In-Chamber Primer Force and Case Pressure Measurements of the 5.56-mm Cartridge

Transcription

1 In-Chamber Primer Force and Case Pressure Measurements of the 5.56-mm Cartridge by John J. Ritter, Richard A. Beyer, and Anthony Canami ARL-TR-5862 January 2012 Approved for public release; distribution is unlimited.

2 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.

3 Army Research Laboratory Aberdeen Proving Ground, MD ARL-TR-5862 January 2012 In-Chamber Primer Force and Case Pressure Measurements of the 5.56-mm Cartridge John J. Ritter, Richard A. Beyer, and Anthony Canami Weapons and Materials Research Directorate, ARL Approved for public release; distribution is unlimited.

4 REPORT DOCUMENTATION PAGE Form Approved OMB No 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 ( ), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 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) January REPORT TYPE Final 4. TITLE AND SUBTITLE In-Chamber Primer Force and Case Pressure Measurements of the 5.56-mm Cartridge 3. DATES COVERED (From - To) 01 March August a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) John J. Ritter, Richard A. Beyer, and Anthony Canami 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-WML-D Aberdeen Proving Ground, MD PERFORMING ORGANIZATION REPORT NUMBER ARL-TR 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 is unlimited. 13. SUPPLEMENTARY NOTES 14. ABSTRACT A custom, in-house designed breech for an M16A1 Mann barrel, 5.56 mm, has been instrumented to measure the force output of an unsupported primer. In addition, the Mann barrel was designed to accept gauges at two diametrically opposed locations to simultaneously measure mid-case chamber pressure. This unique apparatus provides an advanced diagnostic tool to obtain insight into the interactions between the primer and propellant bed of the 5.56-mm cartridge during ignition, and allow for better characterization and understanding of the cartridge s interior ballistics. Experiments were performed to investigate the role of the primer, temperature conditioning, and charge weights to determine their respective effects on chamber pressures and primer forces. This systems level approach provides detail into the primer-propellant interactions never before observed. Experimental results indicate the primer force measurement is an invaluable tool in evaluating the interior ballistics of the 5.56-mm cartridge, and provides early time information that pressure measurements alone overlook. Whereas standard midcase or case mouth pressure measurement diagnostics begin when the propellant gas generation is first observed, this novel diagnostic approach shifts the initial data acquisition timeframe back to when the firing pin strikes the primer. 15. SUBJECT TERMS 5.56-mm, primer, force 16. SECURITY CLASSIFICATION OF: a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACT UU 18. NUMBER OF PAGES 38 19a. NAME OF RESPONSIBLE PERSON John J. Ritter 19b. TELEPHONE NUMBER (Include area code) Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 ii

5 Contents List of Figures List of Tables Acknowledgments v vi vii 1. Introduction 1 2. Approach Fixture Design Fixture Prove Out Experiment Setups Initial Primer/Propellant Experiment RP Primer and Modified Primer Stake Pressure Gauge Shielding Cartridge Temperature Effects Charge Weight/Ullage Effects Results/Discussion Initial Primer/Propellant Experiment, Apparatus Prove Out RP Primer and Modified Primer Stake Pressure Gauge Shielding Cartridge Temperature Effects Charge Weight/Ullage Effects Other Considerations Summary Conclusion Recommendations References 22 iii

6 List of Symbols, Abbreviations, and Acronyms 24 Distribution List 25 iv

7 List of Figures Figure 1. Detail view of 5.56-mm instrumented barrel and breech....2 Figure 2. Exploded view of 5.56-mm instrumented Mann barrel...3 Figure 3. Primer force apparatus mounted in the environmental chamber....5 Figure 4. Sample data of pressure, force, and interferometer measurements....7 Figure 5. Force measurement data; the thicker black line is the average....7 Figure 6. Timeline of interior ballistics based on force gauge output....9 Figure 7. Effects from varying cartridge parameters Figure 8. Detail view of primer function data Figure 9. Comparison of primers with varying outputs Figure 10. Circular stake (left) and four point stake (right) Figure 11. Four point stake vs. circular stake Figure 12. Representative pressure and interferometer records Figure 13. Detailed view of pressure and interferometer records Figure 14. Temperature effects on pressure and force Figure 15. (a) Fired cases from the hot temperature firings. From L to R: Normal, misaligned with some case erosion, higher pressure with erosion, highest pressure (~4 ksi above hot average) with total erosion around pressure port, and approximately 60 ksi with case erosion (b) Misaligned, unsupported, ruptured cartridge case Figure 16. Effects of charge weight on pressure and force output v

8 List of Tables Table 1. Matrix of experiments conducted....6 vi

9 Acknowledgments The authors would like to thank Paul Conroy for his valuable insight into the data analysis and Joe Colburn for initial experiments performed. vii

