Thermobaric weapons distinguish themselves from conventional explosive weapons by using atmospheric oxygen, instead of carrying an oxidizer in their explosives. They are also called high-impulse thermobaric weapons (HITs), fuel-air explosives (FAE or FAX) or sometimes fuel-air munitions, heat and pressure weapons, or vacuum bombs. They produce more explosive energy for a given size than do other conventional explosives, but have the disadvantage of being less predictable in their effect (influenced by weather).
A thermobaric weapon (or solid fuel-air explosive) uses the gaseous products (H2, H2O, CO and CO2) of an initial explosion for an afterburning of reactive solids. Because their reaction with atmospheric oxygen only produces solid oxides the blast wave is primarily generated by heat of combustion ("thermobaric") instead of expanding explosion gases. This makes thermobaric explosives more effective in oxygen deficient environments such as tunnels, caves or underground bunkers. Rather than providing protection as they would from conventional explosive ammunition, structure interior walls, particularly cement or other hard surfaces, magnify and channel the shockwaves created by a thermobaric detonation. The stronger the walls, the higher the pressure’s reflective effect.
A thermobaric explosive consists of a container of a finely powdered solid fuel of differing particle size mixed with a low percentage of oxidizer and binder. The solid fuel could be an explosive metal powder or reactive organic. A high explosive charge is placed in the middle of the mixture.
A thermobaric weapon is initiated upon dropping or firing, and the explosive charge (or some other dispersal mechanism) bursts open the container and disperses the fuel in a cloud, and ignites the mixture in a single event. The heat released by the oxidizer gases then helps ignite the smaller solid particles mixed with the compressed hot air behind the shock leading the blast wave. This sustains a hot environment which allows 100% fuel combustion to be achieved. If fuel particles have a size distribution, smaller particles get ignited in a short period of time, providing heat for the combustion of the larger particles. Smaller particles burn rapidly and remain tied to the local gas, while the larger particles move more freely and mix with new oxidation sources, allowing a more sustained combustion than provided by a single particle size.
In confined spaces, transition to full detonation is not required for enhanced blast, if the solid fuel is ignited early in the dispersion process. A series of reflective shock waves generated by the detonation mixes the hot detonation gasses with metal particles and compresses the metal particles at the same time. These actions provide the chemical kinetic support to maintain a hot environment, causing more metal to ignite and burn. This late time metal combustion process produces a significant pressure rise over a longer time duration (10–50 msec). This is a phase generally referred to as after burning or late-time impulse which can occur outside of where the detonation occurred, resulting in more widespread damage. This is known as an aerobic reaction and draws in all of the unburnt fuel and atmospheric air, and creates a vacuum in the detonation environment.
Fuel-air explosives represent the military application of the vapor cloud explosion and dust explosion accidents that have long bedeviled a variety of industries. An accidental fuel-air explosion may occur as a result of a boiling liquid expanding vapor explosion (BLEVE), for example when a tank containing liquefied petroleum gas bursts. Silo explosions, caused by the ignition of finely-powdered atmospheric dust, are another example.
Fuel-air explosives disperse an aerosol cloud of fuel which is ignited by an embedded detonator to produce an explosion. The rapidly expanding wave front due to overpressure flattens all objects within close proximity of the epicenter of the aerosol fuel cloud, and produces debilitating damage well beyond the flattened area. The main destructive force of FAE is high pressure. More importantly, the duration of the overpressure gives it an edge over conventional explosives and makes fuel-air explosives useful against hard targets such as minefields, armored vehicles, aircraft parked in the open, and bunkers.
There are dramatic differences between explosions involving high explosives and vapor clouds at close distances. For the same amount of energy, the high explosive blast overpressure is much higher and the blast impulse is much lower than that from a vapor cloud explosion. The shock wave from a TNT explosion is of relatively short duration, while the blast wave produced by an explosion of hydrocarbon material displays a relatively long duration. The duration of the positive phase of a shock wave is an important parameter in the response of structures to a blast.
The effects produced by FAEs (a long-duration high pressure and heat impulse) are often likened to the effects produced by low-yield nuclear weapons, but without the problems of radiation. However, this is inexact; for all current and foreseen sub-kiloton-yield nuclear weapon designs, prompt radiation effects predominate, producing some secondary heating; very little of the nominal yield is actually delivered as blast. The resulting injury dealt by either weapon on a targeted population is nonetheless great.
Some fuels used, such as ethylene oxide and propylene oxide, act like mustards. A device using such fuels can be dangerous if the fuel fails to completely ignite; the device is at risk of producing the effects of a chemical weapon.
The overpressure within the detonation can reach 430 lbf/in² (3 MPa, 30 bar) and the temperature can be 4500 to 5400 °F (2500 to 3000 °C). Outside the cloud the blast wave travels at over 2 mi/s (3 km/s). Following the initial blast (compression) is a phase in which the pressure drops below atmospheric pressure (rarefaction) creating an airflow back to the center of the explosion strong enough to lift and throw a human. It draws in the unexploded burning fuel to create almost complete penetration of all non-airtight objects within the blast radius, which are then incinerated. Asphyxiation and internal damage can also occur to personnel outside the highest blast effect zone, e.g. in deeper tunnels, as a result of the blast wave, the heat, or the following air draw.
