Army Research Laboratory 

Aberdeen Proving Ground, MD 210054066 

ARL-TR-2371 November 2000 

Electrothermal-Chemical (ETC) 

Closed-Chamber Interrupted-Burning 

Tests With JA2 and M30 Solid Propellants 

Avi Birk, Miguel Del Guercio, Amy Rinkennon, Douglas E. Kboker, 

and Pamela Kaste 

Weapons and Materials Research Directorate, ARL 

Auuroved for nublic release; distribution is unlimited. 
Abstract 

Interrupted- and noninterrupted-burning tests were conducted with cyhndrical perforated 

grains of JA2 and M30 in a closed bomb with a loading density of approximately 0.2 g/cm3.. Both 

conventional black-powder and plasma igniters were used. The plasma igniter was an ablating 

capillary, and the electrical energy density was about 0.7 kJ/g of propellant. The diameters of the 

collected grains yielded the actual burn distance at the time of the interrupted burning. The 

experimental pressure traces and the conventional burn-rate coefficients of the propellants were 

used to calculate the theoretical depth burned assuming no plasma-induced burn-rate 

modifications. Overlapping pressure traces at several interrupted pressures and from comparison 

to the calculated-versus-measured burn distances indicated that there is burn-rate enhancement 

during the plasma pulse, but not much once the pulse has ended. In contrast to the JA2 burn 

rates, both ambient and cold (-20 “C) M30 burn rates, deduced from the noninterrupted tests 

using the BRLCB code, were enhanced even after the plasma turn-off, thus contradicting the 

interrupted test results. However, vivacity analysis of the noninterrupted tests indicated that the 

M30 grains exhibited increased surface area (possible fragmentation) because of the plasma 

interaction, an effect that would cause erroneous results from the BRLCB. Indeed, simulating the 

noninterrupted M30 tests using the XNOVAKTC code and assuming partial fragmentation of the 

propellant charge yielded vivacities that mimicked the experimental ones. 

ii 
Table of Contents 

Page 

1. 

2. 

2.1 

2.2 

2.3 

2.4 

2.5 

3, 

3.1 

3.2 

3.3 

4. 

5. 

List of Figures .f....*.*...*..**~..f.f..~f~**....*...~..*.~.~.ff ff.1...~******........*~~.~*..~..***.*...*.**~.~...... 

List of Tables *....*..~.*.......****...~~*. . . . . ..~+..~.*.****.*.*.....*...........~....*...*..*.**...*..**........*... 

Introduction f....**....*...f..I.~*..*****. . . .*.f..~f.~...~.*ll...****...~...** ..t..l..l..*.....*..*..*........*....* 

Experimental .*........*.~.*....ff.**...*....~~ . . . . ..~.~.+.......*.***..~*..*..................*..**.*.. . . . . . . . . . . . . . 

Test Fixture, and Procedure ............................................................................... 

Test Matrix ....................................................................................................... 

Grain Archeology.. ........................................................................................... 

Bum Distances ................................................................................................. 

Bum Rates Obtained From Nonintermpted Tests in the ETC Closed 

chamber ........................................................................................................... 

Discussion ,.....~.~...I........~... . ..~~f..*......*..*...~..f.**..~..~.~.~~.t...*.........t.1..~..~.***........~...~~ 

Bum Rates and Vivacity . ..~......*f**.********.......... . . . ..f.....**.........**....~..f..*...*.~**.*.f.f. 

Analysis of the Measured Bum Distances *.***....~~.~........~*.*.*......**..~......**......*.*~~- 

Miscellaneous Observations ~..~*.~~.~~~~..~~.~.....~...~~~.~~.~..~~~~.~.~~.~ ff...fl.l*..**..*~*~*~~.f.f.. 

Concluding Remarks .*..*..*f...ff..****..........~....I~.* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...**.. 

References.. ,. . . . . . . . . . . . . . . . . . 11.f.................~,........*.~~..**..*........~ ..~~~.~~.....~.~~~~......~~..... 

Distribution List f.*.**...*..*.f..........***....**.~.. . . ..l~.f....*****..*.....*~*..~~..~.~******.......**.....**** 

Report Documentation Page .. . ..~....~1.~...~................~.... . . . ..~...~...~.....~~.~.*...~~~.....~~~~. 

V 

vii 

1 

2 

2 

5 

8 

10 

15 

16 

16 

19 

20 

21 

25 

27 

29 

. . . m 
bl’IENlTONALLY LEFT BLANK. 

iv 
List of Figures 

Fim 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

Expansion-Chamber Arrangement .~~~.~~*.~....~.~.........~~~~.....~~.~...~.~..~.~~~~...~.~.~..~..~~~.~

Propellant Packaging .f..ff.....f......f..~ff.............t..ttt.t..tt**~.*..**.*.***...........*..*.**....*....~~~ 

Scoring Scheme Used to Identify the Packaging Position and Orientation of the 

Grains %or to Closed-Bomb Analysis . . . . ..*.........**...**..~..*~.......~~~.I.l.....................*.* 

Samples: (a) Conventionally Ignited With Black Powder and (b) Plasma-Ignited 

With Mylar-Based Capillaries ...tt~~t.....*....1.~~..~~.~..**.*...**..............~~*~...*....*..*....*....... 

Pressure Profiles of the Interrupted-Burning Tests .. ..~...........*~~~...~.....~.........~.~~*~~..~. 

Example of Calculation of Burn Distance From Known Burn Rate and 

Experimental Pressure .*.*..***..*.**.***.......*...*....*......*..*.*.*...~.............****...*.*...*.........*- 

Pressure and Power Traces for the Tests of Table 2 *.*.**...*.....*..*....~*.~.........I..*..**.... 

Bum Rates From Noninten-upted Tests in the ETC Closed chamber ..f.,.l..I..l......... 

Vivacities of the Tests Shown in Figure 8 . . . . ..~...***.*****.*......*~*~...~.~....f..1t...*1****.*t..*.. 

Synthetic Vivacities Based on XNOVAKTC Simulations . . . . ..~..*..*...f.f..*..l.........*..*.. 

