Dynamic Pressure Testing of a 155-mm Howitzer Ballistic 

Window Using a Closed Bomb Vessel

by John J. Ritter, Barrie E. Homan, and William Thalman

ARL-TR-5266 August 2010

Approved for public release; distribution unlimited. 
NOTICES

Disclaimers

The findings in this report are not to be construed as an official Department of the Army position 

unless so designated by other authorized documents.

Citation of manufacturer’s or trade names does not constitute an official endorsement or 

approval of the use thereof.

Destroy this report when it is no longer needed. Do not return it to the originator.
Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5066

ARL-TR-5266 August 2010

Dynamic Pressure Testing of a 155-mm Howitzer Ballistic 

Window Using a Closed Bomb Vessel

John J. Ritter and Barrie E. Homan

Weapons and Materials Research Directorate, ARL

and

William Thalman

Armament Research, Development and Engineering Center

Picatinny Arsenal, NJ 07806

Approved for public release; distribution unlimited. 
ii

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid 

OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

August 2010

2. REPORT TYPE

Final

3. DATES COVERED (From - To)

September 2008 to August 2009

4. TITLE AND SUBTITLE

Dynamic Pressure Testing of a 155-mm Howitzer Ballistic Window Using a 

Closed Bomb Vessel

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

John J. Ritter, Barrie E. Homan, and William Thalman

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Research Laboratory

ATTN: RDRL-WMS

Aberdeen Proving Ground, MD 21005-5066

8. PERFORMING ORGANIZATION

 REPORT NUMBER

ARL-TR-5266

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution unlimited. 

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The U.S. Army Research Laboratory (ARL) was tasked by the Armament Research, Development and Engineering Center 

(ARDEC) to dynamically pressure test a chamber window design that is required to withstand dynamic loads up to 63.3 ksi. 

The window is a part of a proposed system to replace the chemical primer with a Diode Pumped Laser Ignition System (DPLIS) 

for use on the M777 LW155 towed howitzer. A modified closed bomb vessel, manufactured by Harwood Engineering and 

initially designed for propellant characterization, is used to simulate a launch environment and place the window under the 

requisite dynamic loading. The propellant used for testing was MACS M232A1. While the closed bomb is primarily used for 

obtaining propellant burning rates, this report is intended to illustrate the versatility of the closed bomb for use in 

nonconventional testing.

15. SUBJECT TERMS

Closed Bomb, Ballistic Window, Laser Ignition, Pressure Test

16. SECURITY CLASSIFICATION OF: 

17. LIMITATION

 OF 

 ABSTRACT

UU

18. NUMBER

 OF 

 PAGES

24

19a. NAME OF RESPONSIBLE PERSON

John J. Ritter

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified

19b. TELEPHONE NUMBER (Include area code)

(410) 306-0667

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18
iii

Contents

List of Figures iv

List of Tables iv

1. Introduction 1

2. Background 1

3. Equipment Setup 2

4. Calculating Charge Weights and Volume 4

5. Test 5

6. Results 10

7. Conclusions 11

8. References 13

Appendix. Modified Closed Bomb Drawings 15

Distribution List 22
iv

List of Figures

Figure 1. Double-ended closed bomb apparatus. ........................................................................... 3

Figure 2. (a) Test window and (b) exploded view of chamber head and window. ........................ 3

Figure 3. Modified closure cap/steel slug insert for closed bomb volume reduction. ................... 4

Figure 4. Failures of the window.................................................................................................... 7

Figure 5. Vent valve failure, showing a) erosion around vent valve and b) erosion and 

elongation of closed bomb valve seat........................................................................................ 9

Figure 6. Pressure-time trace of closed bomb event. ................................................................... 11

Figure A-1. The steel slug insert. .................................................................................................. 15

Figure A-2. The modified closure (head)..................................................................................... 16

Figure A-3. The modified face plate for closure. .......................................................................... 17

List of Tables

Table 1. Peak pressure data. ........................................................................................................... 8
1

1. Introduction

Closed bomb vessels have been incorporated for many years, primarily to characterize the linear 

burning rate of propellants (1). However, being a high-pressure test vessel, such vessels can 

also be used to simulate the pressures experienced in a gun barrel while accurately measuring 

the pressure within the vessel. The main components of the closed bomb are the thick-walled 

steel vessel, which contains the event; the propellant to produce the pressure; a squib, which 

consists of an electric match and a small quantity of black powder to ignite the propellant; a 

pressure gauge to measure the chamber pressure in the vessel, and a data acquisition system to 

record the data.