Based on the known properties of flammable substances and explosives, it is possible to use conservative assumptions and calculate the maximum distance at which an overpressure or heat effect of concern can be detected. Distances for potential impacts could be derived using the following calculation method [described in Flammable Gases and Liquids and Their Hazards]:
where D is the distance in meters to a 1 psi overpressure; C is a constant for damages associated with 1 psi overpressure or 0.15, n is a yield factor of the vapor cloud explosion derived from the mechanical yield of the combustion and is assumed to be 10 percent (or 0.1) and E is the energy content of the explosive part of the cloud in joules. E can be calculated from the mass m of substance in kilograms times the heat of combustion Qc in joules per kilogram as follows:
Combining these two equations gives:
Vapor cloud explosion modeling historically has been subject to large uncertainties resulting from inadequate understanding of deflagrative effects. According to current single-degree of freedom models, blast damage/injury can be represented by pressure-impulse (P-I) diagrams, which include the effects of overpressure, dynamic pressure, impulse, and pulse duration. The peak overpressure and duration are used to calculate the impulse from shock waves. Even some advanced explosion models ignore the effects of blast wave reflection off structures, which can produce misleading results over- or under-estimating the vulnerability of a structure. Sophisticated software used to produce three-dimensional models of the effects of vapor cloud explosions allows the evaluation of damage experienced by each structure within a facility as a result of a primary explosion and any accompanying secondary explosions produced by vapor clouds.
Arguably, the introduction of flamethrowers in the trench warfare of World War I could constitute the first use of a primitive "vacuum weapon", in that they could suffocate people protected from the direct weapon effects inside a pillbox or bunker. Other such effects were seen to occur in the firestorms that followed the Allied bombing raids at Dresden and elsewhere.
In the form that exists today, these devices (often dubbed Fuel-Air Munitions) are said to have been developed in the 1960s and used by the United States during the Vietnam War to destroy VietCong tunnels, clear forest for helicopter landing sites and to clear minefields. FAMs are certainly in published literature available to English-speaking readers by the mid-1970s.
The Soviet armed forces also developed FAE weapons, including thermobaric warheads for shoulder-launched RPGs (RPO-A Shmel Bumblebee /РПО-А "Шмель"/). Russian forces have a wide array of these weapons and reportedly used them against Chinese forces in a 1969 border conflict, and certainly used them in Afghanistan and in Chechnya. Russian troops report that a single RPO-A round in an urban environment has an equivalent effect to a 152 mm artillery round. TOS-1 "Buratino" is another Russian Army FAE weapon system, composed of a multiple rocket launcher mounted on a T-72 chassis. The TOS-1 was the main thermobaric delivery system that the Russians used against Grozny in the Second Chechen War.
Current US FAE munitions include:
Thermobaric and fuel-air explosives have been used by terrorists since the 1983 Beirut barracks bombing in Lebanon which used a gas-enhanced explosive mechanism, probably propane, butane or acetylene. The explosive used by the bombers in the 1993 World Trade Center bombing was based on the FAE principle, using three tanks of bottled hydrogen gas to enhance the blast.In 2002, Jemaah Islamiyah bombers used a shocked dispersed solid fuel charge, based on the thermobaric principle, to attack the Sari nightclub in the 2002 Bali bombings.
In 2003, United States Marines used a thermobaric version of their Shoulder-Launched Multipurpose Assault Weapon, called a Shoulder-Launched Multipurpose Assault Weapon-Novel Explosion (SMAW-NE), in the Invasion of Iraq. One team of Marines reported that they had destroyed a large one-story masonry type building with one round from 100 yards. The thermobaric explosive used in this weapon, PBXIH-135 or a variant, was developed at the Naval Surface Warfare Center (NSWC) Indian Head Division and had previously been used in BLU-118/B air-dropped bombs against al Qaeda and Taliban forces in Afghanistan in early March, 2002.
Introduced to the Afghanistan conflict, the XM1060 40-mm grenade is perhaps the first small-arms thermobaric device released in a U.S. theater of war. Developed and fielded in just under five months by the Picatinny Arsenal, the XM1060 was delivered to U.S. forces in Afghanistan on April 30, 2003. The grenade was designed to be used with existing battlefield delivery systems presently in use by squad-level field forces.
The 48-lb (22 kg) AGM-114N Hellfire Metal Augmented Charge introduced in 2003 in Iraq contains a thermobaric explosive fill, utilizing fluoridated aluminium layered between the charge casing and a PBXN-112 explosive mixture. When the PBXN-112 detonates, the aluminium mixture is dispersed and rapidly burns. The resultant sustained high pressure is extremely effective against enemy personnel and structures.