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6 Advanced Gun Propellants CURRENT RESEARCH FOCUS The status of advanced gun propellants was obtained from presentations to the committee given by or discussions held with Army and Navy researchers from ARDEC, ARL, NSWC/IH, and ONR.~-3 In order to meet the requirements of future warfighting concepts that call for the use of smaller, lighter, more lethal forces with minimal logistics tails, both Army and Navy researchers recognize the need to develop new and improved gun propellants. Medium- and large-caliber guns will continue to play a major role in these concepts, but barrel sizes must be reduced and munitions must be made smaller, lighter, more lethal, and longer-range. For example, to meet the needs of the Army's Future Combat System, the size of the main tank gun, currently at 120 mm, must be significantly reduced, but at the same time muzzle energy must be increased by 25 percent. The Navy will depend more on 5-in. guns that fire extended-range munitions. To reduce the logistics burden further, gun tube wear must be reduced. New energetic materials and geometries must be used if these needs are to be met. Additionally, health and safety concerns establish needs for environmentally safer, or "green," munitions that are insensitive to harsh handling and less vulnerable to attack. Advances in propellants alone cannot meet all of these needs. There must be synergistic design of the barrels, breaches, recoil systems, munitions, and propellants. As in the case of new energetic materials research and development, the number of U.S. researchers actively working on the formulation and development of advanced gun propellants is a group of fewer than 25 dedicated individuals. Researchers in this area are concentrated at ARDEC, AR L, NSWC/IH, and a small number of industrial corporations. Significant research and development efforts in high-performance gun propellants are going on throughout the world, most notably in Germany and Switzerland. The efforts of foreign A.B. Forch Horst. 2002. Review of Army Advanced Energetic Materials Programs and Facilities, U.S. Army Research Laboratory Weapons and Materials Research Directorate. Aberdeen Proving Ground, Md. May 7. 2 M.N. Maddinec, J. Pertucci, J. Brough, and D. Cichra. 2002. Discussion of advanced energetic materials at Indian Head. Teleconference with the committee held on May 31. 3 J.M. Goldwasser, ONR. 2001. Presentation to the committee. July 31 28
AD VANCED G UN PROPELLANTS 29 researchers have led to the development of propellants with reported performance advantages over currently fielded U.S. propellantsthe performance of the former is relatively flat over a wide temperature range.4 The most promising research in progress for Army applications involves the use of new layered propellants that use new energetic ingredients. Navy researchers are likewise very interested in the use of layered propellants to enhance performance, but are also strongly motivated to find ways to reduce barrel erosion and thereby enhance barrel life. Both Army and Navy approaches employ advances in propellant-processing technologies that have matured significantly during the past decades. Additional details of these two research thrusts are discussed below. It should be noted that no significant research in gun propellants is being conducted by the lJ.S. Air Force. CURRENT GUN PROPELLANTS All those who made presentations on gun propellants noted that over the past 30 years, the basic ingredients in fielded propellants have remained the same. The most important of these ingredients include nitrocellulose, nitroguanidine, nitroglycerin, and other nitrate esters. No propellants using new energetic molecules have been fielded. What has improved during this time period is the processing, formulation, and manufacturing of widely used, legacy molecules. Examples of uses for these materials are shown in Table 6-1. TABLE 6-1 Comparison of Formulations (in percentage) for Propellant Materials Propellant Material M1 M30A2 Low-Vulnerability Ammunition (LOVA) Nitrocellulose 83.11 88.00 27.00 4.00 Nitroglycerine O 0 22.50 0 Nitroguanidine O 0 46.25 0 Cyclotrimethylene trinitramine (RDX) ~ O O 0 76 00 cycl otetra m ethyl e n etetra n itra m i n e ~ H MX) Dinitrotoluene (DNT) Dibutylphthalate (DBP) + diphenylamine (DPA) Diethylene glycol dinitrate (DEGDN) Ethyl centralite Potassium nitrate 9.77 5.87 o o o 7.82 2.93 o o o o o o 1.50 2.75 o o o 0.40 o Other 1.25 1.25 0 19.60 SOURCE: N. Eldredge, Picatinny Arsenal, 2001. Presentation to the committee. December 13. New, recently fielded U.S. propellants have compositions and performance characteristics very similar to those of the formulations listed in Table 6-1. For example, Propulseur d'Appoint a Poudre (PAP 7993) solid propellant (a joint development between ARDEC and industry) is very similar to M1 but uses an environmentally acceptable plasticizer to replace dinitrotoluene (DNT). While this propellant shows improvement in the environmental area, it does not provide any added performance. 4 R.L. Simmons and B. Beat-Volgelsanger.2000. Introduction to NitroChemie El Gun Propellant. Presentation to 37th JAN NAF Combustion Meeting, Monterey, Calif., November. CPIA Publication 701:201-205.