Since the closed bomb is nothing more than a high-pressure vessel, it can be modified for a 

variety of uses. One such use, which was tasked to the U.S. Army Research Laboratory (ARL) 

by the Armament Research, Development and Engineering Center (ARDEC), was to prove the 

survivability of ballistic window designs intended for use in laser ignition. The purpose of the 

window is to replace the chemical primer system on a M777 LW155 towed howitzer with a 

laser ignition system; therefore, the window is required to survive a high-pressure environment. 

In order to accomplish this, we performed slight modifications on an existing closed bomb in 

inventory such that it would accept the window housing. We then incorporated the M232A1 

MACS propellant in order to generate the requisite pressures in the bomb. 

2. Background

The chamber window being tested is used to protect and provide an optical path for the laser 

head of the Diode Pumped Laser Ignition System (DPLIS), which is a proposed replacement for 

chemical primers on the M777 LW155 towed howitzer. The original chamber window,

developed under the Crusader and Future Combat Systems programs, was structurally robust 

but required a specialized cleaning system, which is unavailable on the towed platform.

Improved windows were designed in conjunction with finite element analysis software in an 

effort to improve ease of cleaning by the gun crew. For economical purposes, closed bomb 

pressure testing was performed to simulate the live-fire environment of the gun chamber and 

determine the new windows’ survivability. This included using a closed bomb apparatus and a 

hydraulic drop hammer to generate appropriate pressure-time profiles. A number of designs 

have been proposed and tested with varying degrees of success. 
2

The window needed to meet a requirement of surviving pressures up to 63.3 ksi for 55 shot 

cycles in order to be qualified. Meeting this test qualification would make the window’s design 

acceptable to go forward with gun testing where maximum pressures seen are 61.3 ksi.

Testing of optical, ballistic windows is not a particularly novel idea; however, previous tests 

used a different apparatus (2). The main difference between past and present testing is that the 

previous test apparatus was designed with a controlled leak to ensure the window experiences 

peak pressure for only tens of milliseconds. Previous conclusions indicate a critical 

characteristic of ballistic window design is how they are assembled and seated into their 

housing.

3. Equipment Setup

The equipment used for testing consisted of a Harwood Engineering double-ended closed bomb 

vessel (figure 1), a Kistler Type 607C high pressure sensor, a Kistler model 5100B1 dual-mode 

charge amplifier, fffg black powder, electric matches, M232A1 MACS propellant, and the 

XLCB data acquisition and reduction program (3, 4). However, in this experiment the only 

function of the XLCB program was to record a pressure-time history for each shot. Propellant 

properties and characterizations were irrelevant for this set of experiments.

This particular experiment was unique in the fact that a double-ended bomb was needed in place 

of the standard 200-cc bomb. The primary differences between the two apparatuses is that the 

200-cc bomb has a closure plug on one end to accommodate the firing mechanism and a venting 

valve on the other end of the bomb, while the double-ended bomb has a closure plug on each 

end and is vented through the center portion of the vessel body. The double-ended vessel was 

necessary in order to exploit one of the chamber closures to accommodate the test window, as 

shown in figure 2. The other chamber closure would be used in its standard fashion as the 

location of the ignition source. 

The double-ended bomb had a chamber volume of ~465 cc and closure components identical to 

the Harwood 200-cc closed bomb, which is widely used for standard propellant testing (5). 

