# Military

## Insensitive High Explosives [IHE]

In an effort to improve munitions survivability and safety, the Department of Defense (through the Joint Requirements Oversight Council) several years ago established a policy requiring all new munitions be capable of withstanding accidents, fires, or enemy attack. One method of addressing this requirement, the use of "Insensitive Munitions" (IM), including propellants and explosives, was mandated. Thus a new class of IM explosives has been developed over the past decade. Because these IM formulations differ somewhat from each other in a variety of ways (physical properties, explosive output, manufacturing process and cost, sensitivity and toxicity, etc.,) the explosive selection process for a given munition has become more complex. They must meet MIL-STD-2105, Hazard Assessment Tests, Non-Nuclear Munitions.

The US Air Force is developing an insensitive explosive fill for its general use bombs using a mixture of trinitrotoluene (TNT) and aluminum. Since the insensitive fill is not ready to be used in tactical bombs, and there is no available TNT in the stockpile, Joint Munitions Command (JMC) Bombs/Energetics Division awarded an indefinite delivery/indefinite quantity (IDIQ) contract for supply of TNT over a 5-year period to Alliant Ammunition and Powder Co. (AAPC). Virgin TNT will be supplied from a National Technology Industrial Base source, reclaimed and OCONUS TNT. The facility that produces the virgin TNT can be easily modified to produce other energetic materials, notably insensitive explosives. The IDIQ is delivering sufficient quantities of TNT to meet increased requirements. Partnering with major contractors has proved beneficial for current program execution. New partnerships are now being established with AAPC for TNT and General Dynamics Tactical and Ordnance Systems for bombs. Through these partnerships, communications will be improved, expectations will be better understood, common goals can be set, delivery times improved and problems identified so they can be resolved early on.

#### AFX-757

The Joint Air-to-Surface Stand-off Missile (JASSM) may become the first munition item to obtain insensitive munition (IM) certification and a 1.2.3 hazard classification. Currently, almost all munition items are hazard classified 1.1. This certification and classification reflect improvements in the munition that greatly reduce both the threat for accidental initiation of the item and the severity in case of an inadvertent initiation. The safety implications and reduced costs associated withstorage of such IM-compliant munitions are of significant benefit to boththe US Air Force and Navy customers.

With full support of the Air Force Research Laboratory Munitions Directorate's EnergeticMaterials Branch, the JASSM warhead and All-Up-Round passed some of the most difficult tests for obtaining IM certification and a reducedexplosive hazard classification (1.2.3). After a disappointing failure of the first JASSM warhead during sympathetic detonation testing, engineers from Lockheed Martin asked the Energetic Materials Branch to analyze the failure. Drawing on previous experiencein the development of IM-compliant Mk-82 bombs filled with the newly developed AFX-645 explosive, the directorate recommended a non-standard pallet stacking arrangement. This new stacking arrangement mitigates the energy transferred during sympathetic detonation from onemunition to the next. Lockheed Martin engineers tested this configuration in a hydrocode studyand confirmed that the directorate's suggestions did provide a significant improvement for survival. Lockheed Martin further improved this conceptby positioning the JASSM warheads side by side in a nose-to-tailconfiguration. The engineers placed the warhead casings as close aspossible, preventing a sympathetic detonation from occuring. Lockheed Martin engineers performed a new test using this storage configurationand successfully passed the sympathetic detonation criterion. The directorate and Lockheed Martin have accomplished all required IM classification testing. This is a major milestone since it is the first time amajor Air Force weapon system has passed all required IM testing. The Energetic Materials Branch developed AFX-757, the explosive fill used in JASSM, as a replacement for tritonal in the Miniature MunitionTechnology program. Lockheed Martin, the JASSM contractor, chose AFX-757 for their warhead because of its increased blast energy andpotential insensitivity.

#### DNI Dinitroimidazoles

Dinitroimidazoles, such as DNI (2,4-dinitroimidazole), are a group of very insensitive explosives.