10 INTENTIONALLY LEFT BLANK. viii

11 1. Introduction The genesis of the primer force measurement technique on the 5.56-mm cartridge came from an investigation of dropped primers, where the primer is dislodged from its pocket in the cartridge case during ejection (1). Having a primer pop out of its pocket, either wholly or partially, is undesirable as it can lead to weapon malfunctions. The cause of these dropped primers has led to a variety of root cause speculations. Evidence is available that indicates the pocket was stretching from excess pressure (2). A less explored possibility exists of localized high pressure within the cartridge case just in front of the primer, or possibly some other unknown phenomenon within the cartridge case. However, standard diagnostic techniques are based on measuring mid-case pressure and muzzle velocity. While this provides general insight into how the cartridge is behaving, the method is insufficient in identifying any localized high pressure regions or impulses. Nor does it provide any information on the primer s performance. Previous studies of abnormal primer function in the M mm cartridge showed that extreme impulses could be obtained from the combination of a primer and propellant confined near the primer (3). Exploring this potential cause of dropped primers was most attractive as it has not been extensively researched, and it should provide greater detail into the events occurring inside the cartridge case. In order to accomplish this, a system was designed to measure the total force acting on the back of the primer, parallel to the bore, in conjunction with mid-case pressure measurements. 2. Approach 2.1 Fixture Design The primary challenge in measuring primer force is the fact that both the firing pin and force transducer need simultaneous access to the back of the primer cup. The eventual design consisted of a force washer to measure the load which allows the firing pin to freely pass through the center of the washer to access the primer. The force washer is coupled to the primer through the use of a funnel-like reducer to accommodate the size differences of the load washer and primer. The force washer setup is held firmly in place with a threaded pre-load bolt. Figure 1 shows a detailed illustration of this setup. To obtain accurate measurements, the force gauge must be sufficiently pre-loaded against the primer to prevent it from moving away and losing contact upon the firing pin striking the primer. It should be noted that the force washer s full range of motion is on the order of 10 m, thus the motion of the primer when struck is in fact an extremely small distance. 1

12 Figure 1. Detail view of 5.56-mm instrumented barrel and breech. Using the lessons learned from a preliminary fixture, the full instrumented gun barrel was constructed. Figure 2 shows an exploded view of the essential components. The force washer is mounted between the gauge reducer and pre-load bolt. The small face of the reducer is against the primer; the preload bolt provides firm contact between the reducer and the primer face. In practice, the firing pin is free moving through the preload bolt, the force washer, and the gauge reducer from the solenoid (not shown) which strikes it to the primer. A polyethylene sleeve is used as a bearing around the firing pin to reduce friction and to keep the firing pin motion smooth and aligned. For best results the firing pin should be positioned such that it is against the primer. This will prevent the solenoid from slapping the pin forward and thus striking it twice creating inconsistent ignition data. The gun tube has a 1 in outer diameter with the length and interior dimensions identical to the M16 rifle. The larger outer diameter allows additional flexibility in mounting and instrumentation. Despite the large outer diameter, installation of a mid-chamber pressure transducer, Kistler Model 6215 (4), still required the addition of a pressure gauge collar. Pressure was obtained through a 1/16 in hole drilled into the mid-point of the cartridge case. Mid-case holes were sealed with a DuPont Kapton 1 mil thick tape. For best results the tape should be wrapped one full revolution about the case. This method will provide for the best possible seal and prevent gas leakage. Unless otherwise noted, cartridge cases, primer and propellant were from lot SMQ06H302S581. The propellant of this lot is a modified SMP842. Projectiles are standard M855 from unknown lot. The force transducer selected was the Kistler Model 9011A Load Washer (5). Force transducer selection was driven by the anticipated load, and an annular design that allowed passage of firing pin components through the transducer. As an added measure of certainty, light screens were used to measure projectile velocity at a distance of 9 m from the muzzle. This data should correlate to the pressure data and is used primarily as back up. Additionally, a 55-GHz microwave interferometer (6, 7) was incorporated to measure in-bore projectile motion. 2

13 Figure 2. Exploded view of 5.56-mm instrumented Mann barrel. 2.2 Fixture Prove Out Preliminary experiments were conducted in order to prove out the apparatus as well as empirically determine the necessary preload condition for the force transducer. Kistler recommends that its force transducers always be kept in a loaded condition to ensure valid data. Determining a preload condition was necessary because the force transducer is unloaded a certain amount when the firing pin strikes the primer. If the firing pin imparts enough force the gauge could become fully unloaded showing an output of zero V. At this time it would be impossible to determine the true measurement of the force output as the gauge cannot read below zero. Once the proof of concept was proven out and preload determined, the detailed experiments could be undertaken. A preload of 160 lb was determined to be sufficient for all subsequent experiments. In addition to the preload requirement, the unique experimental apparatus required a characterization of the striking energy imparted on the primer by the firing solenoid. This was to assure the firing pin was striking the primer within the system specifications and therefore eliminates the solenoid as a possible source of failure should the primer or cartridge not function. The #41 primer specification (8) for firing states the following conditions: All-Fire: 3.94 ± 0.02 oz ball dropped from 12 in No-Fire: 3.94 ± 0.02 oz ball dropped from 3 in The all-fire condition results in approximately 0.33 J of kinetic energy transferred to the primer. Through experimentation it was determined that the 132 g solenoid plunger strikes the primer 3

