Volumetric weapons include thermobaric and fuel-air explosives (FAE). Both thermobaric and FAE operate on similar technical principles. In the case of FAE, when a shell or projectile containing a fuel in the form of gas, liquid or dustexplodes, the fuel or dust like material is introduced into the air to form acloud. This cloud is then detonated to create a shock wave of extended duration that produces overpressure and expands in all directions. In a thermobaric weapon, the fuel consists of a monopropellant and energetic particles. The monopropellant detonates in a manner simular to TNT while the particles burn rapidly in the surrounding air later in time, resulting an intense fireball and high blast overpressure. The term "thermobaric" is derived from the effects of temperature (the Greek word "therme" means "heat") and pressure (the Greek word "baros" means "pressure") on the target.
Thermobaric munitions have been used by many nations of the world and their proliferation is an indication of how effectively these weapons can be used in urban and complex terrain. The ability of thermobaric weapons to provide massed heat and pressure effects at a single point in time cannot be reproduced by conventional weapons without massive collateral destruction. Thermobaric weapon technologies provide the commander a new choice in protecting the force, and a new offensive weapon that can be used in a mounted or dismounted mode against complex environments.
The USAF and USN are actively pursuing conventional weapons technology to destroy Nuclear, Biological, and Chemical (NBC) and support/storage facilities while retaining or destroying the agents within the structure and minimizing collateral damage including fatalities. Thermobaric weapons use high-temperature incendiaries against chemical and biological facilities. The USN is working on an Inter-Halogen Oxidizer weapon while the USAF is pursuing a solid fuel-air explosive using aluminum particles. Both of these weapons use an incineration technique to defeat and destroy the CB agents within the blast area.
The Thermobaric Weapon Demonstration is a proposed Advanced Concept Technology Demonstration (ACTD). Under this program, prototype weapons are to be tested under operational conditions for their performance, and leave-behinds are to be delivered to the customer. The program aims to develop a validated means of delivery to/into a tunnel adit [entrance]. Technical risks include the extent to which candidate thermobaric payloads do not perform substantially better than existing high explosives in tunnels.
The Thermobaric [TB] Weapon Demonstration will develop a weapon concept that is based on a new class of solid fuel-air explosive thermobarics.The weapon could be used against a certain type of tunnel targets for a maximum functional kill of the tunnels.
Most of the Hard and/or Deeply Buried Targets (HDBTs), namely tunnels in rock, are so deep that the developmental and current inventory weapons cannot penetrate to sufficient depths to directly destroy critical assets. One of the warfighter's options is to attack the tunnel portals with weapons that penetrate the thinner layer of rock above the portal, or though the exterior doors, resulting in a detonation within the tunnel system. Penetrations through the door systems have the potential to place the warheads deep within the facility. Detonations within a tunnel, even only in a few diameters, have a significant increase in airblast propagation into the facility compared to external detonations. Tunnel layouts range from long, straight tunnels to various types of intersections, expansions, constrictions, chambers, rooms, alcoves, and multiple levels. All of these configurations affect the propagation of airblast.
Air blast propagation within a tunnel system has the potential to cause significant damage to critical equipment and systems. If the critical equipment within a facility can be damaged or destroyed, then the function of the facility can be degraded or destroyed, resulting in a functional kill. Depending on the purpose of the facility and the level of damage, a functional kill can be as permanent as a "structural kill," in which the facility is destroyed in a more traditional manner.
Functional kill from air blast loads is predicated on the ability to accurately determine the blast environment from an internal detonation. The response of critical equipment cannot be calculated without accurate blast loads. Unlike free-field blast loads, a detonation within a tunnel system can have a significant dynamic pressure component. This dynamic pressure component, in conjunction with the overpressure component, makes up the entire pressure-loading history necessary to predict component response.
Thermobaric compositions are fuel rich high explosives that are enhanced through aerobic combustion in the third detonation event. Performance enhancement is primarily achieved by addition of excess metals to the explosive composition. Aluminum and Magnesium are the primary metals of choice. The detonation of Composite Explosives can be viewed as three discrete events merged together. All three explosive events can be tailored to meet system performance needs:
- The initial anaerobic detonation reaction, microseconds in duration, is primarily a redox reaction of molecular species. The initial detonation reaction defines the system’s high pressure performance characteristics: armor penetrating ability.
- The post detonation anaerobic combustion reaction, hundreds of microseconds in duration, is primarily a combustion of fuel particles too large for combustion in the initial detonation wave. The post detonation anaerobic reaction define the system’s intermediate pressure performance characteristics: Wall/Bunker Breaching Capability.
- The post detonation aerobic combustion reaction, milliseconds in duration, is the combustion of fuel rich species as the shock wave mixes with surrounding air. The post detonation aerobic reaction characteristics define the system’s personnel / material defeat capability: Impulse and Thermal Delivery. Aerobic combustion requires mixing with sufficient air to combust excess fuels. The shock wave pressures are less than 10 atmospheres. The majority of aerobic combustion energy is available as heat. Some low pressure shock wave enhancement can also be expected for personnel defeat. Personnel / material defeat with minimum collateral structure damage requires maximum aerobic enhancement and the highest energy practical fuel additives: Boron, Aluminum, Silicon, Titanium, Magnesium, Zirconium, Carbon, or Hydrocarbons.
Thermobaric materials can provide significantly higher total energy output than conventional high explosives. The majority of the additional energy is available as low pressure impulse and heat.