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A thermobaric weapon is an explosive weapon which uses an initial small explosion to disperse reactive material over a relatively large volume. Subsequent heat-producing reaction of this material generates a pressure wave that is responsible for much of the weapon's destructive effect.

Thermobaric explosives are highly effective in confined spaces such as tunnels, caves or underground bunkers. Rather than providing protection as they would from conventional explosive ammunition, structural interior walls, particularly cement or other hard surfaces, magnify and channel the shock waves created by a thermobaric detonation. The stronger the walls, the higher the pressure’s reflective effect.The turbulent mixing of fuel with ambient oxygen is induced by the presence of walls through enhanced mixing from three different types of instabilities as well as from enhanced chemistry from temperature and pressure velocity gradient in differing fuels, creating a piston type afterburn reaction in enclosed structures.

Variants

Thermobaric weapons include fuel-air explosives (FAE or FAX) in which the dispersed material is fuel, and the heat-producing reaction is combustion with atmospheric oxygen. In other cases (metal-augmented charge) the dispersed material contains powdered metals such as aluminum or magnesium, which generate heat by burning in air, or in the gaseous reaction products of the initial explosion, or with the help of oxidizers included in the dispersed powder. Other terms used for this family of weapons are high-impulse thermobaric weapons (HITs), heat and pressure weapons, or vacuum bombs. Thermobaric weapons that depend on atmospheric oxygen can produce more explosive energy for a given size than conventional explosives, but have the disadvantage of being less predictable, being influenced by weather.

Terminology

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.

Mechanism

A thermobaric explosive consists of a container of a finely powdered solid fuel of differing particle size mixed with a low percentage of oxidizer and binder. The solid fuel could be an explosive metal powder or reactive organic. A high explosive charge is placed in the middle of the mixture.

A thermobaric weapon is initiated upon dropping or firing, and the explosive charge (or some other dispersal mechanism) bursts open the container and disperses the fuel in a cloud, and ignites the mixture in a single event. The heat released by the oxidizer gases then helps ignite the smaller solid particles that are mixed with the compressed hot air behind the shock, which leads the blast wave. This sustains a hot environment which allows 100% fuel combustion to be achieved. If fuel particles have a size distribution, smaller particles are quickly ignited, providing heat for the combustion of the larger particles. Smaller particles burn rapidly and remain tied to the local gas, while the larger particles move more freely and mix with new oxidation sources, allowing a more sustained combustion than would be produced by particles of a single size.

In confined spaces, transition to full detonation is not required for enhanced blast, if the solid fuel is ignited early in the dispersion process. A series of reflective shock waves generated by the detonation mixes the hot detonation gases with metal particles and compresses the metal particles at the same time. These actions provide the chemical kinetic support to maintain a hot environment, causing more metal to ignite and burn. This late time metal combustion process produces a significant pressure rise over a longer time duration (10–50 msec). This is a phase generally referred to as after burning or late-time impulse which can occur outside of where the detonation occurred, resulting in more widespread damage.

This is an aerobic reaction that draws in all of the unburnt fuel and atmospheric air, and creates a vacuum in the detonation environment, giving rise to the informal term 'vacuum bomb'. An air shock wave, generated during the fireball expansion, is reflected from the walls of the structure. The reflected shock plays two important roles. First, it stops the temperature decrease of the air and the fireball. It can actually increase the temperature in some places, depending on how the shock waves reflect. Second, it creates two new types of flow instabilities; Richtmyer-Meshkov and Kelvin-Helmholtz instabilities.

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 liquefied petroleum gas bursts. Silo explosions, caused by the ignition of finely-powdered atmospheric dust, are another example.

The detonation of thermobaric explosives (TBX) can be viewed in three stages. The first, an anaerobic stage, is measured in microseconds and breaks down the explosive by a shock wave. The subsequent exothermic molecular reactions go on to propagate the detonation wave. The second stage, measured in hundreds of microseconds, is also anaerobic. This involves reactions between any products that were too large to be involved in the main detonation event. The third stage is aerobic and lasts milliseconds. In this stage more, previously unreacted, fuel particles react with the surrounding air.