30 ADVANCED ENERGETIC MATERIALS ADVANCED GUN PROPELLANT RESEARCH The challenge with respect to propellants research and development is to enhance propellant performance significantly, while taking into account such objectives as reduced pun tube wear. lower flame temperatures of orooellants. and "soft launch" cacabilitv for ~ . ~ . . . . . . . . . . smart munitions, among other attributes. Research and development efforts In propellants have focused on performance, increased survivability, reduced vulnerability and sensitivity, and enhanced safety during transport and use. Efforts should continue in these areas. To reduce the size of munitionsthus allowing reductions in casing, barrel, and breach sizesit is recognized that focusing solely on propellants will not provide an adequate solution. Propellant developers will need to work with gun tube designers to increase the size of the breach relative to smaller gun tubes in order to maintain the volume available within the casing for propellants. Such cooperative efforts among propellant and gun tube designers offer potential for improved systems and should continue. Army Research Activities To improve propellant performance so that it reaches the goal of increasing muzzle energy by 25 percent without increasing barrel wear, ARL and ARDEC are currently exploring energetic formulations based on thermoplastic elastomers (TPEs). These formulations include the use of new, higher-energy fillers (such as CL-20) and nanostructured energetic materials. The primary driver for the use of these new TPE-based propellants has been their excellent performance coupled with relatively low flame temperatures (see Figure 6-1~. In addition to the advantage just noted, TPE propellant may be used in advanced layered geometries. Typical layered geometries are shown in Figure 6-2. When propellants are manufactured in these configurations, a relatively slow burning propellant is used on the outer layers and a faster-burning composition is used in the center core. Propellant geometries are tightly controlled to enable the inner-core propellant to begin burning as the projectile moves down the bore. This allows the pressure to be maintained at a high level for a relatively long duration and often results in a double hump in the pressure time response, as shown in Figure 6-3. 4000 3500 3000 _ c 2500 - LL 2000 NCiNE f'ropel~ -- - Adv?nced TRE . Pro,oellants 900 1 000 1 1 00 1 200 Im petus f Jig ~ 1300 1400 FIGURE 6-1 Calculated impetus and flame temperature for conventional (nitrocellulose ENC] and nitrate esters LI\IE]) and thermoplastic elastomer (TPE) propellants.
ADVANCED GUN PROPELLANTS Disk Slow Burning |~: Rate Outer Layer i/ / ,' ~ \ Strip / I' ~ 1 ~ ~ h ~ ~ ;~ t ~ ~ ~ ~ \\ \\ Fast Burning Rate Inner Layer Wrap FIGURE 6-2 Typical layered geometries of TPE propellants. FIGURE 6-3 Pressure time trace from the firing of a 120-mm gun using layered TPE propellant. The use of these layered propellants results in improved energy transfer to the kinetic energy of the round and hence in higher muzzle energy.5 6 Despite these promising results, no layered propellants are used in currently fielded systems. The new propellants under development by the Army have thermochemistry different from that of traditional nitrocellulose-based propellants. The impact of these differences in combustion products on gun bore erosion is not known. To begin evaluation of the erosivity of new propellants, researchers at ARL have been using a subscale erosion tester. It is anticipated that this device will be very useful in determining comparative erosivity of new and current propellants. At the same time, Army researchers are evaluating tantalum coatings or ceramics on the surface of the barrel bore. To support this effort further, modeling is being done at both the molecular and the macroscopic level to assess thermochemical reactions and surface kinetics, respectively; the results are being validated with experimentation. The advanced layered propellants under development by the Army have exhibited problems with sensitivity. ARL and ARDEC have initiated a collaborative advanced technology directive focused on insensitive high-energy munitions. A key test in assessing the vulnerability of propellants is the pendulum test developed at ARL. In this test, a shaped charge jet attenuated through a conditioning plate of rolled homogeneous armor challenges a propellant sample (see Figure 6-4~. The violence of the reaction is then compared with a baseline such as JA2 (a nitrocellulose gun propellant 5 Joseph A. Lannon, RDC/Picatinny. 2001. Presentation to the committee. July31. 6 N. Eldredge, Picatinny Arsenal. 2001. Presentation to the committee. December 13. 31
32 ADVANCED ENERGETIC MATERIALS developed in Germany). According to recent results reported to the committee, a high-energy layered propellant can be similar in reaction violence to the baseline JA2 if the geometry of the layered propellant is chosen properly.7 The sensitivity of the test result to geometry was large, and more work would have to be done in this area. ~~ [Shaped Charge I ~ :~:~:~:~:~:~ ~ .. Wow_ __! _~ ~ ~ Conditioning Armory ~ _ _ . ... _ _ - _ .~ Pendulum Face FIGURE 6-4 Typical test setup for a pendulum test used to evaluate advanced layered propellant. Chemical propulsion research in the Army is not all internal. A significant amount of research is solicited from academia, national laboratories, and industry. Eleven universities are addressing topics including the theory of energetic reactions, heat and shock pulses on energetic materials, high-temperature energetic kinetics, and seven others. Research in the physics and chemistry of propulsion will further validate molecular and macroscopic modeling. In a parallel and complementary effort, the Army's Electrothermal Chemical (ETC) gun program has been a prolonged technological effort to produce a gun in which electrical energy is used to augment and control the release of the chemical energy of the propellant. The muzzle energy comes entirely from the chemical energy, rather than from the electrical energy. Significant performance enhancements using ETC technology with existing or advanced propellants have been demonstrated. The ETC effort should continue to work on the development of high-energy propellants possessing acceptable vulnerability characteristics. The present program is focusing on the identification of gun propellants with the desired properties and on the extension of ETC technology to medium-caliber guns. Reduction to a fielded system remains many years away. Navy Research Activities As with the Army, the Navy has some very strong points, as well as some shortcomings in its propellant programs. Research in Navy propellant design has suggested areas in which the Navy proposes to continue work. For example, a layered gun propellant approach came out of Navy research in ETC propulsion design. In collaboration with the Army, the Navy is 7 P.C. Braithwaite. 2002. Update on Advanced Gun Propellant Efforts, presentation to the committee during its visit to ATK Thiokol Propulsion, Brigham City, Utah, May 22.