Because the chamber volume was much larger than needed, we designed and implemented a 

modified face plate/steel plug (figure 3) in order to take up volume inside the bomb, thus 

reducing its effective volume. This modification reduced the amount of propellant needed to 

reach desired pressures, and in turn, reduced the amount of stored energy in the closed bomb,

therefore reducing associated safety risks. The appendix provides detailed drawings of the 

closed bomb modification needed in order to satisfy the testing requirements.
3

Figure 1. Double-ended closed bomb apparatus.

a)

b)

Figure 2. (a) Test window and (b) exploded view of chamber head and window.
4

Figure 3. Modified closure cap/steel 

slug insert for closed bomb 

volume reduction.

4. Calculating Charge Weights and Volume

The primary function of the closed bomb vessel is to characterize propellants. Knowing the 

chamber volume as well as the propellant’s mechanical and thermochemical properties, one can 

calculate a burning rate using the empirically measured pressure-time history of the propellant 

(6). In this study, burning rates were of no interest; however, we wanted to calculate the 

chamber volume in order to determine the appropriate charge weights needed to achieve the 

desired chamber pressure. 

Without first knowing the closed bomb volume, the most direct way to calculate the amount of 

propellant needed to provide a desired pressure is to fire a few shots in the closed bomb at 

varying charge weights and calculate the linear relationship of charge weight to pressure. This 

method of determining appropriate charge weights is possible given that the governing equation 

of state for the closed bomb is

MW

m R T

PV p s

s = , (1)

where P is pressure, Vs is system volume, mp is propellant mass, Rs is the system’s gas constant, 

T is the adiabatic flame temperature of the propellant, and MW is the molecular weight of the 

propellant. Here it can be seen that P is linearly proportional to mp, which is just a function of 

propellant mass; while volume, gas constant, temperature, and molecular weight are all 

constants. When employing this method, precaution must be taken to assure one does not over-
5

pressurize the bomb; therefore, a low loading density should be chosen for an estimated bomb 

volume. So while the bomb volume is still unknown, its value is not necessary in calculating 

appropriate charge weights for a given propellant. The charge weights necessary to achieve 

desired pressures for this test were derived from the empirical calibration firings at various 

loading densities and incorporation of equation 1. From this linear fit, a loading charge to 

pressure relationship can then be used for the duration of the test (i.e., 93.6 g propellant for 

60 ksi). 

For future tests, it is best to know the volume of the bomb so one can accurately predict 

pressures, as opposed to engaging in the cumbersome method of firing calibration shots, which 

takes substantial time and effort. We can take equation 1 and substitute 

I

MW

RsT = , (2)

where I is then defined as the impetus of the propellant, which is generally a known 

thermochemical property. From this, we now have a given I and a known P and mp from 

empirical firings, and thus can calculate V. Through these calculations, we determined the 

effective chamber volume (with steel plug) of our bomb to be 235 cc.

5. Test

Over the course of the test, ARDEC provided ARL with various window designs for testing. In 

total, nine different designs were put into the closed bomb and tested, one of which was a 

control window that is known to be able to survive the intended environment. However, this 

window was unusable for application purposes due to the difficult nature of cleaning it.

Initially, each window was put through a seating procedure before undergoing the full up 

pressure shots. The seating procedure consisted of firing nine shots at progressively higher 

pressures: three shots at a low pressure (10 ksi), three shots at a medium pressure (35 ksi), and 

three shots at a high or full up pressure (55 ksi). During the evolution of the test, this 55 ksi 

upper limit was eventually pushed to 60 ksi and ultimately 63.3 ksi. After the window was 

seated, it underwent 55 additional shots at full pressure in order to be deemed gun usable. In an 

effort to save time, the windows were inspected after every third shot to check for compromises 

in integrity. 