Stage One defines the HE's high-pressure shock effects (such as propelling a metal liner or fragments); Stage Two prolongs the high-pressure blast pulse, giving a useful heaving effect needed in building or bunker defeat; and Stage Three produces a long-duration, lower-pressure pulse that can also have a high thermal output, both of which are useful for materiel and personnel defeat.

Stages Two and Three are enhanced in thermobarics. This is accomplished by the addition of various fuels and additional oxygen-carrying chemicals to the explosive. The fuel is normally finely powdered aluminium, but boron, silicon, titanium, magnesium, zirconium, carbon and hydrocarbons can also be used. A typical oxygen-carrying chemical would be ammonium perchlorate. By carefully selecting the HE, fuel and oxidiser, the multiple-target defeat effects of blast, fragmentation and thermal pulse can be brought into effect.

Blast enhancement is mainly due to two reasons. The first is the fact of the wide dispersion of the fuel before combustion, making the initial combustion zone very large in comparison with a standard high explosive (metres compared with millimetres). The second is that although the peak pressure produced is lower, the duration is far longer. This is effective as the ability of buildings and people to survive a given pulse pressure level decreases with increasing pulse duration. The thermal effects of such warheads also dwarf those of classical HE, the temperature of the fireball, the heat flux produced and its duration all being several times larger (some an order of magnitude greater).

Calculations

For vapor cloud explosion there is a minimum ratio of fuel vapor to air below which ignition will not occur. 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, 30 bar) and the temperature can be . 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.

Calculations of enhanced blast explosives(EBX) are based on optical pyrometry of the pyrophoric metals to determine combustion temperature and rate. Depending on the metal particle size, different combustion behaviour can be observed in the detonation products: 315 µm particles present a delayed ignition with low and short emission, while 5 µm particles react almost instantaneously and keep burning for more than 40ms. The presence of AlO at different times indicatesthat aluminium combustion occurs with different delays depending on the particle size and non-monotonousrates during the fireball expansion. By recording the light spectrum emitted by metallized explosives, it is possible to collect information on thepresence of certain species during the fireball expansion. An average apparent temperature can also bedetermined at each integration step, using the classic method of the two-colour pyrometry. Although this technique can generate significant errors in certain conditions, it does not require the determination of emissivity of the observed area. This variable is indeed hardly accessible since it depends on the wavelengthand the chemical species present in the observed area. Previous studies determined the temperature of metallizedexplosive fireball using fixed wavelengths with better time resolution. The ISL spectroscope allowschoosing any pair of two wavelengths out of any specific atomic or molecular emission since all spectra are fullyrecorded during the explosion duration. The two wavelengths chosen for this study are 440 nm and 630 nm,corresponding to the apparent grey emission zones of the spectrum and being in a similar sensitivity range of thespectroscope sensor. Figure 5 presents the estimated fireball temperature evolution during the explosion of four2 kg charges for different aluminium particle sizes (5, 10, 100 and 315 µm).For homogeneous charges, the apparent temperature of burnt products stagnates at approximately 2500Kduring 15ms. In the case of a heterogeneous fireball produced by aluminized charges, the measured temperaturereaches levels between 3000 and 3500K, influenced by the flame temperature of aluminium mixed with air(3400K). Nevertheless the temperature tends to approach the value recorded with homogeneous explosive.

History

The first thermobaric explosions may have been the unintended ignition of flour in flour mills, a phenomenon known since medieval times. Such explosions are the consequence of the rapid burning of a fine fuel (the flour), suspended in air in a confined space.

The introduction of flamethrowers in the trench warfare of World War I 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 Dresdenmarker and elsewhere .

During World War II the ignition of fuel vapour within partially empty aviation fuel tanks caused massive explosions that led to the loss of several carriers including HMS Dasher marker .