14 with a velocity of approximately 4 m/s. This provides roughly 1.06 J of energy to the primer, much more than the required all-fire condition. 2.3 Experiment Setups The primer force measurement apparatus was implemented to perform a variety of experiments in an effort to better characterize primer performance, as well as observe primer-propellant interactions. The experimental variables investigated include varying the primer, propellant, propellant charge weight, temperature conditioning of the cartridge, and pressure measurement technique. 2.4 Initial Primer/Propellant Experiment Various experiments were derived to evaluate the role of the primer in the interior ballistics cycle. The first of these experiments investigated the differences between the standard #41 and an experimental red phosphorous (RP) primer (9, 10). The RP primer is a less brisant, green primer candidate developed by ATK (11). The experiment also investigated the interior ballistic dynamics associated with varying the propellant by exchanging the standard WC844 with an experimental faster burning variant. 2.5 RP Primer and Modified Primer Stake The second primer experiment investigated the effect of non-standard RP primers which generate more particulate matter versus one that produces less particulate matter. The solid products of combustion associated with the more particulate primer can be characterized as producing many, small sized particles, whereas the less particulate primer produces fewer, larger sized particles. It is believed that a primer with more, finer particles will generate a softer, more uniform ignition of the propellant and therefore a more desirable interior ballistic event. In principle, the softer ignition will lead to less propellant bed compaction and create a more optimal propellant burning event. In focusing on the primer, an experiment was undertaking to characterize the effects that the primer stake could have on the primer force measurements. For this, a four point stake was used on the standard no. 41 primer and compared to the more traditional circular stake on a no. 41 primer. The theory here being the four point stake requires a larger force to dislodge the primer from the pocket and therefore different forces may be seen. 2.6 Pressure Gauge Shielding In conjunction with the second primer test, the apparatus was used to investigate the differences associated with pressure measurement techniques. Since the apparatus was equipped with two mid-case pressure ports, two pressure measurements could be obtained from a single cartridge. One method of data acquisition was to equip a gauge with a diaphragm protector Type The other method was to use the heat shield in conjunction with insulating grease between the gauge face and cartridge. Grease is an alternative method to the heat shield in mitigating the 4

15 effects of temperature on the gauge. Differences in data acquisition were noted and will be discussed in the following section. 2.7 Cartridge Temperature Effects It is well known that temperature differences greatly influence the performance of propellants. For this set of experiments the effects of temperature were taken into consideration while all other parameters remained constant. The cartridges were prepared with a nominal 25.0 grain charge weight then fired at temperatures of 50 F, 70 F (ambient), and 125 F. The test sequence was 10 shots at ambient, 25 at hot, 25 at cold, then another 10 at ambient. The temperatures were chosen to mimic the performance specifications of the M855 cartridge (12). Unfortunately due to equipment limitations the cold was limited to 50 F, as opposed to the specification s 65 F. In order to fire at temperature the chamber and breech of the apparatus was conditioned in an environmental chamber along with the ammunition for a minimum of 4 h. It is essential to condition the apparatus along with the ammunition due to rapid heat transfer between the temperature conditioned ammunition and the ambient temperature gun chamber (2). Figure 3 shows the Mann barrel and breech assembly in the ready fire position while in the environmental chamber. Figure 3. Primer force apparatus mounted in the environmental chamber. 5

16 2.8 Charge Weight/Ullage Effects Another potential area of interest is how the primer-propellant interaction behaves given a differing amount of ullage in the cartridge case. This was investigated by varying the amount of propellant in a series of experiments. The nominal charge weight for all other experiments conducted was 25.0 gr; therefore ample data existed for that charge. The standard M855 has a charge weight of 26.1 gr (13). In order to obtain sufficient data, ten shots each at charge weights of 24.0, 26.0, and 27.0 gr were investigated to characterize the relationship of primer force to cartridge free volume. A summary of the experiments performed is shown in table 1. Table 1. Matrix of experiments conducted. Primer Force Measurements Matrix Series Name Primer Propellant Projectile Charge (gr) WC844, Initial No. 41, RP Experimental Fast Pressure Ports Used Cartridge Condition M855 Nominal One A RP primers Less Particulate, More Particulate SMP842* M855 Nominal Two A Primer staking Pressure gauge shielding Cartridge temperature effects No. 41 (Circular Stake), No. 41 (4-Point Stake) SMP842* M855 Nominal Two A No. 41 SMP842* M855 Nominal Two A No. 41 SMP842* M855 Nominal One A, H, C Charge weight/ullage effects No. 41 SMP842* M855 24, 26, 27 One A Note: SMP842* is an early version of production SMP842. A Ambient, C Cold, H Hot. Nominal charge is 25 gr. 3. Results/Discussion Throughout the wide range of experiments conducted, the apparatus performed as designed producing reliable and repeatable pressure and force measurements. Generally speaking the force curve was proportional to the pressure curve with varying amplitudes and time delays for the various experimental variables. Sample data can be seen in figure 4 where the red line is the force gauge output, the green is the mid-case pressure, and the blue line is interferometer data showing projectile movement in bore. Amplitudes are presented here as arbitrary units. In reality the scale is 10,000 pounds per square inch per volt (psi/v) for pressure and 400 pound per volt lb/v for force. Figure 5 is a 10 shot sample showing the repeatability of the experiment s force measurements. The thicker black line is the average. 6