AD VANCED G UN PROPELLANTS 33 looking at low-CO-content propellants with high-nitrogen compounds such as 1,5-diazido-3- nitraza pentane (DANPE). The theoretical advantage of using materials such as DANPE is illustrated in Table 6-2. These numbers are especially impressive when compared with those for current propellants such as JA2, which has similar flame temperatures and an impetus of 1151 J/g. TABLE 6-2 Theoretical Benefits of Systems Using DANPE Ingredients Impetus (J/g) Tv (K) Gas MW TNAZ + DANPE (40/60) 1439 3490 20.16 RDX + DANPE (55/45) 1425 3497 20.40 CL-20 + DANPE (40/60) 1419 3527 20.67 NOTE: The acronyms are spelled out in Appendix C. SOURCE: R.L. Simmons. 1996. Guidelines to Higher Energy Gun Propellants. Paper 22 in Proceedings of the 27th International Conference of Institute of Chemical Technology, KarlsruLe, Germany. June 25-28. The Navy is also evaluating new energetic ingredients such as Field Operating Activity (FOA) Organic Explosive 7 (FOX-7y, FOA Organic Explosive 12 (FOX-12), dihydrazinotetrazine (HzTz), triaminoguanidinium azobitetrazolate, and bis-aminotetrazolyl-tetrazine (BTATz) as new ways of tailoring propellant burning rates to meet the demands of layered propellants.s 9 As with the Army propellants, the first layer provides a relatively cool burn, while the second layer releases much higher energy. This approach maintains a stable gun pressure that increases muzzle energy, as described above, but at the same time it is predicted not to increase tube wear significantly. More research is needed to see what happens at the boundary layer of these two materials, especially when they are made of dissimilar compounds. The Navy has also had success in using energetic thermoplastic binders to develop green propellants.~° These binders are costly, so the Navy is looking at twin screw continuous extrusion processes to compensate for the higher material costs by reducing manufacturing costs. Using this technology, Navy researchers at NSWC-IH have successfully processed sufficient TPE-based propellants through a twin screw extruder to support a series of 5-in. gun firings. Figure 6-5 shows the extrusion of a slab material through a twin screw extruder (TSE). In this case a single material is being extruded. Research is currently being pursued at NSWC-IH to explore the possibility of using two separate TSEs to extrude layered propellant in a single operation. ~ It should be noted that this technology has been successfully used in the food processing industry for several years. Whether it will be possible to manufacture layered gun propellants effectively and efficiently using this technology remains to be demonstrated. R.J. Cramer. 1998. Advanced Gun Propellants. Presentation at 35th JAN NAF Combustion Meeting, Tucson, Ariz., December 7-11. 9 C. Walsh.2001. Advanced Gun Propellant Formulations. Presentation at Energetics for Naval Gun Ammunition Technical Exchange Workshop, Waldorf, Md., October 23. ~°J.M.Goldwasser,ONR. 2001. Presentationto the committee. July31. it C. Walsh and C. Knott. 2002. Advanced Gun Propellant Formulations. Presentation at the 2002 National Defense Industrial Association Guns and Ammunition Symposium, Panama City, Fla., April 15-16.
34 FIGURE 6-5 Slab extrusion using a twin screw extruder. FINDINGS AND RECOMMENDATIONS ADVANCED ENERGETIC MATERIALS New technologies offer promise in advancing the state of the art in propellants. However, it is the opinion of the committee that reduced, unstable funding has significantly affected these efforts. Limited 6.2 funding in this area reduces the ability to exploit advances in basic research, and most 6.3 funding is being used to better package old technologies. As a result of this lack of funding and focus, the workforce and the facilities for gun propellants continue to age. If this trend persists, technological innovations will be severely hampered and the recruitment of the brighter minds in the field will be difficult. Specific technical recommendations of the committee are as follows: . · The development of high-energy layered propellants with a focus on vulnerability and producibility should be continued. · The exploration of high-nitrogen compounds as novel gun propellant ingredients in a variety of configurations should be pursued, with a requirement for an early demonstration. System-level efforts should be continued at a modest level for barrel wear, chamber design, and modeling. i2 The Army and the Navy have numerous codes that model these systems. However, the committee is not able to specify a particular modeling technique. The perception of the committee is that with the improvements in modeling turbulent flow and erosion phenomena, the continuation of modeling efforts is warranted.