Aside from the window’s presence in the pressure vessel, the operation of the closed bomb was 

identical to any regular propellant characterization test. That is to say, the bomb was cleaned, 

loaded, and fired in the same manner and no special precautions or procedures were necessary 

during testing.
6

During testing none of the window designs were able to survive the high-pressure conditions 

present in the closed bomb. They all showed some manner of fracture, either internal to the 

sapphire crystal or on its surface, as illustrated by figure 4. Table 1 shows a shot by shot test 

summary for the windows. The nomenclature used for the windows was an internal method of 

accounting for design differences. After several window designs failed, window 1005 was 

incorporated into the test matrix as a control window. This window is known to be able to 

survive the launch dynamics of a gun, and if it failed in the closed bomb, then it would indicate 

that the bomb was not accurately replicating the gun conditions and could not be used as a 

reliable indicator of window survivability. Unfortunately, during the testing of window 1005,

there was a mechanical failure in the closed bomb. Figure 5a shows how the vent valve 

catastrophically failed and was launched from the bomb body while the closed bomb was 

pressurized. The failure is likely a result of the valve not being adequately secured to the bomb,

which resulted in a much larger valve surface area for the bomb pressure to interact with, thus 

causing the threads to fail. As a precaution, the vessel was sent back to the manufacturer for 

quality inspection and was returned after no defects were detected. The test was subsequently 

continued with more window designs, which also failed.

The test was ultimately ceased due to a mechanical failure of the closed bomb that rendered it 

unusable. The valve seat on the closed bomb had eroded in such a manner that a proper seal 

between the bomb and valve could not be obtained. This was ultimately discovered upon closer 

inspection of the seating area following consecutive valve failures. Figure 5b shows the 

elongation of the eroded area of the bomb’s valve seat, and the valve stem showing evidence of 

gas erosion from the high-pressure leak around the seating area. Unfortunately, the history of 

this vessel was unknown and no record of usage was available so the failure could not have 

been foreseen.
7

Figure 4. Failures of the window.
8

Table 1. Peak pressure data. 

Detailed Pressure Data

Shot Window

Target 

Pressure 

(ksi)

Measured 

Pressure

(ksi) Comments

1 T2 10 NA

2 T2 10 10.5

3 T2 10 10.5

4 T2 35 34.8

5 T2 35 35.4

6 T2 35 35.5

7 T2 55 NA

8 T2 55 56.4

9 T2 55 56.1

10 N1 10 11.5

11 N1 10 10.7

12 N1 10 11.2

13 N1 35 34.5

14 N1 35 35.7

15 N1 35 NA

16 N1 55 55.1 Window cracked

17 T2 55 54.1

18 T2 55 53.5

19 T2 55 55.3

20 T2 55 55.5 Window cracked

21 T1 10 14.3

22 T1 10 10.6

23 T1 10 10

24 T1 35 34.8

25 T1 35 35.5

26 T1 35 35.5

27 T1 55 53.7 Window cracked

28 1005 10 NA

29 1005 35 35.4

30 1005 60 59.3

31 1005 63 60.9 Valve assembly broke

32 22 35 30.7

33 22 60 NA

34 22 60 NA

35 22 60 50.7

36 22 60 56.2

37 TRA-06 35 NA

38 TRA-06 60 46 New pressure gage incorporated

39 TRA-06 60 47.4 Window cracked

40 BA1 35 26.3

41 BA1 60 NA

42 BA1 60 43.5 Window cracked

43 TRA-03 35 26.7

44 TRA-03 60 43.2 Window cracked
9

Table 1. Peak pressure data (continued).

Detailed Pressure Data

Shot Window

Target 

Pressure 

(ksi)

Measured 

Pressure

(ksi) Comments

45 None 60 NA

46 None 60 42.5

47 None 60 NA

48 None 60 NA Valve assembly broke

49 None 35 29 Valve assembly broke

a)

b)

Figure 5. Vent valve failure, showing a) erosion around vent valve and b) erosion 

and elongation of closed bomb valve seat.
10

6. Results

We recorded pressure-time curves from the shots performed as well as the peak pressure values 