17 Force (Arb. Units) (Arb. Units) Interferometer Force Pressure E E E E E E E E E Time (s) Figure 4. Sample data of pressure, force, and interferometer measurements Shot 1 Shot 2 Shot 3 Shot 4 Shot 5 Shot 6 Shot 7 Shot 8 Shot 9 Shot 10 Force Average E E E E E-04 Time (s) 8.00E E-03 Figure 5. Force measurement data; the thicker black line is the average. 7

18 The force gauge data is a great tool to illustrate the sequence of events in the interior ballistic cycle. The initial negative slope before time zero is where the firing pin impacts the primer and in turn unloads the force gauge creating a negative output. The initial spike at time zero is the primer functioning. At this time there is only speculation as to the second primer spike. It has been suggested that this is a reflective wave from the primer output bouncing off the propellant, but its true meaning is still speculative. It is, however, a real event that is present in all data. After the primer functions there is a lull before the propellant begins generating gas. Once the gas generation begins, the cartridge pressure is transmitted to the force gauge. As can be seen in figure 4 the rise and peak forces seen at the primer is proportional to that seen at the mid-case pressure gauge, this is to be expected. A close up inspection of the peak pressures will show the primer s peak slightly delayed from the mid-case. This is attributed to the response time needed for the force to travel through the apparatus to the load washer. A unique situation was discovered during the experimental process. From the interferometer information, the projectile is seen de-bulleting from the cartridge case prior to pressure build up of the burning propellant. Initial movement occurs at approximately 90 µs. In effect, the output of the primer alone is great enough that it forces the propellant bed forward against the aft end of the projectile in the case and de-bullets it. This forward motion and force corresponds to the lull in the primer force recorded. The lull actually creates a situation where there is less support (force) on the primer than when it is initially struck by the firing pin; illustrated in the data by the force measurement going below the pre-load force. This is direct evidence that the primer functioning is powerful enough to force the cartridge forward in the chamber and in the least relieve the contact stress between the breach face and aft end of cartridge, to include the primer. Eventually the burning propellant will generate enough gas to produce a positive rearward force on the cartridge and primer pocket. If the change in forces from the forward lull and rearward impulse from gas generation is large enough then a cartridge failure may result. In essence the primer is going from an unsupported state to one in which a rapid pressurization of the cartridge will slam the cartridge rearward. This phenomenon could be a direct link to the dropped primer issue experienced with the 5.56-mm cartridge since the primer is not being supported by the breach as previously believed. Figure 6 provides an illustrated view of the events seen by the primer force gauge in figure 4. It is evident from the data that the mid-case pressure gauge will not capture the primer initiation or its output. This is significant in that the events prior to propellant ignition are overlooked by the mid-case gauge. The events which take place in this early interior ballistics stage are primarily in the solid phase and mainly occur in the linear axis. This is a situation the mid-case gauge cannot accommodate as its purpose is to measure radial gas pressures. The information gathered by the force gauge is imperative to characterizing the primer s performance which is a major driver in propellant initiation and overall cartridge performance. Not only is it desirable to have a tailored primer output for optimal ballistics, it is also desirable to have a consistent primer output. Neither of which is measureable with a mid-case pressure gauge. 8

19 t=0, Firing pin strikes primer. t>0, Primer functions, initial spike in force, propellant bed compacted and forced forward. Primer forces likely result in de-bulleting. 50<t<200 µs, Force of propellant being pushed forward overcomes rearward force of primer action, creating a sub pre-load force reading. Case wants to separate from breach face, thus creating an unsupported primer situation. t~100 µs, Propellant pressure first recorded. t>150 µs, Propellant gases become primary force driver. If impulse of rearward force is great enough this could lead to malfunctions. Figure 6. Timeline of interior ballistics based on force gauge output. 3.1 Initial Primer/Propellant Experiment, Apparatus Prove Out Initial experiments were conducted at ambient temperatures where primer, projectile, and propellant configurations were modified to correlate the effects on primer force. Figure 7 shows three pairs of curves from three different cartridge types. For each cartridge type there is a primer force curve, and a chamber pressure curve. All curves shown are the average of five shots. In all cases, the chamber pressure curve is the higher of the two. The force units are signal volts as recorded; the calibration factors for the gauge in this configuration were determined but are not reported here. Type M855 is the standard 5.56 mm round with the standard #41 primer. Type M855 no. 41/Fast Propellant has the same projectile and primer as the M855, but with a different propellant. Type M855 RP is the same as type M855, but uses a less brisant candidate green (non-lead) primer developed by ATK. The primer force measurement during the propellant burn tracks well with the timing and magnitude of the chamber pressure. The initial spikes seen on the figure are that of the primer functioning, with the pressure curves beginning shortly thereafter. 9

20 Pressure, Force (Arb. Units) M855 #41 primer/ Standard Prop M855 RP primer/standard Prop M855 #41 primer/fast Propellant time (msec) Figure 7. Effects from varying cartridge parameters. A more detailed view of the primer function from this figure is shown in figure 8. The consistent action of the no. 41 primer is noted. However, the RP primer has two different characteristics. The first is that its motion from the firing pin strike is less than that with the standard primer. This result is indicative of a mechanical property such as a softer energetic fill or different metal parts. In addition, the force recorded from the primer function is significantly reduced when compared to the standard primers. This softer ignition results in a slower pressure rise and slightly longer action time for the full interior ballistics cycle. This effect can be favorable if all other performance metrics are met such as velocity and action time. 10