4,5-dinitroimidazole (45DNI) crystallizes with two crystallographically unique molecules in the monoclinic space group P21/n (#14) with unit-cell parameters a = 11.5360(8) Å, b = 9.071(1) Å, c = 11.822(1) Å, ß = 107.640(6)°, Z = 8, and has a density of 1.781 g/cm3. The molecular packing consists of infinite one-dimensional chains of 45DNI molecules approximately oriented in the ac direction which are linked by two different hydrogen bonds, {\rm [N(1)-\hbox{-}H(1)\cdots N(31)} and {\rm N(11)-H(11)\cdots N(3)]}. In the lateral directions the chains are held together by molecular forces.

#### LX-17

Only the TATB-based formulations of Livermore's LX-17 and Los Alamos's PBX 9502 are considered "insensitive" high explosives (IHE) for nuclear weapons; others are termed "conventional."

#### MNX-194

MNX-194 in a melt castable, wax binder explosive fill to replace TNT in Army M107/M795 155 mm artillery rounds. Munitions Directorate researchers, funded by the US Army Tank-Automotive and Armaments Command/Armament Research, Development and Engineering Center, developed MNX-194, a qualified replacement for Trinitrotoluene (TNT) in both the M107 and M795 155 mm artillery rounds. Directorate researchers from the High Explosives Research and Development Facility developed three compositionally equivalent versions of MNX-194 in which Cyclotrimethylenetrinitramine (RDX) is the sole energetic component.

The primary difference between the three versions is the source and/or pretreatment of the RDX component. Directorate researchers characterized all of these novel wax binder explosive formulations for both small-scale safety and shock sensitivity, and performance. Their research also shows RDX is more powerful than TNT in similar test configurations. Additional characterization and optimization work is currently under way.

Beyond MNX-194 applications for artillery hardware, the Air Force is considering this formulation as a fill for other Air Force applications. In particular, the directorate is developing an aluminized version of the MNX-194 binder matrix as a potential candidate replacement for the TNT-based fill in Mk-series bombs.

The current usage rates and depletion of the Department of Defense stockpile of TNT is causing many program managers to revisit formulation options such as the directorate's bomb fill replacement effort. With a melting temperature similar to that of TNT (~80°C) and the ease of processing this wax binder system, the Air Force considers MNX-194, or modifications thereof, an ideal contender for any TNT replacement program.

#### PAX

Since the mid 1980s, Picatinny has developed over 24 Picatinny Arsenal Explosive (PAX) formulations. New combinations of energetic "fill" binders and, in some cases, plasticizers continue to be evaluated in search of the Army's next generation more powerful explosive.

One of the most significant challenges to Picatinny was the development of a new main charge melt-pour energetic, PAX-21. This new explosive is designed to be low cost and requires little or no refacilitization for production or projectile filling. It contains no TNT and is slightly less toxic than the Composition-B it replaces. Not only is it safe for use in production, PAX-21 also exhibits good IM and thermal stress characteristics and low shock sensitivity.

PAX-2A was the Army's first high performing IM (insensitive munition) explosive. It is less sensitive to initiation by outside stimuli, but retains all the requisite performance capabilities of the high explosive that was used in the past. It has matured through extensive loading, performance testing and hazard threat analysis testing in various current and future warhead configuration of the Army, Navy, and Air Force munitions systems. This IM explosive is now considered to be a viable alternative to current HMX formulations and has been successfully demonstrated in Hellfire, Javelin, M830A1, HE-WAM, SADARM, and XM915 Dual Purpose Improved Conventional Munitions (DPICM) XM80 grenade submunitions.

The vast majority of cannon lunched unitary warheads use melt pour explosives for cost and surge capability. Traditional melt pour explosives have focused on fragmentation capability. A new family of low cost reduced sensitivity melt pour explosives based on 2,4-dinitroanisole, RDX or HMX and AP has been developed in response to Insensitive Munition [IM] requirements.