Synthetic Burn Rates Using BRLCB for the Simulated Pressures From the 

XNOVAKTC . . ..~..*~*.....l..f...l..ff....f.~..**.f.ff~...~*...~..*******..*.....**....*~.~.....*....~....~..***.*.. 

Paae 

4 

4 

6 

9 

11 

13 

15 

16 

18 

18 

19 

v 
h’TENTIONALLY LEFT BLANK. 

vi 
List of Tables 

Tabk m 

1. Test Matrix .*..f.*..........*.*...*.~*****,*...~f..**...*......*....~*.....**...*...~***.*.. *.....*...**f**..........*. 7 

2. Sensitivity of Calculated Propellant Diameter Decrease to the 

Assumed Burn Rate . . . ..*.*.*.....f........~~....~*.*....~...***...*~ . . ..~~.......~~.........~.....~~...~~~...~~... 14

3. Test Arranged by CMM of the Outer Diameter ..~....~~~...~......~............~~~~......~~....~....~ 22 

vii 
hTENTIONALLY LEFT BLANK. 

. . . 

VI11 
1. Introduction 

The electrothermal-chemical (ETC) closed chamber has been popular as a research tool for 

studying the effects of plasma-propellant interaction [l-4]. Typically, the chamber is filled with 

solid-propellant grains at a loading density of about 0.2 g/cm3, and the propellant is ignitd with 

plasma that is either in the form of a jet generated by an ablating capillary external to the 

pro@ant charge [l, 21 or an exploding wire inside the pro@.lant charge [2,3,4]. Diagnostics 

include current and voltage across the capillary or wire and pressure-time history of the gas in 

the chamber. The electrical Rower and energy input into the chamber are directly calculated 

from the current and voltage measurements, but, because of heat losses, less than 75% of the 

electrical energy actually contributes to the enthalpy increase of the chamber gas [5]. The 

pressure-time history is analyzed using the U.S. Army Ballistics Research Laboratory Closed 

Bomb (BRLCB) code [6] that calculates the propellant surface regression rate versus the 

pressure. The ETC option of the BRLCB code requires the electrical power-time history input. 

A major input to the code is the geometry of the propellant grains. The code assumes that the 

instantaneous regression rate and the initial geometry uniquely determine the burning surface. 

The code output will be erroneous if the burning surface changes because of propellant fracture 

or fragmentation. 

In recent years, regression rates from the ETC closed chamber, as deduced using the 

BRLCB, have indicated ETC augmentation of the burn rate of solid propellants 11, 23. Indeed 

solid-propellant ETC guns achieved performance gains that could not be accounted for by the 

added electrical energy onl-ugmentation of the gas-generation rate had to take place. 

Because the extent of the augmentation has implications with respect to gun performance and its 

sensitivity to initial propellant temperature [7], it is important to validate the BRLCB results in a 

more direct method. This augmentation can result from a change in the inherent bum rate of the 

propellant because of the plasma interaction, or it could be caused by grain fracture or 

f@mentation that would produce an “unscheduled” increase of burning-surface area. Of course, 

for gun applications, the desired enhancement mechanism is the one that is most controllable, 

which favors the inherent burn-rate increase. But Kooker [7], using numerical simulations of 

gun and closed-bomb firings, hypothesized that most of the apparent gas-generation rates in gun 

firings and ETC closed chambers could be explained if some degree of grain fragmentation took 

1 
place. The present work is about the nature of the burn-rate augmentation, and as is shown, it 

lends credence to the grain-fracture hypothesis. 

The impetus for the present effort was the need to provide extinguished propellant grams that 

were ignited with plasma of the ablating capillary type for chemical and morphological analysis. 

Earlier results of some of these analyses were reported by Kaste et al. [8] and are not the subject 

of this paper. The authors analyze here the measured dimensional change of the extinguished 

grains and their pressure-time histories to find the plasma effect on the inherent burn rate. In 

addition, this effort extends Del Guercio’s work [l] with noninterrupted burning of ambient M30 

and JA2 to the study of cold propellants (-20 “C). Also, the validity of BRLCB-based 

derivations of the propellant burn rates is checked using vivacity analysis. Applying the BRLCB 

code, Del Guercio found that the M30 burn rate was significantly augmented, even after the 

plasma was turned off. Here, Kooker’s [7] gram-fracture hypothesis is investigated by applying 

the XNOVAKTC [9] ballistic code for simulation of partially fragmented M30 burning in the 

ETC closed chamber and then deriving synthetic vivacities from the simulated pressures. 

Indeed, the authors fmd that the synthetic vivacities track well with the experimental ones, which 

supports Kooker’s hypothesis. 

2. Experimental 

2.1 Test Fixture and Procedure. The experiments were conducted with the ETC closed 

chamber and pulse-forming network used by Del Guercio [ 11. The plasma was generated and 

sustained by a high-voltage/high-current pulse across a nickel fuse wire, placed along the inside 

of a capillary, that vaporized rapidly causing ablation and ionization of the polyethylene or 

Mylar capillary liner. The plasma duration was about 1.2 ms. The electrical pulse was generated 

with a 400 kJ capacitor-based pulse-forming network with a charging voltage of 4 kV and an 

output energy of up to 30 kJ. ; 

Closed-bomb tests of M30 and JA2 with both conventional and plasma ignitions were 

performed in a 3.8 l-cm-inside diameter (ID) ETC closed chamber with a total volume of 

129 cm’. The chamber was equipped with one or two 607~C4 Kistler pressure transducers. For 

the interrupted-burning tests, the end plug of the ETC closed chamber was replaced with a plug 

t 

2 
containing a stainless steel rupture disc. After rupturing, the disc formed a 2.54-cm opening. A 

240-L expansion chamber (a modified air-compressor tank) was interfaced to the ETC closed 

chamber via a 15-cm-long tube with a 2.5-cm-diameter blowout area. The addition of the 

interface tube increased the ETC chamber volume to 150 cm’, A picture of the expansion 

chamber arrangement is shown in Figure 1. 

The propellant charges consisted of seven-perforated grains of M30 or JA2 having a 

diameter, length, and perforation diameter of approximately 0.75, 1.5, and 0.07 cm, respectively. 

