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.
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.
The term thermobaric is derived from the Greek words for "heat" and "pressure": thermobarikos (θερμοβαρικός), from thermos (θερμός), hot + baros (βάρος), weight, pressure + suffix -ikos (-ικός), suffix -ic.
Other terms used for this family of weapons are high-impulse thermobaric weapons (HITs), heat and pressure weapons, vacuum bombs, or fuel-air explosives (FAE or FAX).
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 km 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.
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