Development of Insensitive Munition [IM] melt pour explosives has been next to none. Picatinny Arsenal and Thiokol Propulsion developed the first melt pour explosive (PAX-21) to exhibit IM properties (currently in production). The PAX-21 success led to increased interest in all areas of IM melt pour explosives I.e., cost, producibility, facilitization, etc.

Family of PAX

• PAX-21- Comp B replacement: RDX, DNAN, AP and trace amounts of MNA (for processability) currently in production
• PAX-24 - TNT replacement: DNAN, AP and MNA
• PAX-25 - Comp B replacement: RDX, DNAN, AP and MNA (different proportions for RDX, DNAN, and AP) better performance than PAX-21
• PAX-26 - Tritonal replacement: DNAN, Al, AP, MNA
• PAX-28 - Unitary warheads: RDX, DNAN, Al, AP, MNA. An equivalency factor of 1.62 was determined between Composition B and PAX-28
• PAX-40 - Octol replacement: HMX, DNAN, MNA
• PAX-41 - Cyclotol replacement: RDX, DNAN, MNA

One advantage is the ease of loading of melt pour explosives into various munition items. Typically less expensive than pressed explosives to manufacture, load and facilitization. Increased IM characteristics without decreasing performance. Performance and shock sensitivity can be tailored for a given system based on particle size and the percentage of ingredients. PAX-40 & 41 exceed COMP B's detonation velocity. PAX-40 & 41 are less shock sensitive than COMP B.

#### PBXIH-135

The navy's insensitive munitions advanced development program for high explosives (IMAD/HE) has developed a new insensitive, cast-cured pbx called PBXIH-135. PBXIH-135 has enhanced internal blast performance and improved vulnerability and penetration survivability characteristics compared to PBXN-109. PBXIH-135 was subjected to insensitive detonating substance (EIDS) testing to include cap, gap, external Fire, slow cookoff, and friability.

The Navy began developing thermobaric explosives in the late 1980's and resumed research and development in the mid 1990's, responding to the need for internal blast explosives to defeat hard and deeply buried structures as evidenced during Operation Desert Storm. NSWC Indian Head scientists developed the PBXIH-135 thermobaric explosive, which not only offers effective blast and thermal effects, but also is extremely insensitive to factors that may cause accidental detonation during transit or storage. The secret to PBXIH-135 is the addition of a precise mixture of aluminum powder, which burns in the hot gases. Long after the initial shock wave, the burning aluminum sends heat and pressure bounding through corridors.

In response to the Sept. 11, 2001 terrorist attacks on the United States, the Defense Threat Reduction Agency (DTRA) organized a 60-day joint project with NSWC Indian Head, the Air Force and Department of Energy to identify, test and integrate a solution to deliver a new capability for tunnel defeat. NSWC Indian Head was responsible for the payload and booster design, as well as loading of the new bombs.

After static and flight tests at full-scale tunnel facilities at the Department of Energy's Nevada test site, the program culminated in December with a successful flight test of a laser-guided weapon, containing Indian Head's PBXIH-135 thermobaric explosive, launched from an F-15E Strike Eagle. NSWC Indian Head, along with DTRA and the Air Force, are engaged in a three-year advanced Concept Technical Demonstration of another thermobaric weapon. Indian Head is developing the new payload, which will have superior performance to that of PBXIH-135.

#### PBXN-109

Two different energetic composite formulations can be used in hard target penetrator warheads: PBXN-109 and AFX-757. Four explosive formulations have been evaluated for the Mk-83 warhead. The four candidate formulations: AFX-777, AFX-757, PBXN-111 and PBXW-129 were tested against the Mk-83 baseline fill, PBXN-109.