The charges consisted of either 28 grains (with a total weight of about 30 g) of temperatureconditioned propellants or 30 grains (32 g) at ambient temperature. The grams were packed 

around a plastic straw and distributed concentrically in two tiers and two rings as pictured in 

Figure 2 which also shows the charge with respect to the rupture disc. For the tests with the 30 

grams, the propellant charge was wrapped with cellophane. For the tests with the 28 grains, the 

propellant charge was encased in a Styrofoam container that weighed about 0.5 g. In the case of 

plasma ignition, the plasma was channeled through a plastic straw that was perforated to 

resemble a piccolo tube to enable uniform distribution of the plasma around the propellant. For 

conventional ignition, an electric match was used to ignite 0.6 g of black powder confined in a 

nonperforated plastic straw. The grains extinguished instantly upon sudden expansion into the 

vacuumed tank. Soft capture of the propellant was achieved with polyurethane foam inside the 

tank. The absence of bum marks on the foam confirmed the sudden extinguishment. 

Temperature conditioning of propellant charges, encased in Styrofoam containers, was done 

in a temperature conditioner cooled to -65 “C. The charge temperature could not be monitored 

during the actual test when the charge was sealed in the ETC closed chamber. Therefore, before 

testing, the warming rates of the propellant charges were calibrated using two thermocouples 

mounted on the periphery and in the middle of the charge. For each type of propellant, two 

calibration tests were conducted with the propellant charge mounted, but not sealed, in the ETC 

chamber. The warming rates were found to be repeatable, and, typically, the charge temperature 

would rise to -20 “C in about 7.5 min. At that time, the temperature difference between the 

middle of the charge and its exterior was less than 10 “C. Therefore, in order to test a charge at 

3 
Figure 1. Expansion-Chamber Arrangement. 

Figure 2. Propellant Packaging. 

4 
t 

-20 “C, the igniter had to fire for about 7.5 min from the time the charge was removed from the 

temperature conditioner. 

The goal was to track the original location of the recovered grains to assess whether this 

location might affect such factors as the quantity or location of residue deposited on the grains, 

level of decomposition or extent of burning on the grains’ surfaces, or any morphological 

features observed. A simple scoring scheme was devised, which would indicate (1) whether the 

grain was in the top tier or bottom tier, which is defmed as that closest to the igniter; (2) whether 

the gram was located in the inner or outer ring around the straw, which simulated the piccolo 

igniter; (3) the end of the grain, which faced downward (i.e., toward the igniter); and (4) the part 

of the gram that faced radially outward (i.e., away from the igniter). This was accomplished as 

shown in Figure 3, which shows the appearanck of a top tier and bottom gram as viewed end-on 

from the surface closest to the blow-out disc. A single score was indicative of top-tiered grams; 

a score through one perforation was used to mark grains adjacent to the straw (“inner” ring, 

typically 5 grains); and a score through two perforations indicated that the grain was located in 

the outer ring (typically 10 grams). Two scores indicated that the grams were located in the 

bottom tier (closest to the igniter). Scores along perforations on consecutive radii indicated that 

the grains were in the outer packing ring, while those grains with two scores along alternate radii 

indicated that the gram was packaged adjacent to the straw. The grams were positioned so that 

the scoring pointed radially outward. Scoring was achieved with a razor and was sufficiently 

deep to be evident in the recovered grains but did not induce fracture or other artifacts. 

After firing, grains were separated according to where they were recovered: remaining in the 

ETC chamber; loose in the expansion chamber in which the grams were extinguished; or 

embedded in the polyurethane foam used to line the extinguishing chamber. For each trial, a 

matrix was established in which the number of grains from each original and final position were 

recorded; 

2.2 Test Matrix. The experimental test matrix is given in Table 1. The tests are ordered 

chronologically. From test 11 on, Mylar-lined capillaries were used instead of polyethylene 
TOP TIER BOTTOM TIER 

Figure 3. Scoring Scheme Used to Identify the Packaging Position and Orientation of the 

Grains Prior to Closed-Bomb Analysis. 

because they provided somewhat more consistent power-time behavior of the plasma discharge. 

The Mylar capillaries resulted in less char deposition on the outer surface of the grams and fewer 

electrical shorts across the electrodes. Moreover, polyethylene consists only of C and H, while 

Mylar contains C, H, and 0; so the effect of ignition chemistry could possibly be compared by 

chemical analysis of the recovered grams. 

The propellants from the “BRL Research Series,” dated 1988, and propellant description 

sheets that included dimensions, compositions, and bum rates were available to the authors. 

Nevertheless, deviations were found from the described dimensions, and from test 11 on, special 

care was taken in selecting the grains for the charges. Random M30 grains were found to have 

dimensional variations of the outer diameter, with a standard deviation of 0.08 mm. The JA2 

diameters were better behaved. The standard deviation of the selected grains was about 

0.03 mm. The standard deviations of the grains’ lengths were above 0.1 mm; hence the lengths 

were not analyzed. For both propellants, the diameters of the inner perforations were found to be 

different from those of the outer perforations. Nominal bum rates were initially measured using 

a 500-cm3 closed bomb optimized for this purpose. For each propellant, two tests were 

conducted, and the bum rates were averaged. These are the “nominal bum rates” used to 

calculate the diameter change in Table 1. The nominal rates for JA2 were very close to the ones 

in the Research Series; the M30 rates were somewhat different. The closed-chamber method 

(based on BRLCB) does not give reliable bum rates below 30 MPa. For low pressures, bum-rate 

6 
Table 1. Test Matrix 

Meamd-Outer 

Tmpchnt-ignitim meth~prqtellant temprature: A = 22 k 2 ‘c C = -20 f 5 T. 

%sed cm the nominal burn rates: BR (an/s) = 0.2195 * P(!vIP~)~* for M30. 

BR (cm/s) = 0.1379 * P(IvIP~)~” for JA2. 

TIKHTI Miller’s [lo] srrand burner far JA2 (l-10 MPa): 

%a& m the EK ehamkr. test 6119SlP: 

BR (cm/s) = 0.2745 * P(MPa)as30p. 