(figure 6). All the pressure-time curves behaved as the one illustrated in figure 6; therefore, 

only one pressure-time curve is included for representative purposes. The blue line represents 

the raw recorded data, the pink line is the reduced data, and the green line represents the change 

in pressure of the reduced data. Table 1 shows a recapitulation of the data recorded, along with 

the repeatability in obtaining pressures. From table 1, we notice that beginning with the TRA06 window design, the measured pressure data were substantially lower than the target value, 

along with the fact that capturing the data became more sporadic. While the reason for failing 

to capture those event’s data is not completely known, it is possible that the low-pressure 

readings contributed to this inability to capture data. The data acquisition system is triggered by 

the generation of system pressure, and if the actual pressure is substantially lower than the 

expected pressure then the system may never have reached the trigger threshold. 

The lower recorded pressure values can be attributed to a pressure gauge change during the 

course of testing. The initial gauge became unreliable during testing so we decided to use 

another gauge. At this time, the lower pressure readings appeared and prompted an inquiry into 

which gauge was within calibration and which one was not. In theory, this does not negatively 

affect the data recorded because the gauge calibration values can be updated at a later time into 

XLCB and the program can be rerun to return a more accurate data set. After the gauges 

returned from calibration, we discovered that the original gauge had deviated 25% from its 

previous calibration, while the second gauge had only deviated 1%. Given this fact, we 

determined that none of the data recorded with the original gauge were reliable with such a 

large deviation in calibration; therefore, we did not perform a recalculation of the pressure 

values. 

While not having accurately measured pressure for the majority of the shots is less than ideal, 

practical conclusions can still be made from the data set available. The shots performed with 

the second gauge have been validated with the post-test gauge calibration. Because the bomb is 

a fixed volume and a fixed amount of propellant was incorporated to reach targeted pressures, 

we can deduce that shots performed with the original, unacceptable gauge produced pressures 

similar to that of the later shots, which incorporated the second gauge. This information should 

give solace to all interested parties that the windows were not being over-pressurized and, in 

fact, were likely failing under pressures much lower than acceptable values. 
11

Figure 6. Pressure-time trace of closed bomb event.

7. Conclusions

The double-ended closed bomb was modified to accept a window housing in order to test the 

window’s survivability in a simulated gun environment around 60 ksi. The variety of window 

designs tested did not survive the closed bomb environment. One possible cause for the failures 

was a flawed window design. A second possible cause was an unforeseen difference in pressure 

and thermal environments between a live-fire gun chamber and the closed bomb chamber. The 

main difference between the two is that gun pressures decay quickly, on the order of 

milliseconds, as the projectile moves down the barrel, whereas the closed bomb retains the high 

pressure for a much longer period of time, generally 30–60 s, before venting. Furthermore, the 

pressure dynamics in a closed bomb may not accurately simulate that of a gun chamber, where 

unknown pressure waves may be adversely affecting the window.

A third possible failure mechanism of the windows could have been a result of overpressurizing the closed bomb. This possibility arose when we experienced a discrepancy in the 

pressure recordings when different gauges were used. However, over-pressurization seems 

unlikely due to the fact that post-test gauge calibrations showed the secondary gauge to be 

within specifications and during testing it only recorded values in the 45 ksi range.

Pressure vs Time

0.00E+00

5.00E+01

1.00E+02

1.50E+02

2.00E+02

2.50E+02

3.00E+02

3.50E+02

4.00E+02

-1.00E01-8.00E02-6.00E02-4.00E02-2.00E020.00E+

00

2.00E024.00E026.00E028.00E021.00E011.20E01

Time (sec)

Pressure (MPa)

-4.00E+04

-2.00E+04

0.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

Derivative (MPa/sec)

Pressure data

Processed Data

Pressure Derivative
12

While the closed bomb is a useful apparatus that can be modified for operations other than 

propellant characterization, it is not a be-all, end-all tool for material survivability testing due to 

the potential differences in the high-pressure closed bomb environment when compared to the 

environment it is intended to replicate.
13

8. References

1. Juhasz, A.; Price, C. The Closed Bomb Technique for Burning Rate Measurements at High 

Pressure. Experimental Diagnostics in Combustion of Solids, edited by Boggs and Zinn, 

vol. 63 of Progress in Astronautics and Aeronautics, 1977.

