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.
Some experts draw a distinction between the terms thermobaric weapon and fuel-air explosive based on the primary intended effects. Thermobaric weapons relate to closed-zone convection or air displacement as the primary objective. Other sources use "fuel-air" as a catch-all term, subsuming "thermobaric" as previously detailed; still others use the two terms interchangeably.
Conventional explosive weapons such as the Daisy Cutter incorporate both agent and oxidizer. In contrast, a fuel-air explosive consists only of agent and a dispersing mechanism, using oxygen from the air as the oxidizer.
Bomb Mechanism
A thermobaric weapon works by first dispersing a cloud of powder or liquid explosive using a small charge, then igniting it with a second charge.
The weapon consists of a container of either a volatile liquid or a finely powdered solid, which could be an explosive or metal powder, and two separate explosive charges.
The weapon is initiated upon dropping or firing, and 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. Once the fuel is appropriately mixed, the second charge detonates, propagating an explosion (blast wave) through the cloud.
Newer types of thermobaric weapons do not disperse the fuel before igniting it, but are single-stage bombs having one explosive charge that both ignites and disperses the fuel.
Weapon effects
Fuel-air explosives represent the military application of the vapor cloud explosion and dust explosion accidents that have long bedeviled a variety of industries. An accidental fuel-air explosion may occur as a result of a boiling liquid expanding vapor explosion (BLEVE), for example when a tank containing liquified petroleum gas bursts. Silo explosions, caused by the ignition of finely-powdered atmospheric dust, are another example.
Fuel-air explosives 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 pressure. More importantly, the duration of the overpressure gives it an edge over conventional explosives and makes fuel-air explosives useful against hard targets such as minefields, armored vehicles, aircraft parked in the open, and bunkers.
There are dramatic differences between explosions involving high explosives and vapor clouds 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.
The effects produced by FAEs (a long-duration high pressure and heat impulse) are often likened to the effects produced by low-yield nuclear weapons, but without the problems of radiation. However, this is inexact; for all current and foreseen sub-kiloton-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.
Some fuels used, such as ethylene oxide and propylene oxide, act like mustards. A device using such fuels can be dangerous if the fuel fails to completely ignite; the device is at risk of producing the effects of a chemical weapon.
Calculations
For vapor cloud explosion there is a minimum ratio of fuel vapor to air below which ignition will not occur. Alternately, there is also a maximum ratio of fuel vapor to air, above which ignition will not occur. These limits are termed the lower and upper explosive limits. For gasoline vapor, the explosive range is from 1.3 to 6.0% vapor to air, and for methane this range is 5 to 15%. Many parameters contribute to the potential damage from a vapor cloud explosion, including the mass and type of material released, the strength of ignition source, the nature of the release event (e.g., turbulent jet release), and turbulence induced in the cloud (e.g., from ambient obstructions).
The overpressure within the detonation can reach 430 lbf/in² (3 MPa) and the temperature can be 4500 to 5400 °F (2500 to 3000 °C). Outside the cloud the blast wave travels at over 2 mi/s (3 km/s). Following the initial blast (compression) is a phase in which the pressure drops below atmospheric pressure (rarefaction) creating an airflow back to the center of the explosion strong enough to lift and throw a human. It draws in the unexploded burning fuel to create almost complete penetration of all non-airtight objects within the blast radius, which are then incinerated. Asphyxiation and internal damage can also occur to personnel outside the highest blast effect zone, e.g. in deeper tunnels, as a result of the blast wave, the heat, or the following air draw.
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(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 overpressure 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 m of substance in kilograms times the heat of combustion Qc in joules per kilogram as follows:
