Research and Development of High-performance Explosives

Development of high explosives for military use involves extensive safety considerations and resource limitations due to test facility requirements. At the US Army Armament Research and Development and Engineering Command (ARDEC), Picatinny Arsenal, explosives are evaluated from the research level through full lifecycle monitoring and demilitarization. New explosives that are safer for handling, storage, and loading are continuously evaluated in an effort to provide effective and safe munitions for the warfighter. Recent law dictates that whenever possible, Insensitive Munitions (IM) guidelines and requirements are followed. Therefore, whenever new explosives are synthesized and formulated, performance testing is paramount to ensure they meet user requirements. In this context, the measurement of detonation properties of newly developed PAX-30 is compared with the PBXN-5, a traditional high performance explosive. In particular, measurement of its detonation velocity and detonation pressure, which are important for verification of theoretical models and performance calculations, is shared. The PAX-30 was developed to replace legacy explosives such as PBXN-5 by using reactive aluminum.

Aluminum possesses a high enthalpy of oxidation as aluminum on a per molar basis:

2Al + 3/2 O₂  -> Al₂O₃ (1,670 kJ/mol)

By adding aluminum in place of the shock sensitive explosive ingredients, the formulation is rendered more safe to external shock and hazard insults. This effectively helps fulfill Insensitive Munition (IM) United Nation requirements while at the same time maintaining the performance necessary for military applications.^2,3.4

The facilities to test such items are unique and highly specialized. Some initial tests are performed to screen explosives before handling in large quantities. These tests include thermal characterization with differential scanning calorimetry (DSC) and impact and friction tests. For the DSC tests, a small test sample is heated at a constant rate in an inert atmosphere, and the amount and direction of heat flow is monitored. For impact and friction tests, the sample is subjected to insults from a standardized falling weight (Bundesanstalt fur Materialprufung, or BAM Impact), and for the friction test a standardized ceramic pin and plate (Bundesanstalt fur Materialprufung, or BAM Friction).⁵

Once the formulations are deemed safe for handling, further scale-up is accomplished by proprietary mixing technologies. In short, high explosives fall into three categories:

Melt-cast, in which the binder is a melt-phase material like a wax, trinitrotoluene (TNT), dintroanisole (DNAN), or other meltable material. Energetic or fuel solids can be incorporated with careful consideration of particle size and compatibility.

Cast-cure, in which the binder is a castable polymer, such as hydroxyl-terminated polybutadiene (HTPB), polyacrylate, or other epoxy-type plastic that is liquid in its unreacted state, but upon initiation solidifies to a solid. Solids are incorporated into the matrix during its liquid state.

Pressed, in which the solids loading is very high, often approaching nearly 95% by weight, with a binder that is added to coat the solids using a lacquer or extrusion process.

Once pressed or cast, the materials are machined using standard methodologies to obtain proper geometry for a desired test. In this paper, PAX-30 and PBXN-5 are high performance pressed explosives. The formulations are made through a slurry-coating process, in which energetic nitramine crystals (HMX, RDX, or CL-20) and aluminum particles are suspended in an aqueous solution. A lacquer with the proprietary binder is then added. Upon lacquer addition, the polymer coats the explosive crystals, the suspension is heated under vacuum to drive off the solvent, and the particles are then filtered and dried. The granule-like particles are then pressed to the configuration desired.

Detonation Velocity

In order to determine the detonation velocity, one must monitor the arrival of the detonation front in the material. A detonation is defined as a self-sustaining instantaneous rise in pressure and temperature that is faster than the sound speed in the material. It becomes self-sustaining once the temperature and pressure are sufficient to provide exothermic reactions behind the propagating reaction front. Such behavior is realized by incorporating oxidizing moieties such as nitrate groups in certain materials of the formation. Two examples known as RDX (cyclo-1,3,5-trimethylene-2,4,6-trinitramine) and HMX (cyclotetramethylenetetranitramine) are shown in Figure 1, which by and large are the most used energetic materials in the US DoD (Department of Defense). Note the oxygen balance of the molecules, which results in the self-propagating exothermic reaction behind the shock front.