Two test series involving the static detonation of a new design Hellfire missile warhead, now designated as a type N thermobaric warhead, were conducted in 2002 to determine fragment spatial, mass, and velocity distributions. The data from the type N tests are compared with the performance of a hellfire type M blast-frag warhead (BFWH) loaded with the conventional explosive PBXN-109. Of particular interest in the tests was the assessment of thermobaric phenomena with regard to warhead effectiveness.

The cookoff of energetic materials involves the combined effects of several physical and chemical processes. These processes include heat transfer, chemical decomposition, and mechanical response. The interaction and coupling between these processes influence both the time-to-event and the violence of reaction. The prediction of the behavior of explosives during cookoff, particularly with respect to reaction violence, is a challenging task. To this end, a joint DoD/DOE program has been initiated to develop models for cookoff, and to perform experiments to validate those models. In this paper, a series of cookoff analyses are presented and compared with data from a number of experiments for the aluminized, RDX-based, Navy explosive PBXN-109.

Computational tools are being developed to predict the response of Navy ordnance to abnormal thermal (cookoff) events. The Naval Air Warfare Center 1 (NAWC) and Naval Surface Warfare Center (NSWC) are performing cookoff experiments to help validate DOE computer codes and associated thermal, chemical, and mechanical models. Initial work at the NAWC is focused on the cookoff of an aluminized, RDX-based explosive, PBXN-109 that is initially confined in a tube with sealed ends. The tube is slowly heated until ignition occurs. The response is characterized using thermocouples, strain gauges, and high-speed cameras. A modified version of this system is being developed at the NSWC. The designs of these cookoff systems are relatively simple to facilitate initial model development. An effort is being made to achieve a wide range of results for reaction violence.

Lawrence Livermore National Laboratories (LLNL) and Sandia National Laboratories (SNL) are developing computer codes and materials models to simulate cookoff for ordnance safety evaluations. The computer program ALE3D from LLNL is being used to simulate the coupled thermal transport, chemical reactions, and mechanical response during heating and explosion 2 . SNL is employing multiple computer codes in a parallel effort 3,4,5 . For the analysis of PBXN-109 cookoff, Schmitt et al.6 performed an initial survey of measured materials properties and provided estimates for several others.

#### RS-RDX Reduced Sensitivity RDX

In the late 1990s SNPE (SME now Eurenco) marketed an "insensitive" form of RDX (IRDX®) produced by the Woolwich synthesis. It employed proprietary recrystallization process. This produced RDX that displayed reduced sensitivity to shock initiation as measured by Large Scale Gap Test. In 2001 Army pursued an FCT program to evaluate IRDX® in 155 mm projectile with the goal to determine whether IRDX® would improve the IM characteristics of the projectile and to determine whether the SNPE crystallization process could be implemented at Holston (Bachmann synthesis) to produce reduced sensitivity RDX. SNPE recrystallized HSAAP RDX into IRDX®. The US Army, Navy and AF evaluated 2 formulations using both IRDX® and HSAAP IRDX®: Wax-based melt castable explosive (Air Force); and Cast curable PBX explosive (Navy). They conducted performance and IM testing. Upon aging, the HSAAP RDX recrystallized by SNPE did not retain the original "insensitive" characteristics.

Subsequently, other manufacturers claimed to produce forms of RDX that exhibit reduced sensitivity to shock relative to conventional RDX as produced by the Bachman process. In 2003, Army published a sources sought solicitation. The following manufacturers are now claiming to make Reduced Sensitivity RDX (RS-RDX): EUROENCO (aka SNPE): IRDX®; Australian Defence Industries (ADI): Grade A RDX; Royal Ordnance Defence (RO): Type I RDX; and Dyno Nobel: RS-RDX. Of these, only Dyno employs the Bachman process.

What makes RDX insensitive? No definitive explanation has been offered for the insensitivity of the RDX from these manufacturers. Crystal quality of some sort appears to be involved. No parameter that can be measured on the crystalline material has been identified that will enable one to distinguish between relatively more sensitive and relatively less sensitive forms.