BR(cm/s) = 0.2525 l P(MPa)a’slg. 

“Based cm the El’C chamber. test 6049SlP: BR(cm,k) = 0.2678 l P(MPa)‘ntn. 

7 
Table 1. Test Matrix (continued) 

102.6 (0.075,0.035,0.075) m Mylar 

27 NUO-ETC/C 6169s 1P 0.116, (0. 13‘Id) 0.222,0.2%, 0.227 

(0.040,0.013,0.030) (ii) 

26, 1.3 27.2 

35.1 (0.131”) MY~ 

‘Prqdlant-igniti~ method&opllant temperature: A = 22 k 2 T, C = -20 f 5 T. 

bsasd cm the ncuninal bum rates: BR (cm/s) = 0.2195 * P(MPa)a7Bm for M30. 

BR (cd.) = 0.1379 * P(MPa)aw12 for JA2. 

Trcnn Miller’s [lo] stnu~~I burner for JA2 (l-10 MPa): BR (cm/s) = 0.2745 * P(h@a)-. 

dsased on the EK chamber. test 6119SlP: BR(un/s) = 0.2525 * P@iPa)a7s1g. 

%sed an the ETC chamber, test 6049SlP: BR(cm/s) = 0.2678 * P(MPa)afM. 

data f/ram strand-burner tests were considered to be more reliable. For JA2, Miller [lo] has 

obtained sub-10 MPa data that are significantly different from our “‘nominal” data. Calculated 

diameter changes, based on Miller’s burn-rate coefficients, are given for the low-pressure JA2 

(test numbers 8 and 10 in Table I) to compare to the changes based on the nominal coeflicients. 

For the interrupted tests, the peak pressure obtained depended on the rupture disc used. 

Rupture discs with nominal burst pressures of 35 and 70 MPa were used in a single or doublemount configuration. The electrical parameters in Table 1 were intended to be held constant, but 

the repeatability of the peak power, pulse shape, and pulse width was less than desired. 

l 

I 

2.3 Grain Archeology. In the following discussion, the top tier refers to the 15 grains 

closest to the rupture disc, and the bottom tier refers to the 15 grains closest to the igniter. The 

closed bomb is referred to as bomb, and chamber refers to the expansion chamber used to 

interrupt the burning. 

8 
More grains were recovered when extinguished at 35 MPa than at 60-70 MPa. At 35 MPa, 

grains remaining in the bomb were primarily from the bottom outer region; grains from the 

middle all passed through the bomb and entered the chamber and the foam, whether from the top 

or bottom tiers. At higher pressures, only the bottom outer grains were found in the bomb. 

The grains recovered from the plasma-ignited samples were visibly different from the 

conventionally ignited samples. In the case of plasma-ignited samples, the top were light cream 

in color, and the bottom were covered with a black residue. The residue was less dense for the 

Mylar-based capillary samples than for the polyethylene-based capillary samples. The residue 

appeared mainly on the grains originally located in the bottom tier, nearest the igniter. 

Conventionally ignited samples (35 MPa) also exhibited some residue, presumably from the 

black-powder ignition, but the coating was less dense than for the plasma case, and it was mainly 

limited to the bottom middle grains. The color variations persisted into the higher pressures. 

The white virgin M30 grains turned to a golden-brown color in the extinguished samples 

(75 MPa, Figure 4a) from the conventional ignition. In contrast, most recovered grains from the 

plasma ignition were blackened (100 MPa, Figure 4b). However, the recovered plasma-ignited 

M30 grains that were shielded from the direct blast of the plasma had a light cream color. The 

reasons for the color variations are not understood at this time. 

Figure 4. Samples: (a) Conventionally Ignited With Black Powder and (b) Plasma-Ignited 

With Mylar-Based Capillaries. 
The top tier grains were all found in either the chamber or the foam. At 35 MPa, those found 

in the foam were virtually the same color as the virgin material except for areas that had a light 

coating of black powder residue. In contrast, grains from the same firing but originahy located 

in the outer ring of the bottom tier and recovered in the closed bomb were distinctly yellowed. 

At 35 MPa, the number of grains remaining in the bomb were comparable for plasma- and 

black-powder-ignited samples and mainly originated from the bottom tier, middIe section. For 

plasma-ignited samples, these grains had the greatest level of residue from the capillary. 

At 62-78 MPa, fewer grains (about 20 of the 30 originally packaged) were recovered for 

either conventionally or plasma-ignited samples. Most of the recovered grains were from the 

bottom tier and found in the chamber (not foam). The unrecovered grains primarily originated 

from the top tier; the top tier grains that were recovered were mainly found in the foam. At this 

pressure, in most cases no grains remained in the bomb for either ignition system although 

occasionahy one remains. For plasma-ignited samples at 70-78 MPa, most of the top-tier outer 

ring samples were driven into the foam. In general, most samples embedded in the foam were 

lighter in color (i.e., not as golden brown) than those found elsewhere in the chamber. In 

contrast, very few bottom-tier samples were found in the foam, and they tended to be blackened 

from the plasma residue. 

For 100 MPa and ambient temperature, only plasma-ignited trials were performed. There 

was little recovery from the top tier of grains and none of the middle samples (top or bottom 

tier). Only bottom, outer grains were found (9 of 10 original), and these were found in the 

chamber. No samples remained in the bomb at 100 MPa. 

2.4 Burn Distances. For the same rupture pressure, burning occurred to a greater extent in 

the samples ignited with black powder than with the plasma; with the plasma-ignited samples, a 

significant fraction of the total pressure was due to plasma injection rather than propellant 

burning. This initial plasma pressurization occurs rapidly, leaving a shorter effective time for 

propellant burning to occur when compared to conventional ignition (see Figure 5). This 

occurred whether Mylar or polyethylene capillaries were used in the plasma ignition. This is 

reflected by the extent of grain regression. For example, for M30 grains recovered nominally at 

10 
110.0 

100.0 

90.0 

80.0 

70.0 

60.0 

50.0 

40.0 

30.0 

20.0 

10.0 

0.0 

file-prop:Hant-plasma dsrgy in KJ-pdk power in Md 

---me-- 

.-.-.-.- 

. ..-. 