21 Pressure, pressure/force Force M855 #41 primer/ Standard Prop M855 RP primer/standard Prop M855 #41 primer/fast Propellant Start of pressure curve curve time (msec) Figure 8. Detail view of primer function data. 3.2 RP Primer and Modified Primer Stake A prior experiment investigated the primer output of an experimental RP based primer vs. a traditional no. 41 primer (14), with a follow up investigation looking at various RP formulations. From these experiments the primers could be categorized based on the amount of particulate matter the primer produced. From this, two particular lots were characterized and labeled more particulate and less particulate. The more particulate primers generated more hot embers that were smaller in physical size, whereas the less particulate primers generated less embers of larger physical size. The theory being that a less forceful primer output would result in less propellant bed compaction and in turn produce a more optimal interior ballistic event. Five shots of each primer were tested with the resulting averages seen in figure 9. 11

22 Pressure, Force (Arb. Units) More, Force More, P Up More, P Down Less, Force Less, P Up Less, P Down E E E E E E E E E E-03 Time (s) Figure 9. Comparison of primers with varying outputs. From the data it can be seen that the primers generated greatly different cartridge performance. The primer with more particulate is less brisant in nature and would produce a softer initiation of the propellant. The primer had such little brisance that it actually did not produce an initial spike upon functioning. This was the only experiment where this phenomenon was observed. Despite this, the primer had no difficulty igniting the propellant, and no misfire or hang-fire events occurred. This less brisant primer also caused a significant delay in propellant ignition, 300 µs, when compared to the more brisant primer. Also of interest, is the fact that the less brisant primer generated about 5 ksi less peak pressure resulting in a 13 m/s reduction in muzzle velocity. When compared with M855 cartridges tested, the more brisant primer had the same muzzle velocity as the standard M855 with no. 41 primers. The differences in pressure-time curves as well as velocities can be attributed to the performance of the primer. Under normal circumstances the projectile de-bullets, the propellant bed is compacted, and more volume is created in the cartridge case from the bullet motion. With the less brisant primer this sequence of events is altered, if not all together eliminated, thus different interior to exterior ballistic dynamics. 12

23 Force, Pressure (Arb. Units) In addition, a set of experiments was conducted to determine if there was a discernible difference between a circular stake primer and the newer four point stake (figure 10) that is designed to create a greater interference fit thus resulting in fewer dropped primers. The data of the five shot averages is presented in figure 11. While it appears the four point primer stake sees more force and pressure, these results may be the result of statistical sampling errors. The velocity average for the four point stake primers is 5 m/s higher which supports the higher pressure results. Further investigations are needed to reach a more definitive conclusion. Figure 10. Circular stake (left) and four point stake (right) Modified Stake Force Modified Stake Pressure Standard Stake Force Standard Stake Pressure E E E E E-04 Time (s) 8.00E E E E-03 Figure 11. Four point stake vs. circular stake. 13

24 Pressure (10 ksi) Volts (55 GHz) 3.3 Pressure Gauge Shielding As a complimentary experiment to the primer force measurements the apparatus was incorporated with dual mid-case pressure ports 180 opposite each other. In this configuration one transducer designated P-up, was configured with just the Kistler heat shield. The second transducer, designated P-down, was configured with the heat shield (Model 6565A), and the volume in front of the transducer was filled with grease before each test. The grease-filled port was expected to measure the stress wave and solid particle motion (15) initiated by the primer that would be transmitted through the propellant bed in the tightly packed M855 cartridge. A plot of representative pressure and interferometer records are shown in figure 12. In all tests, the P-down gauge (grease-filled, red line) responded before the un-greased P-up gauge (green line). This is consistent with primer-induced stress transmitted through the propellant bed, before ignition of the grains. It is expected that this effect is less-pronounced in cartridges with lower loading densities Pressure, "Up" Interferometer Pressure, "Down" Time (ms) Figure 12. Representative pressure and interferometer records. 14

25 Pressure (ksi) Volts (55 GHz) It was also observed that the peak pressure measured by the greased gauge was always several thousand pounds per square inch lower than the ungreased gauge, and lagged slightly behind the ungreased gauge during depressurization. It is suspected that this is due to compression of the grease column in front of the transducer. This experiment also utilized a 55-GHz microwave interferometer to measure projectile displacement. The data presented in figure 13 indicates that the projectile begins to move (arrow) just after initial stress is seen in the greased gauge, and the projectile appears to have moved approximately 1 mm (16) by the time gas pressure is observed at the ungreased gauge Pressure, "Up" Pressure, "Down" Interferometer -1.0 Time (ms) Figure 13. Detailed view of pressure and interferometer records. This experiment provided further evidence that the propellant bed is moving with a significant force upon primer initiation. The solid body motion is transmitted through the grease to the pressure gauge prior to that of the ungreased gauge. In hindsight the experiment should have been conducted without a heat shield on the greased gauge. The Kistler manual indicates a heat shield shall be used or insulating grease, not both. The rationale is that the combination of heat shield orifices and grease will noticeably delay the gauge s time response. 15

