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Fuel/Air Explosive (FAE)

U.S. Fuel-Air Explosives
BLU-95 500-lb (FAE-II)
BLU-96 2,000-lb (FAE-II)
Fuel-Air Explosives [FAE] 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 overpressure, useful against soft targets such as minefields, armored vehicles, aircraft parked in the open, and bunkers.

Fuel/air explosive represent the military application of the vapor cloud explosions and dust explosions accidents that have long bedeviled a variety of industries.

Accidental vapor cloud explosion hazards are of great concern to the refining and chemical processing industry, and a number of catastrophic explosion accidents have had significant consequences in terms of injury, property damage, business interruption, loss of goodwill, and environmental impact.

And every year, many serious explosions and fires occur in industrial plants as a result of dust. Many materials form dust clouds that can easily ignite and explode, injuring personnel and damaging plant. This is a well-known phenomenon in the coal mining, grain storage, and the woodworking and paper industries. Many miners have been killed and injured and massive production losses have resulted from coal dust explosions in underground coal mining operations. Of the 129 grain dust explosions that occurred nationwide between 1987 and 1997, about half involved corn. Eleven were caused by wheat dust and 10 by dust from soybeans. Billions of tiny, highly combustible particles of grain are generated by grain kernels rubbing together as they move along conveyer belts and shifted between bins. Inside the enclosed chambers, those particles rise in a cloud. When the dust gets in with the right mixture of oxygen and comes in contact with a spark or even an overheated bearing on a conveyer belt, it is extremely explosive.

Almost all organic material in the form of a dust cloud will ignite at temperatures below 500 oC - approximately the same temperature as a newly extinguished match. Cotton, plastics and foodstuffs such as sugar, flour and cocoa can also, under the right conditions, act as explosives. In order for a dust explosion to take place, the dust particles must be of a certain size and the amount of finely granulated material per unit of volume must lie within certain critical values. There is generally a direct correlation between particle size and explosive hazard. The smaller the particle, the more reactive the dust. As the materials become smaller, they disperse and remain suspended more easily, increasing the potential for ignition and propagation of the reaction. Industrial explosion prevention measures include, where possible, providing nitrogen gas purging to ensure that the oxygen concentration is kept below that required for combustion.


TNT generates well over 4,000 psi overpressure in close proximity to the source of the explosion, along with significant radiant heat effects from the explosion's fireball. Conventional high explosive munitions also produce fragments from the munition case, as well as fragments from material in the target area that is broken loose by the high blast overpressures.

There are dramatic differences between explosions involving vapor clouds and high explosives 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.

Although the detonation combustion mode produces the most severe damage, fast deflagrations of the cloud can result from flame acceleration under confined and congested conditions. Flame propagation speed has a significant influence on the blast parameters both inside and outside the source volume.


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]:

D = C x (nE)1/3

where D is the distance in meters to a 1 psi overpressure; C is a constant for damages associated with 1 psi overpressures 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 of substance in kilograms times the heat of combustion (hc) in Joules per kilogram as follows:

E = mass x hc

Combining these two equations gives:

D = 0.15 x (0.1 x mass x hc)1/3

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.

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