,-,-,-~- 

--m-,.-m 

0 1 5 6 

Time, ms 

7 a 9 IO 11 

Figure 5. Pressure Profiles of the Interrupted-Burning Tests. 
35 and 60 MPa, the grain regression measured for plasma-ignited samples (Mylar capillaries) 

was 0.23 and 0.33 mm, while for conventionally ignited samples, the regression was greater at 

0.37 and 0.62 mm, respectively. 

The burn distances of the extinguished grains were measured using calipers for the outer 

diameter and drill bits to gauge the size of the perforations. The outer-diameter burn distances 

were emphasized more because the burning in the perforations, though their diameters were still 

small, might have been influenced by erosive flow; the M30 grains were slightly banana-shaped; 

the set of drill bits had uneven and relatively large size increments; and the drill bits could not 

always be passed through the perforations. The diameter of each collected propellant grain was 

measured, and the measurements were averaged. Not all of the grains were found intact. The 

authors did not find sign&ant burn-distance variances with respect to the packaging locations. 

The measured burn distances were compared to theoretical distances calculated from the 

experimental pressure curves. The method of calculation is demonstrated in Figure 6, where the 

“nominal” burn-rate coefficients (a, n) are used. If the calculated burn distances are less than 

0.08 mm from the measured ones, the authors consider that there is no burn-rate enhancement 

due to the plasma interaction. If the burn rate has been enhanced, then the calculated burn 

distance is smaller than the measured burn distance. Figure 6 also demonstrates how to obtain 

post-plasma burn distances. The figure considers identical propellants fired under similar plasma 

peak powers and energies for which the pressure-time histories were comparable, with the 

exception that they were extinguished at two different pressures. Typically, during the plasma 

pulse, the pressure would rise to about 30 MPa; therefore, comparing calculated regression with 

the measured propellant regression between two pressures beyond 30 MPa yields the extent of 

the intrinsic post-ETC burn-rate augmentation. This method circumvents the uncertainty 

regarding the burn-rate coefficients at pressures below 30 MPa. 

The sensitivity of the burn-distance calculations to the assumed burn rates is demonstrated in 

Table 2 for the three pairs of tests shown in Figure 7. Unfortunately, the experimental power 

pulse shapes were somewhat variable, and it was a matter of serendipity to obtain both 

overlapping pressure and power traces. The two electrical power-time histories for the JA2 pair 

12 
110.0 

100.0 

90.0 

80.0 

70.0 

60.0 

50.0 

40.0 

30.0 

20.0 

10.0 

0.0 

t I I , I I , 

B Calculated diameter change due 

f 

to burning: 3 --- - 

/ 

I DC(t) =2*j a * P(t) ” dt 

Ir 

0 

I For JA2: a = 0.1379, n = 0.9412 

i 

Al extinguishment: 

DC(A) = 0.08i mm ; DC(B) = 0.526 mm 

i Post ETC diameter change: i I DC(B) - DC(A) = 0.445 mm 

0 1 5 6 

Time, (t), ms 

7 8 9 10 11 

Figure 6. Example of Calculation of Burn Distance From Known Burn Rate and Experimental Pressure. 
Table 2. Sensitivity of Calculated Propellant Diameter Decrease to the Assumed Burn Rate 

Based on ETC 

cbamber cow. Based on 

Nominal Based 

on soo-cm3 

Ignition Test soo-cm3 Closed 

6119SlP 

Conventional Measured Closed Chamber (7G250 MPa) 

Ignition M30 Test (std dev) 0.2195 * P”.78m 0.2524 * PO.7519 

(lEZ9 

From Propellant 

Data Sheet 

0.2454 * PO.7 ’ 0.2856 * PO.717 

b-w 

5199SlP 0.367 (0.097) 0.390 0.422 0.416 0.440 

5209SlP 0.618 (0.105) 0.631 0.674 0.661 0.702 

Difference 0.251 0.241 0.252 0.245 0.262 

- 0.251 - 0.241= 0.010 - - - 

Based on ETC Based on ETC 

Char&r Cow. Chunk-ETC 

Nominal Based 

on SOO-cm3 

IgnitionTest Ignition Test 

6119SlP 6109SlP From Propellant 

Plasma Ignition Measured Closed Chamber Data Sheet 

M30 Test (std dev) 0.219s * Po.78o3 0.2524 * po.751 

(70-250 MPa& (;ygSfpy$> 

. 0.2856 * PO.717 

(mn.0 

5219SlP 0.760 

(O*W 

0.666 0.694 0.978 0.684 

5139SlP 0.230 (0.W 0.167 0.177 0.314 0.183 

Difference 0.530 0.499 0.517 0.664 0.501 

- 0.530 - 0.499 = 0.031 0.530 - 0.664 = -0.134 - 

Nominal Based 

on SOO-cm3 From Propellant 

Plasma Ignition Measured Closed Chamber - - Data Sheet 

JA2 Test (std dev) 0,1379 * Po.9412 0.133 * po.9s 

(nrm) 

9298S2P 0.70 (0.071) 0.526 - - 0.514 

9298SlP 0.183 0.081 - - (0.039) 0.078 

Difference 0.517 0.445 - - 0.436 

- 0.517 - 0.445 = 0.062 - - - 

14 
90.0 

m 70.0 

ii 

i 60.0 

i 0 50.0 

lz 

40.0 

30.0 

20.0 

10.0 

0 1 2 3 4 5 6 7 8 9 10 11 

Time, ms 

Figure 7. Pressure and Power Traces for the Tests of Table 2. 

of tests did not overlap as well as the M30 pair of tests. Therefore, although the JA2 pressure 

traces overlap well, the calculated post-ETC burn distance is less reliable for the JA2 than for the 

M30. In Table 2, for each pair of tests, the post-ETC regression between the peak pressures is 

calculated as described in Figure 5. The differences between the measured and calculated postETC regressions (e.g., 0.03 1 mm for M30) are remarkably small--an indication of minimal postETC burn-rate enhancement. Table 2 also includes calculations based on burr-rate coefficients 

obtained (using BRLCB) from noninterrupted tests in the ETC chamber (Figure 8). When 

coefficients from an M30 ETC test are used, the post-ETC difference between measured and 

calculated for M30 is too negative (-0.134 mm). Obviously, the BRLCB burn-rate deduction for 

the M30 ETC test is erroneous. 