26 3.4 Cartridge Temperature Effects Cartridges along with the entire apparatus were conditioned in an environmental chamber to the desired hot (125 F) and cold ( 50 F) temperatures for a minimum of 4 h. In total 25 cartridges were fired each at hot and cold, and 20 fired at ambient. Ambient firings were conducted outside the box at a nominal range temperature of 72 F. As expected the hot conditioned rounds produced a substantially higher peak pressure than cold and ambient, as seen in figure 14. The hot rounds essentially produced a scaled up output of the ambient rounds as expected. This holds true in both primer force and mid-case pressure outputs. The most intriguing information gleaned from the experiment is the behavior of the cold rounds. First, the primer output can be seen as identical to that of the ambient rounds, whereas one might expect it to be reduced. Second, the force output occurs much earlier than ambient or hot, which implies the propellant is igniting earlier in the ballistic cycle. This is most likely the byproduct of propellant grain fracture. Examining the cartridge s impulse (area under the force curve) can also provide valuable information to the interior ballistics of a cartridge. The impulse of the cold cartridge is significantly higher in magnitude than the other two temperatures. From this higher impulse one would believe that more adverse cartridge conditions would be seen such as a higher rate of malfunctions or inconsistent performance. The pressure curve also supports this assertion that there is a greater impulse through the greater gas generation rate, dp/dt, which is highest in the cold cartridge. An interesting phenomenon was noticed while firing the hot conditioned rounds. When the peak pressure approached 60 ksi the pressure port of the cartridge case would begin to erode. As the pressure went higher more erosion was seen. Figure 15a shows the types of erosion seen from none to near complete, gauge port hole sized. It should also be noted that cartridge alignment is critical when measuring pressure. If cartridge and pressure port holes are not aligned the gases want to take the path of least resistance and therefore will erode the cartridge case at lower pressures. An example of this is shown in figure 15a in the second cartridge from the left. If total misalignment occurred and the cartridge case was unsupported at the gauge port hole then the case pressure would shear off a disk the diameter of the port hole. An example of this is shown in figure 15b. During the course of performing the experiments a best practice was developed for sealing the pressure hole. Ideally the cartridge should have the tightest fit in the chamber in order to create the best seal and prevent gas leakages. For our setup one full revolution of sealing tape was ideal, and implemented some time after this set of experiments. 16

27 Pressure, Force (Arb. Units) Ambient Force Ambient Pressure Cold Pressure Cold Force 4.00 Hot Pressure E E E E E E-04 Time (s) Figure 14. Temperature effects on pressure and force. a b Figure 15. (a) Fired cases from the hot temperature firings. From L to R: Normal, misaligned with some case erosion, higher pressure with erosion, highest pressure (~4 ksi above hot average) with total erosion around pressure port, and approximately 60 ksi with case erosion (b) Misaligned, unsupported, ruptured cartridge case. 17

28 Pressure, Force (Arb. Units) 3.5 Charge Weight/Ullage Effects Figure 16 shows the results of the experiment conducted to investigate the effects of charge weight on the performance of the primer and propellant. The charge weights examined were 24.0, 26.0, and 27.0 gr. Twenty-five grains was not part of this set as that was the nominal charge of the other experiments in this study. Twenty-six grains is the M855 standard charge weight gr Force 24 gr Pressure 26 gr Force 26 gr Pressure 27 gr Force 27 gr Pressure E E E E E-04 Time (s) Figure 16. Effects of charge weight on pressure and force output. The lower charge cartridges had lower pressure and primer force, which is to be expected. Initial primer force was relatively unaffected by the different charge weights with the 27.0 gr cartridge having a slightly higher primer force output. This could be a direct result of the reduced ullage and the fact that the initial primer output acts upon the propellant bed. A more tightly packed cartridge will have a higher resistance to the primer output and therefore the system will see more force from the primer. The only other information gained is that it appears that the pressurization rate of the light cartridge, 24.0 gr, was lower than that of the other two. This is also expected as a propellant s burn rate is directly proportional to overall system pressure, therefore, lower overall pressure will produce a slower burn rate. 18