Figure 1. RDX (cyclo-1,3,5-trimethylene-2,4,6-trinitramine, left) and HMX (cyclotetramethylenetetranitramine, right).  Please click here to view a larger version of this figure.

One way to determine the speed of the detonation front is to monitor its position as a function of time. Fiber-optic detonation velocity (FODV) testing is performed to determine the detonation velocity of an explosive material. An acrylic fixture was designed to hold the explosive sample, and locate the optical fibers at known distances down the charge length. The standard test uses a 5-inch long by 0.75-inch diameter explosive sample with five total optical fibers; the bottom fiber is located 0.50-inch from the bottom of the charge and each successive fiber is located 1-inch above the next. The holes drilled in the acrylic fixture are two-stepped holes. The larger diameter hole is sized to fit the core and cladding of the optical fiber and the smaller diameter hole serves as a confined air space. As the detonation progresses through the explosive sample, the shock wave produced excites the confined air space producing a short, bright flash that can be observed with the fiber-optics.

The fiber-optics used for this test possess an inexpensive plastic core. Due to the destructive nature of the test and the consistency of the air shock, higher quality fibers were not found to be necessary to maintain high quality velocity data. The test facility at Picatinny Arsenal uses summed photodiodes to translate the light from the detonation into voltage. The amplitude of the voltage spike is unimportant for the purposes of this test. A 1-GHz oscilloscope is connected to the photodiode summing box, although that sampling rate is far beyond what is necessary for this test. The optical fiber “peaks” can be either determined by first rise of the signal or peak values. Given the distance between optical fibers and the time difference between detonation arrival, detonation velocity is then determined.

Detonation Pressure

Detonation pressure is estimated by measuring the dent depth in a standard steel plate resultant from the explosive’s detonation. Dent depths are well correlated to known pressure values for a variety of explosive compounds. Usually, since most explosives satisfy the Chapman-Jouguet (CJ) condition for a detonation to occur, the detonation pressure is typically referred to as CJ pressure, and it will be from this point forward in this article. The charge assembly is placed on top of a steel plate, called a “witness plate”, and the detonation results in a dent in the plate. The dent depth at the standard 0.75-inch charge diameter for numerous explosive materials with known detonation pressures is then compared to the test dent depth. Detonation pressure by the plate dent is a reliable method with many years of documented data for acceptable correlations. However, a detonation is a dynamic, fast chemical reaction, and in recent years it has become desirable to utilize tools with higher resolution to observe the pressure-time history.

To directly measure the detonation pressure of an explosive, Photonic Doppler Velocimetry (PDV) can also be used. This laser interferometer system was developed by Lawrence Livermore National Laboratory and utilizes a 1,550 nm CW laser source. By directing the laser at a moving target and collecting the Doppler-shifted light, the resulting beat frequency can be analyzed to provide a velocity trace of the target. Unlike traditional high-speed photographic techniques, these velocity traces provide a continuous record of the target’s velocity as a function of time. This measurement technique has gained significant attention in the last few years and is becoming ubiquitous in DoD and Department of Energy (DoE) explosive characterization labs.

In order to calculate the CJ pressure of a new explosive, a PDV system can be used to measure the particle velocity between the explosive and a polymethyl methacrylate (PMMA) window. A very thin foil, usually aluminum or copper, is placed at this interface to act as a reflective surface. This foil should be thin enough to prevent significant shock wave attenuation while being thick enough to prevent detonation light from passing through. Typically, a foil thickness of 1,000 angstroms is ideal for most experimental setups. Given the particle velocity in the PMMA and the detonation velocity of the explosive, the detonation pressure can be calculated with Hugoniot shock matching equations.⁶

While the FODV test at 0.75” charge diameter is an established standard at ARDEC, PDV-based tests are continually undergoing refinement. Depending on the explosive formulation, either one or both tests can be used to characterize detonation velocity and detonation pressure.