2.5 Burn Rates Obtained From Noninterrupted Tests in the ETC Closed Chamber. 

The burn-rate plots based on the BRLCB code were “wiggly.” In the range of 75 to 250 MPa 

(which is post ETC), when plotted on log-log scale, it became apparent that the plots could be 

curve-fit to straight lines. The results of the curve fittings are shown in Figure 8. The results for 

15 
--...-.,l. M30-ETC (-20 C), (608981P) 

‘-*-‘-*- M3&EK (22 C), (6109SlP) 

- MB&Conv (-2OC), (6049Sl P) 

------. M304onv(22 C), (6119SlP) 

--- JA&Canv (-20 C), (5289$2P) 

- -- - . J/t-ETC (-20 C), (528991 P) 

electrical energies input into the BRLCB ate 75% - 

of those measured across the capillaries 

7 8 2 ld 2 3 

Pressure, MPa 

Figure 8. Burn Rates From Noninterrupted Tests in the ETC Closed Chamber. 

the conventional ignition were close to the results from the 500-cm3 closed bomb (see Table 2). 

As expected, ambient M30 burned faster than cold propellant. There was only minor burn rate 

augmentation for the plasma-ignited JA2, and even this augmentation would disappear if the 

assumed electrical energy transfer were less than 75%. More importantly, the slopes of the ETC 

JA2 and conventional JA2 bum-rate curves are virtually parallel. In contrast, the M30 ETC bum 

rates are markedly enhanced, and the slopes are significantly smaller than those of the 

conventional M30. As already suggested, the ETC M30 bum-rate curves are inconsistent with 

the measured grain regression and may be erroneous. This is elaborated upon in section 3. 

3. Discussion 

3.1 Burn Rates and Vivacity. The technique for calculating vivacity, as applied to closedchamber pressure data, is well established [l 1 J. Ignoring scaling factors, the vivacity is 

popularly defined as the instantaneous time derivative of the experimental pressure divided by 

that pressure. For a certain grain geometry, the plot of the vivacity versus the instantaneous 

pressure yields a curve with distinct characteristics. For example, a seven-perforated grain like 

16 
the test propellant should yield a curve that is linear and has a slight positive slope. (The curve is 

not linear during the ignition/flame-spreading phase and during the final slivering of the grain.) 

A more energetic propellant (e.g., JA2 versus M30) would yield a steeper curve. The curve has a 

positive slope if the burning-surface area is progressive and a negative slope if it is regressive. 

The BBLCB gives reliable burn rates only if the vivacity curve behaves as expected. It is 

unlikely that changes in the burning-rate pressure indexes (or slopes in Figure 8) are real if the 

vivacity curves imply unusual changes in burning-surface area. The experimental pressure itself 

can not reveal much about the burning-surface trends. The contradiction between the measured 

burn distances and the ones calculated based on the BRLCB results for the ETC M30 led us to 

plotting the vivacities of the Figure 8 tests. The plots are shown in Figure 9. 

Interestingly, the vivacity curves are as expected for all tests save the plasma-ignition tests of 

the M30 propellant. The experimental vivacities for the M30 ETC tests indicate a regressive 

burning-surface area. The immediate suspicion is that the more brittle M30 propellant (as 

compared to JA2) breaks apart to some extent because of the plasma impact, and a few of the 

M30 grains break into fragments. In order to prove this suspicion, the authors decided to 

simulate the ETC closed-chamber tests using the XNOVAKTC ballistic code. Input to the code 

included the “nominal” burn rates (used in Table 2) and simplified experimental power-time 

histories. The M30 propellant-grain geometry, as well as a combination of “fines” (small 

spheres, as coined by Kooker [7]) and seven-perforated grains, was inputted. Only 3 or 6 grams 

out of 28 were converted into fines. The simulated pressure plots from the XNOVAKTC were 

then processed into synthetic vivacity curves, These curves are shown in Figure 10. The 

simulated pressures and the synthetic vivacities have somewhat higher values than the 

experimental ones, but this is mostly because the heat loss routine in the XNOVAKTC 

underestimates the actual heat loss in the ETC closed chamber. As can be seen, the pressure 

plots for the conventional versus ETC are markedly different, but the vivacities are quite similar. 

This is because the vivacity is a measure of the burning-surface geometry. For the case without 

fines, the ETC vivacities are slightly lower compared to the conventional vivacities. This is just 

like what was obtained experimentally for JA2 (Figure 9). The synthetic vivacities trend like the 

experimental M30 vivacities if fmes are involved. Therefore, the conclusion was that the ETC 

M30 experiments involved surface area changes similar to the creation of fines. 

17 
350.0 

262.5 

VI 

: 

n- 

- 175.0 

67.5 

0.0 

49s.1 P! 

(-20 C), (dO99Sl P)’ 

Wonv (22 C), (6119Sl P) 

MBO-ETC (22 C), 61 OQSI ---.. PI , 

!-ETC (-20 C), (5299W P) 

. . . . 0.015 0 020 .+ JAP-Conv (-20 C), (5289S2P) 

0.2 0.3 0.4 0.5 0.6 0.7 

P I Pmax 

Figure 9. Vivacities of the Tests Shown in Figure 8. 

400 

360 

300 

250 

E 

n- 200 

5 

150 

100 

50 

o( 

28 grains (30.5 9 ) of 7-pert M30 (BRL Research Series): 

L= 15.6 mm, D= 7.67 mm, d= 0.60 mm 

FINES: Spheres with 0 = 1.58 mm 

1 Grain = I .06964 g tr 

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

PIPmax 

Figure 10. Synthetic Vivacities Based on XNOVAKTC Simulations. 