29 3.6 Other Considerations Frictional forces between the cartridge case and barrel chamber were not taken into consideration. It has been shown that less frictional contact between these two surfaces results in higher axial primer forces (17). These cartridges likely had varying coefficients of friction (due to moisture, oil, or other debris) but the magnitude of friction from these sources is completely offset by the steel pressure port plug secured to the cartridge case to ensure there was no gap that could cause the case to rupture as can be seen in figure 15b. The only exception being the experiments that utilized both pressure ports which occurred during the gauge technique evaluation in conjunction with the primer particulate experiment. Considerations for temperature sensitivity of the Kistler pressure and force gauges were not taken. In the temperature range of the experiments undertaken the temperature extremes investigated would alter the output of the gauges by less than two percent. Since the experiments were more qualitative in nature, it was deemed unnecessary to incorporate the gauges temperature coefficient of sensitivity. Finally, with small calibers, in this case 5.56 mm, the volume of the pressure chamber (cartridge) is increased by the additional volume of the pressure gauge port. Because of the increased volume compared to a cartridge s volume without port, lower pressures will be produced. Since the volume is constant throughout the experiments no corrections for additional chamber volume were taken. Future experiments may look into the quantitative effect of the apparatus s pressure port volume in order to minimize its effects in data measurements. 4. Summary An experimental fixture was designed and manufactured in house to measure the force output of an unsupported 5.56-mm cartridge primer. The system was successfully implemented with experiments conducted to determine how various external factors affect the interior ballistics of the cartridge. A systematic approach was developed to investigate the role of the primer and propellant composition, cartridge temperature, primer staking method, and propellant charge weight. From the experiments it was shown that the primer performance plays an instrumental role in the overall interior ballistic performance of a cartridge. Information obtained from the primer force measurements demonstrate that in the first 100 µs after the firing pin strikes the primer that the primer will initiate and push the solid phase propellant against the aft end of the projectile. At this time a partial de-bulleting can occur. The amount of this force imparted on the projectile by the solid phase propellant can be influential in the later time performance of the cartridge. The extent of this movement is all dependent upon the cartridge temperature, primer performance, propellant properties, and ullage. 19

30 At approximately 150 µs after the primer is struck the primary force driver becomes the burning of the propellant charge. The manner in which the propellant is initiated via the primer and subsequently functions will also have a significant impact on the overall interior ballistic performance. Therefore, when evaluating the interior ballistic performance of a cartridge one must take into account numerous factors (primer performance, propellant performance, temperature, charge weight, etc.), and understand that changing one will have a non-linear affect on the cartridge s performance. 5. Conclusions It has been successfully demonstrated that the primer output can be measured using our custom, in-house apparatus. These measurements have provided great insight into the performance of the primers as well as the interactions between the primer and propellant. Since the primer contributes a significant influence on the overall cartridge performance, it would be beneficial to take a system level approach as opposed to an individual component approach when designing a cartridge. The force gauge adds a valuable tool in evaluating primer and cartridge performance. It captures data from the initial stages of the interior ballistic cycle that the mid-case pressure gauge will not capture. From the experiments performed it is evident that the solid propellant motion from primer action is significant, and influences overall cartridge performance with the potential for being responsible for cartridge malfunctions. The data collected from the primer force gauge can be used to evaluate primers that are suspect in malfunction scenarios, such as dropped primers. It can also be used to evaluate new primers and/or propellants to determine their level of performance in a new or existing weapon system. 6. Recommendations Pressure measurement techniques should be revisited in order to conduct the experiment with a greased gauge without heat shield per Kistler recommendation. The volumetric effects of the pressure gauge port hole should also be investigated. More data should be collected to investigate the primer stake method, and any effects it may produce in the cartridges performance. An area not closely examined is the role of the projectile de-bulleting during primer function. Such an occurrence creates inconsistencies in the cartridge volume thus altering the propellant s performance. Finally, novel experiments may be undertaken with the apparatus to investigate concepts such as the role of the cartridge s flash hole or multipoint ignition of the propellant. 20

31 To conclude, when evaluating primer performance and output, whether for an experimental or suspect primer, that the subject apparatus should be incorporated into the evaluation process. With experimental primers or even propellants, the apparatus can provide great insight into the interactions and performance of both the primer and propellant. This will benefit the experimentalist in their pursuit of the most optimized primer/propellant combination and, ultimately, optimized cartridge performance. 21

32 7. References 1. Schmidt, J. R.; Beyer, R. A. Experimental Studies of Primer Pushout Forces in 5.56-mm Ammunition, 56th JANNAF Propulsion Meeting, Las Vegas, NV, April Schmidt, J. R.; Beyer, R. A.; Brosseau, T. L. Small-Caliber Dropped Primer Phenomenology: Interior Ballistic and Push-Out Force Measurements of 5.56-mm Rounds; ARL-TR-5408; U.S. Army Research Laboratory: Aberdeen Proving Ground, MD, December Beyer, R. A.; Horst, A. W. Linking Ignition and Transient Projectile Forces in a Medium Caliber Cannon, 42nd JANNAF Combustion Meeting, Newton, MA, May Kistler Operating Instructions, Quartz High-Pressure Sensor Type 6215, Kistler Group, Winterthur, Switzerland, 2008 ( 5. Kistler Product Data Sheet, Load Washers, Type 9001A 9071A, Kistler Group, Winterthur, Switzerland, 2009 ( 6. Vest, D. C.; Anders, J. E.; Grollman, B. B.; DeMare, B. L. Ballistic Studies With a Microwave Interferometer Part I; BRL Report No. 968; U.S. Army Ballistic Research Laboratory: Aberdeen Proving Ground, MD, September Vest, D. C.; Anders, J. E.; Grollman, B. B.; Smith, H. C.; Kashihara, T. Ballistic Studies With a Microwave Interferometer Part II; BRL Report No. 1006; U.S. Army Ballistic Research Laboratory: Aberdeen Proving Ground, MD, September MIL-P-46610E. Percussion Primers for Small Arms Ammunition Specification, 5 November Beyer, R. A.; Colburn, J. W. In-chamber force and Pressure Measurements at 5.56 mm Caliber, 57th JANNAF Propulsion Meeting, Colorado Springs, CO, May Beyer, R. A.; Colburn, J. W. Primer Force and Chamber Pressure Measurements at 5.56 Caliber, 26th International Ballistics Symposium, Miami, FL, September Busky, R. Insensitive Munitions: Pyrotechnics Substitution for Explosives at Lake City, ATK Small Arms Systems, NDIA IM/IE Technical Symposium, Tucson, AZ, May MIL-C-63989C. M855 Military Specification, 15 February TM Army Ammunition Data Sheets, Small Caliber Ammunition, FSC 1305, 29 April

