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A thermobaric weapon, which includes the type known as a "fuel-air bomb", is an explosive weapon that produces a blast wave of a significantly longer duration than those produced by condensed explosives. This is useful in military applications where its longer duration increases the numbers of casualties and causes more damage to structures.

Mechanism

Thermobaric explosives rely on oxygen from the surrounding air, whereas most conventional explosives consist of a fuel-oxygen premix (for instance, gunpowder contains 15% fuel and 75% oxidizer). Thus, on a weight-for-weight basis they are significantly more energetic than normal condensed explosives. Their reliance on atmospheric oxygen makes them unsuitable for use underwater, at high altitude or in adverse weather. However, they have significant advantages when deployed inside confined environments such as tunnels, caves, and bunkers.

In contrast to condensed explosive where oxidation in a confined region produces a blast front from essentially a point source, here a flame front accelerates to a large volume producing pressure fronts both within the mixture of fuel and oxidant and then in the surrounding air.

Thermobaric explosives apply the principles underlying accidental unconfined vapor cloud explosions (UVCE), which include those from dispersions of flammable dusts and droplets. In previous times they were most often encountered in flour mills and their storage containers, and later in coal mines, but now most commonly in discharged oil tankers and refineries, the most recent being at Buncefield in the UK where the blast wave woke people 150 kilometresmi) from its centre.

A typical weapon consists of a container packed with a fuel substance, in the center of which is a small conventional-explosive "scatter charge". Fuels are chosen on the basis of the exothermicity of their oxidation, ranging from powdered metals such as aluminium or magnesium, or organic materials, possibly with a self-contained partial oxidant. The most recent development involves the use of nanofuels.

A thermobaric bomb's effective yield requires the most appropriate combination of a number of factors; among these are how well the fuel is dispersed, how rapidly it mixes with the surrounding atmosphere and the initiation of the igniter and its position relative to the container of fuel. In some cases separate charges are used to disperse and ignite the fuel. In other designs stronger cases allow the fuel to be contained long enough for the fuel to heat to well above its auto-ignition temperature, so that, even its cooling during expansion from the container, results in rapid ignition once the mixture is within conventional flammability limits.

It is important to note that conventional upper and lower limits of flammability apply to such weapons. Close in, blast from the dispersal charge, compressing and heating the surrounding atmosphere, will have some influence on the lower limit. The upper limit has been demonstrated strongly to influence the ignition of fogs above pools of oil. This weakness may be eliminated by designs where the fuel is preheated well above its ignition temperature, so that its cooling during its dispersion still results in a minimal ignition delay on mixing.

In confinement, a series of reflective shock waves are generated, which maintain the fireball and can extend its duration to between 10 and 50 msec as exothermic recombination reactions occur.[ Further damage can result as the gases cool and pressure drops sharply, leading to a partial vacuum, powerful enough to cause physical damage to people and structures. This effect has given rise to the misnomer "vacuum bomb". Piston-type afterburning is also believed to occur in such structures, as flame-fronts accelerate through it.[
The overpressure within the detonation can reach 430 psi (3.0 megapascals) and the temperature can be 4,500 to 5,400 °F (2,500 to 3,000 °C). Outside the cloud the blast wave travels at over 2 miles per second (3.2 km/s).


Current US FAE munitions include:

* BLU-73 FAE I
* BLU-95 500-lb (FAE-II)
* BLU-96 2,000-lb (FAE-II)
* CBU-55 FAE I
* CBU-72 FAE I

The XM1060 40-mm grenade is a small-arms thermobaric device, which was delivered to U.S. forces in April 2003.[43] Since the 2003 Invasion of Iraq, the US Marine Corps has introduced a thermobaric 'Novel Explosive' (SMAW-NE) round for the Mk 153 SMAW rocket launcher. One team of Marines reported that they had destroyed a large one-story masonry type building with one round from 100 yards (91 m).
The 48-pound (22 kg) AGM-114N Hellfire Metal Augmented Charge introduced in 2003 in Iraq contains a thermobaric explosive fill, using 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 people and structures.[45]

Improvised uses

Thermobaric and fuel-air explosives have been used in guerrilla warfare since the 1983 Beirut barracks bombing in Lebanon which used a gas-enhanced explosive mechanism, probably propane, butane or acetylene.[46] The explosive used by the bombers in the 1993 World Trade Center bombing incorporated the FAE principle, using three tanks of bottled hydrogen gas to enhance the blast.[47][48] In 2002, Jemaah Islamiyah bombers used a shocked dispersed solid fuel charge,[49] based on the thermobaric principle,[50] to attack the Sari nightclub in the 2002 Bali bombings.