#### TATB (triamino-trinitrobenzene)

One of the most important accomplishments made by nuclear weapons laboratories' chemists in the past two decades has been the formulation of powerful conventional high explosives that are remarkably insensitive to high temperatures, shock, and impact. These insensitive high explosives (IHEs) significantly improve the safety and survivability of munitions, weapons, and personnel.

The Department of Energy's most important IHE for use in modern nuclear warheads is TATB (triamino-trinitrobenzene) because its resistance to heat and physical shock is greater than that of any other known material of comparable energy. The Department of Energy currently maintains an estimated five-year supply of TATB for its Stockpile Stewardship and Management Program, which is designed to ensure the safety, security, and reliability of the U.S. nuclear stockpile. The Department of Defense is also studying the possible use of TATB as an insensitive booster material, because even with its safety characteristics, a given amount of that explosive has more power than an equivalent volume of TNT.

The compound 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is a reasonably powerful high explosive (HE) whose thermal and shock stability is considerably greater than that of any other known material of comparable energy. The high stability of TATB favors its use in military2 and civilian applications3 when insensitive high explosives are required. In addition to its applications as a HE, TATB is used to produce the important intermediate benzenehexamine. Benzenehexamine has been used in the preparation of ferromagnetic organic salts and in the synthesis of new heteropolycyclic molecules such as 1,4,5,8,9,12-hexaazatriphenylene (HAT) that serve as strong electron acceptor ligands for low-valence transition metals.

In addition to its military uses, TATB has been proposed for use as a reagent in the manufacturing of components for liquid crystal computer displays. The use of TATB to prepare components of lyotropic liquid-crystal phases for use in display devices is the subject of a German patent. There is also interest in employing the explosive in the civilian sector for deep oil well explorations where heat-insensitive explosives are required. Despite its broad potential, the high cost of manufacturing TATB has limited its use. Several years ago, TATB produced on an industrial scale in the U.S. was priced at $90 to$250 per kilogram. Today it is available to customers outside DOE for about $200 per kilogram. In response to a need for a more economical product, chemists at Lawrence Livermore have developed a flexible and convenient means of synthesizing TATB as well as DATB (diamino-trinitrobenzene), a closely related but less well known IHE developed by the U.S. Navy. The initial phase of this work was funded by the Department of Defense (U.S. Navy) to explore the chemical conversion of surplus energetic materials to higher value products as an alternative to detonation. The Lawrence Livermore process--also called the VNS (vicarious nucleophilic substitution) process--should be able to produce TATB for less than$90 a kilogram on an industrial scale in about 40% less manufacturing time. The process also offers significant advantages over the current method of synthesis in environmental friendliness, for example, by avoiding chlorinated starting materials.

#### TNAZ

1,1,3 Trinitroazetidine (TNAZ) is a material that is more powerful, but less-sensitive than HMX. The advent of the new high-energy explosive CL-20 and TNAZ present the possibility of increased performance high explosives with reduced sensitivity. A nitrogen-rich compound, TNAZ can itself be melted and moulded. But money was an issue. It costs just a few tens of dollars to produce a kilogram of HMX or RDX, but about \$200 to create the same amount of TNAZ.

Most of the effort for producing the next generation of energetic materials is currently centered around the production of 1,3,3-trinitroazetidine (TNAZ). Researchers have evaluated five synthetic routes for producing TNAZ. The two most likely methods to manufacture TNAZ in a sustainable green manufacturing process are those due to Axenrod, and Coburn and Hiskey.

work funded by ARDEC led to the synthesis and process for the commercial scale-up of 3,3,1-trinitroazetidine (TNAZ), a strained ring Heterocyclic nitramine. TNAZ is one of the few new energetic materials found to be thermally stable above its melting point. However, in formulations studies, it has been found that TNAZ has high volatility that will severely inhibit its utility in military explosive and propellant applications. Further limitations to its use include the processing, polymorph, and material costs.