, D.. CLAmMONT. A. R.. Jr.. ERKMAN.J. O.. and EDWARDS. D. L. Combustion & Flame 13. 104 (1969). 3, PRICE.D.. Elel,enth ~l,mposiam (International) on Combustion, p. 693. The Combustion Institute: Pittsburgh (1967). 4. ROEERrS. A., Naval Ordnance Station. Indian Head. Md.. private communication. 5. ARDEN, E, A.. POWLING. J.. and SMITH, W. A. W.. Combustion & Flame 6, 21 (1962). 6, GLAZKOVA.A. P.. and TERESHKIN.I. A., Russ. J, eh.l'S. Chem. 35, 795 (1961). 7. APIN. A. YA., VOSKOBOYNIKOV.1. M., and SOSNOVA,G. S.. Zh. Pril~t. Mekhan. i Tekhn. Fi:, 5. tl5 (1963) (through DDC: AD 614 773). 8. GOR'KOV,V. A., ~nd I{.URBANGALINA.R. K., Combllslion. Explosion. and Sllock Waves 2(2), 12 0966).

9. BELYAEV.A. F., ](~OROTKOV.A. 1.. OBMENIN, A. V.. and SULIMOV, A. A., Fi:. Goreniya F~ryva 4(2). 238 (I 968). 10. ERKMAN. J. O., and PRICE, D., Naval Ordnance Lab. Rept. NOLTR 69-235 (May 25. 1970). 11. PRICE, D.. CLAIRMONT,A. K , Jr., ERKMAN,J. O.. and EDWARDS. D. J.. Naval Ordnance Lab. Rept. NOLTR 68-182 (December 16. 1968). 12. HURWlTZ. H.. "'Ruby Code Computation." NOL, private communication. 13. PEARSON, G. S., Oxidation t~ Combustion Revs. 4 (I), 1(1969). 14. BOnOLEV.V. K.. Dokl. ,4kad. Nauk SSSR 57. 789 (I 947). Translated by U. S. Joint Publications Res. Services, JPRS 4026. 15. PRICE, D.. and JAFFE, I.. A/HA J. 1. 389 (1963). See also "Current Calibration" in IPRtcE, D.. JAFFE, I_ and ROBERSON,(3. E., Ind. Chim. Beige 1967. 32 (Spcc. No.). pp. 506-510. 16. HALL. A. R.. and PEARSON, G, S.. Rocket Propulsion Establishment Rept. No. 67/I. Ministry of Teehno|.. Londor~ (January. 1967). 17. BAI~nMAN. N. N.. and BELYAEV,A. F,. Inst. Chem. Phys. Acad. Sci, USSR (Moscow), 1967 (through RPI Transl. No. 19. November, 1967). 18. JACOBS,P. W. M.. and WHITEItEAD,H. M.. Chem. Revs. 69, 551 (1969). 19. ARDEN. E. A.. POWLING, J.. and SMITH. W. A. W.. Combustion & Flame 6, 21 (1962). 20. HALL, A, R., and PEARSON, G. S., Twe(fth Symposhml (International) on Combustioa. p. 1025. The Combustion Institute: Pittsburgh (1969). 21. ADAMS. G. K.. NEWMAN.'B. H.. and ROmNS, A. B.. Eighth Symposium (International) on Combustion. p. 693. Williams & Wilkins: Baltimore (1962). 22. ELWELL, R. B.. IRWIN. O. R.. and VAIL. R. W.. Jr.. Project SOPHY--Sotid Propellant Hazards Program, Tecb. Kept. Air Force Rocket Propulsion Lab. 67.21 I. Vol. I (August. 1967).

( R e c e i v e d D e c e m b e r , 1970)