18 
The simulated pressures shown in Figure 10 were input into the BRLCB in order to obtain 

synthetic burn-rate curves for comparison with the experimentally deduced burn rates shown in 

Figure 8. The burn rates are given in Figure 11.. Indeed, the synthetic burn rates, which include 

a small number of fines, bracket closely the M30-ETC-ambient experimental burn rate. This 

lends additional support to the propellant-fracture hypothesis. 

f 

28 grains (30.5 g ) of 7-perf M30 (BAL Research Series): 

L= 15.6 mm, D- 7.67 mm, cb 0.60 mm 

FINES: Spheres with D - 1.66 mm 

1 Grain I 1.06664 g 

I 

7 8 9 102 2 

Pressure, MPa 

Figure 11. Synthetic Burn Rates Using BRLCB for the Simulated Pressures From the 

XNOVAKTC. 

3.2 Analysis of the Measured Burn Distances. Now that it has been established that, for 

our test conditions, there is no significant post-plasma burn-rate enhancement,here remains the 

question of whether there is enhancement during the plasma pulse. For this purpose, the test data 

of Table 2 were recorded according to “calculated minus measured” (CMM) change of the outer 

diameter and tabulated in Table 3. First, it is apparenthat the conventional CMM is very small 

and positive while all of the ETC CMMs are negative. The fact that the conventional CMMs are 

so small bears out the choice for the nominal burn rates assumed in the calculations. The actual 

19 
ignition of all the grains in the propellant charge is not instantaneous as is assumed in the 

calculations of the burn distances. If delayed ignition were factored into the calculations, the 

calculated burn distances would be smaller, and the CMM values would be larger negative 

numbers falling outside the standard deviation range of the measurements. A negative CMM 

larger than the standard variation clearly indicates burn-rate enhancement. To investigate 

burn-rate enhancement during the plasma pulse, we restrict attention to the tests that were 

interrupted soon after the termination of the plasma (i.e., to peak pressures below 50 MPa). For 

these tests, the CMM was more negative when the peak power and energy were higher. This is 

another indication of burn-rate enhancement during the plasma event. 

Concentrating on the tests with peak pressures in excess of 50 MPa, we found that the CMMs 

did not become progressively more negative with progressively higher blowout pressures. This 

indicates that there is no post-ETC burn-rate enhancement. 

3.3 Miscellaneous Observations. The number of grains collected after each test depended 

on the propellant and the peak pressure. Because M30 is more brittle than JA2, more JA2 grains 

were collected than M30 grains. Because of higher forces, fewer grains were collected at higher 

peak pressures. Of particular interest is the number of grains collected at the low peak pressures. 

At these pressures, the grains are less likely to break during the flow into the 

expansion/collection chamber. It appeared that fewer ETC M30 grains were collected than 

conventional M30 grains. For example, test 16 in Table 2 is conventionally fired M30; it 

reached a peak pressure of 36.1 MPa, and 28 grains out of the original 30 were collected intact. 

Test 14 is ETC-fired M30; it reached a peak pressure of 34.4 MPa, and 26 grains out 30 were 

collected. Bearing in mind that the measured burn distance of test 16 is 0.390 mm while the 

measured burn distance of test 14 is 0.135 mm, the grains of test 16 should have been more 

fragile than those of test 14 (because of their thinner webs). Yet, more grains were collected in 

test 16 than in test 14. Also, for the low peak pressures, the number of M30 ETC grains tends to 

correlate with the peak powerdewer grains were collected at higher peak powers. The above 

observations are, admittedly, statistically weak, more experiments are needed. Nevertheless, the 

observations buttress the ETC grain-fracture hypothesis. 

20 

f 
The capillary material’s effect on the bum-rate enhancement could not be determined from 

the present data. A measure of the ETC burn-rate enhancement is the percent inmase of actual 

burn distance from the calculated burn distance, as given in the right column of Table 3. As the 

column indicates, soon after the plasma pulse was turned off (ie., peak pressures below 

40 MPa), the enhancement was in the range of 40 to 126%. The fact that the enhancement did 

not grow progressively with time (and pressure) indicates that little enhancement occurred after 

the plasma was turned off. 

Resources limited the authors to only a few tests with the cold-conditioned grains. Because 

of increased propellant brittleness at the cold temperature, fewer grains were collected than in the 

ambient tests with comparable peak pressures. Considering that the propcllant-fracture 

hypothesis is more relevant to brittle propellants, more burn-rate enhancement was expected with 

cold M30 ETC than with ambient M30 ETC. This did not occur (see Figure 8); perhaps the 

reason is that lower peak power was obtained in the cold test (see test 22 vs. test 23 in Table 2). 

4. Concluding Remarks 

The purpose of this effort was to determine the extent that solid-propellant burning rate is 

modified when interacted with plasma. For this purpose, an ETC closed chamber was used 

The ETC closed-bomb tests employed standard M30 and JA2 propellants having common 

cylindrical seven-perforated geometries. The plasma was hydrocarbon-based in a form of a jet 

emanating from a capillary. The electrical density was rather high, about 0.7 kJ/g of propellant. 

The tests included interrupted and noninterrupted burning, conventional (black-powder) ignition 

and .plasma ignition, and ambient and cold propellants. Test data were in the form of pulse 

power and pressure traces and measurements of burn distances on extinguished propellants. 

Data analysis invoked burn-distance calculations, burn-rate deductions using the BRLCB code, 

vivacity plots, and ballistic simulations using the XNOVAKTC code. Based on the analysis, the 

following was found: 

l There is sign&ant (-120%) burn-rate enhancement during the plasma pulse. 

21 
Table 3. Tests Arranged by CMM of the Outer Diameter 

9118S2P JA2-ETC 71 28.1 28 1.2 Polyeaykne -0.049 22.2 

6159SlP JA2-ETC 102.6 25.9 29 1.3 Mylar -0.063 8.4 

9298SlP JA2-EI’C 38 28.4 25 1.3 Polyethylene - 0.102 126 

10018SlP JA2-ETC 35 20 30 0.7 Polyerhylene -0.113 67.7 

9298S2P JA2-ETC 93 29.4 25 1.3 Polyethylene -0.174 33.1 

l There is no significant bum-rate enhancement in the absence of plasma. 

l The plasma-propellant interaction can cause surface-area increase consistent with the 

hypothesis of grain fracture. 

l The above applies also to cold propellants (-20 “C). 

