Thermobaric weapon

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Thermobaric weapons distinguish themselves from conventional explosive weapons by using atmospheric oxygen, instead of carrying an oxidizer in their explosives. They are also called high-impulse thermobaric weapons (HITs), fuel-air explosives (FAE or FAX) or sometimes "fuel-air munitions", "heat and pressure" weapons, or vacuum bombs. They produce more explosive energy for a given size than do other conventional explosives, but have the downside of being less predictable in their effect.

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[edit] Terminology

The term thermobaric is derived from the Greek words for heat ("therme",Θερμό) and pressure ("baros", Βάρος) respectively.

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 explosives such as the Daisy Cutter incorporate both agent and oxidizer. In contrast, a fuel-air explosive consists only of agent and a dispersing mechanism, taking as oxidizer oxygen from the air.

[edit] Mechanism

Thermobaric weapons work by first expelling a cloud of explosive mist using a small charge, then igniting it with a second charge.

They consist 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 detonated 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 which have one explosive charge that both ignites and disperses the fuel.

[edit] Weapon effects

Fuel-air explosives represent the military application of the vapor cloud explosions and dust explosions 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—although 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.

Some fuels used, such as ethylene oxide and propylene oxide, are highly toxic. A device using such fuels is very dangerous even if the fuel fails to ignite; the device then becomes essentially a chemical weapon.

[edit] 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, at 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. Following the initial blast is a phase in which the pressure drops below atmospheric pressure 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 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.

[edit] History

Arguably, the use of flamethrowers in the trench warfare of World War I (the flamethrower was a German invention[citation needed]) could constitute the first use of a primitive "vacuum weapon", in that they could suffocate people protected from the direct weapon effects inside a pillbox or bunker. Other such effects were seen to occur in the firestorms that followed the Allied bombing raids at Dresden and elsewhere.

In the form that exists today, these devices (often dubbed Fuel-Air Munitions) are 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 minefields. However, it is not clear that this is entirely the case; in particular, the very large parachute-delivered "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 (it seems to be filled with ANFO, a mixture of ammonium nitrate and jet fuel, instead). FAMs are certainly in published literature available to English-speaking readers by the mid-1970s (Cite to come).

In 2003, United States Marines used a thermobaric version of their Shoulder-Launched Multipurpose Assault Weapon, called a Shoulder-Launched Multipurpose Assault Weapon - Novel Explosion (SMAW-NE), in the Invasion of Iraq. One team of Marines reported that they had destroyed a large one-story masonry type building with one round from 100 yards. [1] The thermobaric explosive used in this weapon, PBXIH-135 or a variant, was developed at the Naval Surface Warfare Center (NSWC) Indian Head Division and had previously been used in BLU-118/B air-dropped bombs against al Qaeda and Taliban forces in Afghanistan in early March, 2002.

The Soviet armed forces also developed FAE weapons, including thermobaric warheads for shoulder-launched RPGs (RPO-A Shmel Bumblebee /Russian: РПО-А "Шмель"/). Russian forces have a wide array of these weapons ([2]) and reportedly used them against Chinese forces in a 1969 border conflict, and certainly used them in Afghanistan and in Chechnya. Russian troops report that a single RPO-A round in an urban environment has an equivalent effect to a 152 mm artillery round. TOS-1 "Buratino" is another Russian Army FAE weapon system. It's a multiple rocket launcher on the T-72 chassis. The TOS-1 was the main thermobaric delivery system that the Russians used against Grozny in the Second Chechen War.

An FAE system from Israel was developed for minefield clearing. The system uses a small rocket-propelled thermobaric charge which explodes over the minefield and activates exposed or buried mines.

Current US FAE munitions include

BLU-73 FAE I BLU-95 500-lb (FAE-II) BLU-96 2,000-lb (FAE-II) CBU-55 FAE I CBU-72 FAE I

[edit] External links

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