A thermobaric weapon, aerosol bomb, or vacuum bomb is a type of explosive that uses oxygen from the surrounding air to generate a high-temperature explosion. In practice, the blast wave typically produced by such a weapon is of a significantly longer duration than that of a conventional condensed explosive. The fuel–air explosive is one of the best-known types of thermobaric weapon.

Most conventional explosives consist of a fuel–oxidizer premix (black powder, for example, contains 25% fuel and 75% oxidizer), but thermobaric weapons are almost 100% fuel and so are significantly more energetic than conventional condensed 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 destructive when used against field fortifications such as foxholes, tunnels, bunkers, and caves, partly because of the sustained blast wave and partly by the consumption of the oxygen inside it.

Many types of thermobaric weapons can be fitted to hand-held launchers.

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.

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

Mechanism

In contrast to a condensed explosive, which 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 dispersions of flammable dusts and droplets. Previously, such dust explosions were most often encountered in flour mills and their storage containers and later in coal mines but now most commonly in partially or fully empty oil tankers and refinery tanks and vessels, including an incident at Buncefield in the UK in 2005 in which the blast wave woke people 150 kilometres (93 mi) from its centre.

A typical weapon consists of a container packed with a fuel substance, the centre 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 requires the most appropriate combination of a number of factors like 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, a 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 additional 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 misnomer “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 (and possibly ionizes it, depending on whether a fused quartz dispersal charge container was employed) 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 antipersonnel effect of the blast wave is more severe in foxholes and tunnels and in enclosed spaces, such as bunkers and caves.

FAEs were first developed by the United States for use in the Vietnam War. In response, Soviet scientists quickly developed their own FAE weapons, which were reportedly used against China in the Sino-Soviet border conflict and against the Mujahideen in Afghanistan. Since then, research and development has continued, and Russian forces now field a wide array of third-generation FAE warheads.

Effects

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, thus 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 developments

The first attempts occurred during the Second World War by the German Luftwaffe and Wehrmacht, the inventor being Mario Zippermayr.

Soviet and Russian developments

Thermobaric weapons were developed in the 1960s in the Soviet Union and the United States; however, the Soviet armed forces extensively developed FAE weapons, such as the RPO-A, and Russia used them in Chechnya. The Russian armed forces have developed thermobaric ammunition variants for several of their weapons, such as the TBG-7V thermobaric grenade with a lethality radius of 10 m (33 ft), which can be launched from an RPG-7. The GM-94 is a 43 mm (1.7 in) pump-action grenade launcher designed mainly to fire thermobaric grenades for close-quarters combat. The grenade weighed 250 g (8.8 oz) and contained 160 g (5.6 oz) of explosive, its lethality radius is 3 m (9.8 ft), but due to the deliberate “fragmentation-free” design of the grenade, a distance of 4 m (13 ft) is considered safe.

The RPO-A and upgraded RPO-M are infantry-portable RPGs, designed to fire thermobaric rockets. The RPO-M, for instance, has a thermobaric warhead with a TNT equivalence of 5.5 kg (12 lb) and destructive capabilities similar to a 152 mm (6 in) high-explosive fragmentation artillery shell. The RShG-1 and the RShG-2 are thermobaric variants of the RPG-27 and RPG-26 respectively. The RShG-1 is the more powerful variant, with its warhead having a 10-metre (33 ft) lethality radius and producing about the same effect as 6 kg (13 lb) of TNT. The RMG is a further derivative of the RPG-26 that uses a tandem-charge warhead, with the precursor HEAT warhead blasting an opening for the main thermobaric charge to enter and detonate inside. The RMG’s precursor HEAT warhead can penetrate 300 mm of reinforced concrete or over 100 mm of rolled homogeneous armour, thus allowing the 105 mm (4.1 in)-diameter thermobaric warhead to detonate inside.

Other examples include the SACLOS or millimeter-wave radar-guided thermobaric variants of the 9M123 Khrizantema, the 9M133F-1 thermobaric warhead variant of the 9M133 Kornet, and the 9M131F thermobaric warhead variant of the 9K115-2 Metis-M, all of which are anti-tank missiles. The Kornet has since been upgraded to the Kornet-EM, and its thermobaric variant has a maximum range of 10 km (6 mi) and has a TNT equivalence of 7 kg (15 lb). The 300 mm (12 in) 9M55S thermobaric cluster warhead rocket was built to be fired from the BM-30 Smerch MLRS. A dedicated carrier of thermobaric weapons is the purpose-built TOS-1, a 24-tube MLRS designed to fire 220 mm (8.7 in) thermobaric rockets. A full salvo from the TOS-1 will cover a rectangle 200 by 400 m (220 by 440 yd). The Iskander-M theatre ballistic missile can also carry a 700 kg (1,540 lb) thermobaric warhead.

