Thermobaric weapon

A thermobaric weapon, also called an aerosol bomb, or a vacuum bomb, is a type of explosive munition that works by dispersing an aerosol cloud of gas, liquid or powdered explosive. This allows the chemical combustion to proceed using atmospheric oxygen, so that the weapon does not need to include an oxidizer.

The fuel is usually a single compound, rather than a mixture of multiple substances. Many types of thermobaric weapons can be fitted to hand-held launchers, and can also be launched from airplanes.

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) '-ic'.

Other terms used for the family of weapons are high-impulse thermobaric weapons, heat and pressure weapons, vacuum bombs, and fuel-air explosives (FAE).

Mechanism

Most conventional explosives consist of a fuel–oxidiser premix, but thermobaric weapons consist only of fuel and as a result are significantly more energetic than conventional explosives of equal weight. Their reliance on atmospheric oxygen makes them unsuitable for use under water, at high altitude, and in adverse weather. They are, however, considerably more effective when used in enclosed spaces such as tunnels, buildings, and non-hermetically sealed field fortifications (foxholes, bunkers).

The initial explosive charge detonates as it hits its target, opening the container and dispersing the fuel mixture as a cloud. The typical blast wave of a thermobaric weapon lasts significantly longer than that of a conventional explosive.

In contrast to an explosive that uses oxidation in a confined region to produce a blast front emanating from a single source, a thermobaric flame front accelerates to a large volume, which produces pressure fronts within the mixture of fuel and oxidant and then also in the surrounding air.

Thermobaric explosives apply the principles underlying accidental unconfined vapor cloud explosions, which include those from dispersion of flammable dusts and droplets. Such dust explosions happened most often in flour mills and their storage containers, grain bins (corn silos etc.), and earlier in coal mines, prior to the 20th century. More recent unconfined vapor cloud explosions happen most often in partially or completely empty oil tankers, refinery tanks, and vessels, such as the Buncefield fire in the United Kingdom in 2005, where the blast wave woke people 150 kilometres (93 mi) from its center.

A typical weapon consists of a container packed with a fuel substance, the center of which has a small conventional-explosive "scatter charge". Fuels are chosen on the basis of the exothermicity of their oxidation, ranging from powdered metals, such as aluminum or magnesium, to organic materials, possibly with a self-contained partial oxidant. The most recent development involves the use of nanofuels.

A thermobaric bomb's effective yield depends on a combination of a number of factors such as how well the fuel is dispersed, how rapidly it mixes with the surrounding atmosphere and the initiation of the igniter and its position relative to the container of fuel. In some designs, strong munitions cases allow the blast pressure to be contained long enough for the fuel to be heated well above its autoignition temperature so that once the container bursts, the superheated fuel autoignites progressively as it comes into contact with atmospheric oxygen. Conventional upper and lower limits of flammability apply to such weapons. Close in, blast from the dispersal charge, compressing and heating the surrounding atmosphere, has some influence on the lower limit. The upper limit has been demonstrated to influence the ignition of fogs above pools of oil strongly. That weakness may be eliminated by designs in which the fuel is preheated well above its ignition temperature so that its cooling during its dispersion still results in a minimal ignition delay on mixing. The continual combustion of the outer layer of fuel molecules, as they come into contact with the air, generates added heat which maintains the temperature of the interior of the fireball, and thus sustains the detonation.

In confinement, a series of reflective shock waves is generated, which maintain the fireball and can extend its duration to between 10 and 50 ms as exothermic recombination reactions occur. Further damage can result as the gases cool and pressure drops sharply, leading to a partial vacuum. This rarefaction effect has given rise to the term "vacuum bomb". Piston-type afterburning is also believed to occur in such structures, as flame-fronts accelerate through it.

Fuel–air explosive

A fuel–air explosive (FAE) device consists of a container of fuel and two separate explosive charges. After the munition is dropped or fired, the first explosive charge 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). The cloud of fuel flows around objects and into structures. The second charge then detonates the cloud and creates a massive blast wave. The blast wave can destroy reinforced buildings, equipment, and kill or injure people. The blast wave's antipersonnel effect is magnified in confined spaces, such as foxholes, tunnels, bunkers and caves.

Effects

Conventional countermeasures such as barriers (sandbags) and personnel armor are not effective against thermobaric weapons. A Human Rights Watch report of 1 February 2000 quotes a study made by the US Defense Intelligence Agency:

The [blast] kill mechanism against living targets is unique—and unpleasant. ... What kills is the pressure wave, and more importantly, the subsequent rarefaction [vacuum], which ruptures the lungs. ... If the fuel deflagrates but does not detonate, victims will be severely burned and will probably also inhale the burning fuel. Since the most common FAE fuels, ethylene oxide and propylene oxide, are highly toxic, undetonated FAE should prove as lethal to personnel caught within the cloud as with most chemical agents.

According to a US Central Intelligence Agency study,

the effect of an FAE explosion within confined spaces is immense. Those near the ignition point are obliterated. Those at the fringe are likely to suffer many internal, invisible injuries, including burst eardrums and crushed inner ear organs, severe concussions, ruptured lungs and internal organs, and possibly blindness.

Another Defense Intelligence Agency document speculates that, because the "shock and pressure waves cause minimal damage to brain tissue ... it is possible that victims of FAEs are not rendered unconscious by the blast, but instead suffer for several seconds or minutes while they suffocate".

Development

German

The first attempts occurred during World War I when incendiary shells (in German 'Brandgranate') used a slow but intense burning material, such as tar-impregnated tissue and gunpowder dust. These shells burned for approximately 2 minutes after the shell exploded and spread the burning elements in every direction. In World War II, the German Wehrmacht attempted to develop a thermobaric weapon, under the direction of the Austrian physicist Mario Zippermayr.

The weapon was claimed by a weapons specialist (K.L. Bergmann) to have been tested on the Eastern front under the code-name "Taifun B" and was ready for deployment during the Normandy invasion in June 1944. Apparently, canisters of a charcoal, aluminum and aviation fuel would have been launched, followed with a secondary launch of incendiary rockets. It was destroyed by a Western artillery barrage minutes before being fired just before Operation Cobra.

United States

FAEs were developed by the United States for use in the Vietnam War. The CBU-55 FAE fuel-air cluster bomb was mostly developed by the US Naval Weapons Center at China Lake, California.

American FAE munitions in use include the following:

• BLU-73 FAE I
• BLU-95 500 lb (230 kg) (FAE-II)
• BLU-96 2,000 lb (910 kg) (FAE-II)
• CBU-72 FAE I
• AGM-114 Hellfire missile (AGM-114N MAC)
• XM1060 grenade
• SMAW-NE round for rocket launcher

The XM1060 40-mm grenade is a small-arms thermobaric device, which was fielded by US forces in Afghanistan in 2002, and proved to be useful against targets in enclosed spaces, such as caves. Since the 2003 invasion of Iraq, the US Marine Corps has introduced a thermobaric "Novel Explosive" (SMAW-NE) round for the Mk 153 SMAW rocket launcher. One team of Marines reported that they had destroyed a large one-story masonry type building with one round from 100 yards (91 m). The AGM-114N Hellfire II, uses a Metal Augmented Charge (MAC) warhead, which contains a thermobaric explosive fill that uses aluminum powder coated or mixed with PTFE layered between the charge casing and a PBXN-112 explosive mixture. When the PBXN-112 detonates, the aluminum mixture is dispersed and rapidly burns. The result is a sustained high pressure that is extremely effective against personnel and structures.