22 
The conclusion regarding the fracturing of the grains is not based on truly hard evidence; 

rather it is based on careful analysis of the data. Mechanisms for the burn-rate enhancement 

during the plasma interaction will be suggested after the chemical and morphological analysis of 

the extinguished grains, Based on the fmdings, burn-rate enhancement should be incorporated 

during the plasma pulse and/or partial charge fracture in numerical simulations of experimental 

ETC gun firings. 

23 
hlENTIONALLY~BLANK. 
5. References 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

Del Guercio, M. “Propellant Burn Rate Modification by Plasma Injection.” 34th JANZVAF 

Combustion Subcommittee Meeting, pp. 3-2, West Palm Beach, FL, 1997. 

Woodley, C. R., and S. Fuller. “Apparent Enhauced Burn Rates of Solid Propellants Due to 

Plasmas.” 16th International Symposium on Ballistics, pp. 153-162, San Francisco, CA, 

1996. 

Proud, W. G., and N. K. Bourne. ‘The Electrothermal Enhancement of ProI&lant Burning 

by Plasma Injection.” Propellants, Explosives & Pyrotechnics, ~0122, pp. 212217, 1997. 

Haak, I-L K, A. M. Voronov, and T. H. G. G. Weise. ‘The Interaction of Electrothermally 

SuppIied Energy With Compact Solid Propellants.” EML Symposium, Edinburgh, Scotland, 

1998. 

Oberle, W. F., and G. P. Wren. “Radiative and Convective Heat Loss in ElectrothermalChemical (ETC) Closed Chambers.” 35th JAMVAF Combwtion Subcommittee Meeting, 

pp. 22%236, Tucson, AZ, 1998. 

Oberle, W. F., and D. Kooker. ‘BRLCB: A Closed-Chamber Data Analysis Program.” 

ARL-TR-36, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD, January 

1993. 

Kooker, D. E. “Burning Rate Deduced From ETC Closed-Chamkr Experiments: 

Implications for Temperature Sensitivity of Gun Systems.” 35th JMAF Combustion 

Subcomatee Meeting, pp. 201-217, Tucson, AZ, 1998. 

Kaste, P. J., A. E. Kinkennon, R. A. Pesce-Rodriguez, M. A. Del Guercio, D. D. Devynck, 

A. Birk, S. L. Howard, and M. A. Schroeder. “Chemical Analysis of Extinguished Solid 

Propellants From an Interrupted Closed Bomb With Plasma Igniter.” 35th JANNAF 

Combustion Subcommittee Meeting, Tucson, AZ, 1998. 

Gough, P. S. ‘The XNOVAKTC Code.” BRL-CR627, U.S. Army BaIlistics Research 

bboratory, Aberdeen Proving Ground, MD, February 1990. 

10. Miller, M. Personal communication. U.S. Army Research Laboratory, Aberdeen Proving 

Ground, MD, 1999. 

11. Klingaman K W., and J. K Domen. “The Role of Vivacity in Closed Vessel Analysis.” 

JAhWAF Propellant Development and Characterization Subcommittee Meeting, Patrick 

AFB, FL, 1994. 

25 
J 

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28 
REPORT DOCUMENTATION ‘PAGE Fom, Approved 

OMB No. 0704-0188 

Eletrothermal-Chemical (ETC) Closed-Chamber Interrupted-Burning Tests With 

JA2 and M30 Solid Propellants 

Avi Birk, Miguel Del Guercio, Amy Kinkennon, Douglas E. Kooker, and 

Pamela Kaste 

U.S. Army Research Laboratory 

ATT-N: AMSRL-WM-BE 

Aberdeen Provin 

REPORT NUMBER 

ARL-TR-237 1 

12a. DISTRIBUllONIAVAllAE4Ll~ STATEMENT 12b. DISTRIBUTION CODE 

Approved for public release; distribution is unlimited. 

13. ABSTRACT(Maximum 200 words) 

Interrupted- and noninterrupted-burning tests were conducted with cylindrical perforated grains of JA2 and 

M30 in a closed bomb with a loading density of approximately 0.2 g/cm’. Both conventional black-powder and plasm 

igniters were used. The plasma igniter was an ablating capillary, and the electrical energy density was about 0.7 kJ/g ol 

propellant. The diameters of the collected grains yielded the actual burn distance at the time of the interrupted burning, 

The experimental pressure traces and the conventional bum-rate coefficients of the propellants were used to calculate 

the theoretical depth burned assuming no plasma-induced burn-rate modifications. Overlapping pressure traces ai 

several interrupted pressures and from comparison to the calculated-versus-measured burn distances indicated that there 

is burn-rate enhancement during the plasma pulse, but not much once the pulse has ended. In contrast to the JA2 burr 

rates, both ambient and cold (-20 “C) M30 burn rates, deduced from the noninterrupted tests using the BRLCB code, 

were enhanced even after the plasma turn-off, thus contradicting the interrupted test results. However, vivacity 

malysis of the noninterrupted tests indicated that the M30 grains exhibited increased surface area (possible 

~agmentation) because of the plasma interaction, an effect that would cause erroneous results from the BRLCB. Indeed 

;imulatittg the noninterrupted M30 tests using the XNOVAKTC code and assuming partial fragmentation of the 

Jropellant charge yielded vivacities that mimicked the experimental ones. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

32 

:losed bomb, ETC, interrupted burning, M30 propellant, JA2 propellant, vivacity, 16. PRICE CODE 

>um rate 

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATlON IQ. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT 

OF REPORT OF THIS PAGE OF ABSTRACT 

UNCLASSIFED UNCLASSIFIED UNCLASSIFIED UL 

NSN 754041-280-5500 C,lrdl,rl E.-.- Me IDaw ‘) en\ 

29 “.LA..“YI” 1 “ll &I” \‘\“. L-0, Prescribed by ANSI Std. 23946 298-I 02 
INlENllONAUYLEFTBUNK. 

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