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A high-impulse thermobaric weapon (HIT), also known as a fuel-air explosive (FAE or FAX), a "heat and pressure" weapon, or a "vacuum bomb," consists of a container of a of volatile liquid, in some designs including a finely powdered explosive component as a slurry, and (typically) two separate explosive charges. After the munition is dropped or fired, the first explosive charge (or some other dispersal mechanism) bursts open the container at a predetermined height and disperses the fuel in a cloud that mixes with atmospheric oxygen (the size of the cloud varies with the size of the munition, and the shape varies with the dispersal mechanism). The cloud of fuel flows around objects and into structures. The final explosive charge then detonates the cloud, creating a massive blast wave.
Note on terminology
From the open literature it appears that some experts draw a distinction between these two terms based on the primary intended effects: "thermobaric" relating to closed-zone convection or air displacement as the primary objective, and "fuel-air" for use as area-denial or "daisy cutter"-like ordnance through blast and combustion, in a role somewhat similar to that of cluster bomb weapons. Other sources use "fuel-air" as the general case, subsuming "thermobaric" as previously detailed; still others use the two terms interchangeably. The term "thermobaric" appears to be of Russian/Soviet origin.
The overpressure within the detonation can reach 3 MPa (430 psi) and the temperature can be 2,500 - 3,000°C. Outside the cloud the blast wave travels at over 3 km/s. The vacuum created both pulls in loose objects and drags the burning fuel to create almost complete penetration of all non-airtight objects within the blast radius. Asphyxiation and internal damage can occur to personnel outside the highest blast effect zone, e.g. in deeper tunnels.
The effects produced by FAEs (a long high duration pressure and heat impulse) are often likened to the effects produced by low-yield nuclear weapons, but without the problems of radiation--though this is inexact; for all current and foreseen subkiloton yield nuclear weapon designs, prompt radiation effects predominate, producing some secondary heating--very little of the nominal yield is actually delivered as blast. The significant injury dealt by either weapon on a targeted population is nonetheless great.
In the form that they exist today, these devices (then dubbed Fuel-Air Munitions) have been said to have been developed in the 1960s and used by the United States during the Vietnam War to destroy Viet Cong tunnels, clear forest for helicopter landing sites and to clear mine fields. However, it is not clear that this is entirely the case; in particular, the very large parachute-dellivered "Daisy Cutter" bomb used for this purpose was suspected to have been such a weapon but the current published details indicate that it was not. FAMs are certainly in published literature available to English-speaking readers by the mid-1970s (Cite to come).
The Soviet armed forces also developed FAE weapons, including thermobaric warheads for shoulder-launched RPGss (RPO-A Shmel Bumblebee). 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.Weapon Effects
The blast wave destroys unreinforced buildings and equipment and kills and injures personnel. The antipersonnel effect of the blast wave is more severe in foxholes, on personnel with body armor, and in "stiff" enclosed spaces such as caves, buildings, and bunkers.History
Arguably, the use of flamethrowers in the Pacific Theater of World War II could constitute the first use of a primitive "vacuum bomb", in that they could suffocate people outside of the direct weapon effects. Other such effects were seen to occur in the firestorms at Dresden and elsewhere.