Many Russian Air Force munitions also have thermobaric variants. The 80 mm (3.1 in) S-8 rocket has the S-8DM and S-8DF thermobaric variants. The S-8’s 122 mm (4.8 in) brother, the S-13, has the S-13D and S-13DF thermobaric variants. The S-13DF’s warhead weighs only 32 kg (71 lb), but its power is equivalent to 40 kg (88 lb) of TNT. The KAB-500-OD variant of the KAB-500KR has a 250 kg (550 lb) thermobaric warhead. The ODAB-500PM and ODAB-500PMV unguided bombs carry a 190 kg (420 lb) fuel–air explosive each. The KAB-1500S GLONASS/GPS guided 1,500 kg (3,300 lb) bomb also has a thermobaric variant. Its fireball will cover a 150 m (490 ft) radius and its lethal zone is a 500 m (1,600 ft) radius. The 9M120 Ataka-V and the 9K114 Shturm ATGMs both have thermobaric variants.

In September 2007, Russia exploded the largest thermobaric weapon ever made. Its yield was reportedly greater than 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 American-developed Massive Ordnance Air Blast (MOAB) bomb, which has the backronym “Mother of All Bombs” and once held the title of the most powerful non-nuclear weapon in history. The Russian bomb contains a charge of approximately 7 tons of a liquid fuel, such as pressurized ethylene oxide, mixed with an energetic nanoparticle, such as aluminium, surrounding a high explosive burster that when detonated created an explosion equivalent to 39.9 tons of TNT.

American developments

Current American FAE munitions 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-55 FAE I
• CBU-72 FAE I

The XM1060 40-mm grenade is a small-arms thermobaric device, which was delivered to US forces in April 2003. 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, first used by US forces in 2003 in Iraq, uses a Metal Augmented Charge (MAC) warhead, which contains a thermobaric explosive fill that uses aluminium powder coated or is mixed with PTFE 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 result is a sustained high pressure that is extremely effective against people and structures.

Spanish BEAC thermobaric bomb project

In 1983, a program of military research was launched with collaboration between the Spanish Ministry of Defence (Directorate General of Armament and Material, DGAM), Explosives Alaveses (EXPAL) and Explosives Rio Tinto (ERT) with the goal of developing a Spanish version of a thermobaric bomb, the BEAC (Bomba Explosiva de Aire-Combustible). A prototype was tested successfully in a foreign location out of safety and confidentiality concerns. The Spanish Air Force has an undetermined number of BEACs in its inventory.

Indian development

Based on the high-explosive squash head (HESH) round, a 120 mm thermobaric round was developed, which packed thermobaric explosives into the tank shells to increase the effectiveness against enemy bunkers and light armoured vehicles.

The design and the development of the round was taken up by Armament Research and Development Establishment (ARDE). The rounds were designed for the Arjun MBT. The TB rounds contains fuel rich explosive composition called thermobaric explosive. As the name implies, the shells, when they hit a target, produce blast overpressure and heat energy for hundreds of milliseconds. The blast overpressure and heat energy causes collateral damage to enemy fortified structures like bunkers and buildings and for soft targets like enemy personal and light armoured vehicles.

History

Military use

The TOS-1 system was test fired in Panjshir Valley during the Soviet-Afghan War in the late 1980s. MiG-27 attack aircraft of the 134th APIB also used ODAB-500S/P fuel-air bombs against Mujahideen forces in Afghanistan, but they were found to be unreliable and dangerous to ground crew.

Unconfirmed reports suggest that Russian military forces used ground-delivered thermobaric weapons in the storming of the Russian parliament during the 1993 Russian constitutional crisis and during the Battle for Grozny (first and second Chechen Wars) to attack dug-in Chechen fighters. The use of TOS-1 heavy MLRS and “RPO-A Shmel” shoulder-fired rocket system during the Chechen Wars is reported to have occurred.

It is thought that a multitude of handheld thermobaric weapons were used by the Russian Armed Forces in their efforts to retake the school during the 2004 Beslan school hostage crisis. The RPO-A and either the TGB-7V thermobaric rocket from the RPG-7 or rockets from either the RShG-1 or the RShG-2 is claimed to have been used by the Spetsnaz during the initial storming of the school. At least three and as many as nine RPO-A casings were later found at the positions of the Spetsnaz. The Russian government later admitted to the use of the RPO-A during the crisis.

According to the British Ministry of Defence, the British military also used thermobaric weapons in its AGM-114N Hellfire missiles (carried by Apache helicopters and UAVs) against the Taliban in the War in Afghanistan.

The US military also used thermobaric weapons in Afghanistan. On 3 March 2002, a single 2,000 lb (910 kg) laser guided thermobaric bomb was used by the United States Air Force against cave complexes in which Al-Qaeda and Taliban fighters had taken refuge in the Gardez region of Afghanistan. The SMAW-NE was used by the US Marines during the First Battle of Fallujah and the Second Battle of Fallujah.

Reports by the rebel fighters of the Free Syrian Army claim the Syrian Air Force used such weapons against residential area targets occupied by the rebel fighters, such as during the Battle of Aleppo and in Kafar Batna. A United Nations panel of human rights investigators reported that the Syrian government had used thermobaric bombs against the rebellious town of Al-Qusayr in March 2013.

The Russia and Syrian governments have used thermobaric bombs and other thermobaric munitions during the Syrian Civil War against insurgents and insurgent-held civilian areas.

Terrorist use

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

References