2. Beyer, R.; Kerwien, H. Evaluation of ALON for Cannon Window Application; ARL-TR2358; U.S. Army Research Laboratory: Aberdeen Proving Ground, MD, October 2000.

3. Wagner, T.; Ritter, J.; Homan, B. Upgrades to the Closed Bomb Facility and Measurement 

of Propellant Burning Rate; ARL-TR-5058, U.S. Army Research Laboratory: Aberdeen 

Proving Ground, MD, January 2010.

4. Homan, B.; Juhasz, A. XLCB: A New Closed-Bomb Data Acquisition and Reduction 

Program; ARL-TR-2491, U.S. Army Research Laboratory: Aberdeen Proving Ground, 

MD, May 2001.

5. Juhasz, A. Round Robin Results of the Closed Bomb and Strand Burner; CPIA-Pub-361, 

July 1982.

6. Oberle, W.; Kooker, D. BRLCB: A Closed-Chamber Data Analysis Program. Part I –

Theory and User’s Manual; ARL-TR-36; U.S. Army Research Laboratory: Aberdeen 

Proving Ground, MD, January 1993.

7. Beyer, et al. United States Patent: 6,212,989. High Pressure, High Temperature Window 

Assembly and Method of Making the Same. April 10, 2001.
14

INTENTIONALLY LEFT BLANK.
15

Appendix. Modified Closed Bomb Drawings

Figures A-1 through A-3 display the diagrams for the modifications made to the closed bomb 

for our experiment.

Figure A-1. The steel slug insert.
16

Figure A-2. The modified closure (head).
17

Figure A-3. The modified face plate for closure.
18

NO. OF

COPIES ORGANIZATION

1 ADMNSTR

ELEC DEFNS TECHL INFO CTR

ATTN DTIC OCP

8725 JOHN J KINGMAN RD STE 0944

FT BELVOIR VA 22060-6218

1 US ARMY RSRCH LAB

ATTN RDRL CIM G T LANDFRIED

BLDG 4600

APG MD 21005-5066

3 US ARMY RSRCH LAB

ATTN IMNE ALC HRR MAIL & 

RECORDS MGMT

ATTN RDRL CIM L TECHL LIB

ATTN RDRL CIM P TECHL PUB

ADELPHI MD 20783-1197

1 DIR USARL

RDRL ROP TECH LIB

PO BOX 12211

RESEARCH TRIANGLE PARK NC

27709-2211

1 COMMANDER

RADFORD ARMY AMMO PLANT

SMCAR QA HI LIB

RADFORD VA 244141-0298

1 DIR BENET WEAPONS LAB

TECH LIB

WATERVLIET NY 12189-4000

1 CDR NAVAL RSRCH LAB

TECH LIB

WASHINGTON DC 20375-1972

NO. OF

COPIES ORGANIZATION

1 COMMANDER

ARDEC

ATTN K KLINGAMAN

AMSTA AR WEE A B321

PICATINNY, NJ 07806

1 COMMANDER

ARDEC

ATTN W. THALMAN

RDAR WSW I B61S

PICATINNY, NJ 07806

1 COMMANDER

NSWC/IHDIV

ATTN KC ELLIOTT

3945 BALLISTICS LANE, SUITE 1

ENERGETICS EVALUATION 

DEPARTMENT

INDIAN HEAD MD 20640-5089

ABERDEEN PROVING GROUND

1 DIR USARL

RDRL CIM G (BLDG 4600)

7 DIR USARL

RDRL WML B

B E HOMAN

RDRL WML C

K L MCNESBY

RDRL WML D

R A BEYER

A L BRANT

J COLBURN

P CONROY

RDRL WMS

J RITTER

TOTAL: 20 (1 ELEC, 19 HCs)