33 14. Williams, A. W.; Brant, A. L. Experimental Investigation of a Red Phosphorus Small- Caliber Primer; ARL-TR-4149; U.S. Army Research Laboratory: Aberdeen Proving Ground, MD, June Williams, A. W.; Brant, A. L.; Kaste, P. J.; Colburn, J. W. Experimental Studies of the No. 41 Primer and Ignition of 5.56-mm Ammunition, ARL-TR-3922; U.S. Army Research Laboratory: Aberdeen Proving Ground, MD, September Rosenberger, T. E.; Martz, R. L. A Method for the Real-Time Measurement of Projectile In- Bore Velocity; BRL-TR-3407; U.S. Army Ballistic Research Laboratory: Aberdeen Proving Ground, MD, September Michlin, A.; South, J.; Brosseau T. Effects of Lubrication and Pressure on Bolt Face Forces; ARL-TR-5377; U.S. Army Research Laboratory: Aberdeen Proving Ground, MD, October

34 List of Symbols, Abbreviations, and Acronyms F degrees Fahrenheit ATK dp/dt g GHz gr J ksi lb lb/v mm m/s oz psi/v RP Alliant Techsystems, Inc. rate of pressurization gram gigahertz grain joule thousands of pounds per square inch pounds pound per volt millimeter meters per second ounce pounds per square inch per volt red phosphorous µs microsecond 24

35 NO. OF COPIES ORGANIZATION 1 DEFENSE TECHNICAL (PDF INFORMATION CTR only) DTIC OCA 8725 JOHN J KINGMAN RD STE 0944 FORT BELVOIR VA DIRECTOR US ARMY RESEARCH LAB IMNE ALC HRR 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB RDRL CIO LL 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB RDRL CIO MT 2800 POWDER MILL RD ADELPHI MD DIRECTOR US ARMY RESEARCH LAB RDRL D 2800 POWDER MILL RD ADELPHI MD

36 NO. OF COPIES ORGANIZATION 4 COMMANDER US ARMY ARDEC AMSRD AAR AEE W E CARAVACA L LOPEZ M KAUFFMAN G. KAHN BLDG 382 PICATINNY ARSENAL NJ COMMANDER US ARMY ARDEC AMSRD AAR AEM I M VOLKMANN A ISMAILOV BLDG 65N PICATINNY ARSENAL NJ COMMANDER US ARMY ARDEC AMSRD AAR WSW F M MINISI K RUSSELL BLDG 65N PICATINNY ARSENAL NJ COMMANDER US ARMY ARDEC M NICOLICH BLDG 65N PICATINNY ARSENAL NJ COMMANDER US ARMY ARDEC S NICOLICH BLDG 3022 PICATINNY ARSENAL NJ US ARMY TACOM ARDEC CCAC AMSTA AR CCL B J MIDDLETON BLDG 65N PICATINNY ARSENAL NJ US ARMY ARDEC RDAR MEM I D GUBERNAT BLDG 65N PICATINNY ARSENAL NJ NO. OF COPIES ORGANIZATION 1 APM SMALL & MEDIUM CALIBER AMMO OPM MAS SFAE AMO MAS SMC G DEROSA BLDG 65N BLDG 354 PICATINNY ARSENAL NJ PM MAS SFAE AMO MAS SMC R KOWALSKI F HANZL P RIGGS J LUCID BLDG 354 PICATINNY ARSENAL NJ ST MARKS POWDER A GENERAL DYNAMICS CO J HOWARD C MEYERS R PULVER PO BOX 222 ST MARKS FL ATK SMALL CALIBER SYS J MACH M LEE D STUBLER T SPEARS PO BOX 1000 INDEPENDENCE MO ABERDEEN PROVING GROUND 29 DIR USARL RDRL WM S KARNA J MCCAULEY P PLOSTINS RDRL WML J NEWILL M ZOLTOSKI RDRL WML A W OBERLE J SOUTH RDRL WML B J MORRIS P KASTE RDRL WML D R BEYER A BRANT S HOWARD J RITTER 26

37 NO. OF COPIES ORGANIZATION A WILLIAMS M NUSCA P CONROY J SCHMIDT A HORST J COLBURN Z WINGARD RDRL WML F D LYON RDRL WML G T BROSSEAU A MICHLIN RDRL WML H T EHLERS L MAGNESS RDRL WMM B R KASTE 27

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