In 1944 the Germans proceeded with the development of a fuel-air bomb, using 40% liquid oxygen mixed with 60% dry brown coal powder. In a test of an 8 kg charge near Doberitz, trees were completely destroyed within a 600 meter radius, with shock effects being felt as far away as 2 km. This was believed to be the beginning of fuel-air and thermobaric weapon development. The extent of the described destruction radius is not plausible for the stated mass of the charge.

In the form that exists today, these devices (also called Fuel-Air Munitions) were developed in the 1960s and used by the United Statesmarker during the Vietnam War to destroy VietCong tunnels , clear forest for helicopter landing sites and to clear minefields . FAMs are in published literature available to English-speaking readers by the mid-1970s.

The Sovietmarker armed forces extensively developed FAE weapons, including thermobaric warheads for shoulder-launched RPG (RPO-A Shmel Bumblebee / /). Russian forces have a wide array of these weapons and used them against Chinesemarker forces in the Sino-Soviet border conflict of 1969 , and used them in Afghanistan and in Chechnyamarker. 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, composed of a multiple rocket launcher mounted on a T-72 chassis. The TOS-1 was the main thermobaric delivery system that the Russians used against Groznymarker in the Second Chechen War.

A FAE system from Israelmarker 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


Thermobaric and fuel-air explosives have been used by terrorists since the 1983 Beirut barracks bombingmarker in Lebanon which used a gas-enhanced explosive mechanism, probably propane, butane or acetylene. The explosive used by the bombers in the 1993 World Trade Center bombingmarker was based on the FAE principle, using three tanks of bottled hydrogen gas to enhance the blast. In 2002, Jemaah Islamiyah bombers used a shocked dispersed solid fuel charge, based on the thermobaric principle, to attack the Sari nightclub in the 2002 Bali bombingsmarker.

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. 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.

Newest U.S. small arms FAE munitions

Introduced to the Afghanistan conflict, the XM1060 40-mm grenade is perhaps the first small-arms thermobaric device released in a U.S. theatre of war. Developed and fielded in just under five months by the Picatinny Arsenal, the XM1060 was delivered to U.S. forces in Afghanistan on April 30, 2003. The grenade was designed to be used with existing battlefield delivery systems presently in use by squad-level field forces.

The 48-lb (22 kg) AGM-114N Hellfire Metal Augmented Charge introduced in 2003 in Iraq contains a thermobaric explosive fill, using fluoridated aluminium layered between the charge casing and a PBXN-112 explosive mixture. When the PBXN-112 detonates, the aluminium mixture is dispersed and rapidly burns. The resultant sustained high pressure is extremely effective against enemy personnel and structures.

Russian test of the largest vacuum bomb

In September 2007 Russia successfully exploded the largest vacuum bomb ever made, leveling a multi-story block of apartment buildings with a power greater than that of the smallest dial-a-yield nuclear weapons at their lowest settings. Russia named this particular ordnance the "Father of All Bombs" in response to the United States developed "Massive Ordnance Air Blast" (MOAB) bomb whose backronym is the "Mother of All Bombs", and which previously held the accolade of the most powerful non-nuclear weapon in history. The bomb contains a 14,000 pound (6,400 kilogram) charge of a liquid fuel such as ethylene oxide, mixed with an energetic nanoparticle such as aluminium, surrounding a high explosive burster.The FOAB is based on the Russian ODAB-500PM and the BLU-82 Daisy Cutter. Shortly after the announcement of the FOAB, the United States Air Force announced the production of the 30,000 pound Massive Ordnance Penetrator, utilizing a 6,000 pound thermobaric mixture encased in a 24,000 pound steel shell.

Afghanistan

In June 2008, the United Kingdommarker revealed that its forces had used thermobaric munitions in Afghanistan. The munitions were delivered by the Hellfire AGM-114N from WAH-64 Apache attack helicopters. American forces have also apparently been employing the weapons in Afghanistan from Apaches and from unmanned drones. The UK stated that the weapon will also be configured to be delivered from its own MQ-9 Reaper drones.

See also



Footnotes

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