Thermobaric Weapons - Mechanism

Mechanism

In contrast to condensed explosive, where oxidation in a confined region produces a blast front from essentially a point source, 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, which include those from dispersions of flammable dusts and droplets. Previously, such explosions 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 kilometres (93 mi) 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. The continual combustion of the outer layer of fuel molecules as they come into contact with the air, generates additional heat which maintains the temperature of the interior of the fireball, and thus sustains the detonation.

In confinement, a series of reflective shock waves are generated, which maintain the fireball and can extend its duration to between 10 and 50 ms 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 (Bad rounding here3.0 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) - 7200 mph.