A thermobaric weapon (also known as an aerosol bomb or Fuel Air Explosive) is a type of weapon the used the oxygen in the surrounding environment to generate a high-temperature explosion. While conventional explosive weapons are 25% fuel and 75% oxidizer, Thermobaric weapons consist of 100%fuel and as a result, are significantly more energetic than conventional explosives of equal weight. A thermobaric weapon consists of a container filled with fuel – which may be a form of aerosolised aluminium, magnesium, or silicon, which is dispersed into the air to mix with atmospheric oxygen and detonated by a small explosive charge at its centre.
This can be launched as a rocket or dropped as a bomb from aircraft. They are used for a variety of purposes and come in a range of sizes – including weapons for use by individual soldiers such as grenades and hand-held rocket launchers.
There are a number of different types of thermobaric weapons, The fuel-air explosive (FAE) is the most widely-used and well-known variant of thermobaric weapons. An FAE warhead consists of the fuel and two explosive charges with the munition fuzed to detonate above the ground with the first charge dispersing the fuel over a wide area.
When it hits its target, the first explosive charge opens the container and widely scatters fuel mixture as a cloud. This cloud can penetrate any building openings or defences that are not totally sealed. A second charge then detonates the cloud, resulting in a huge fireball, a massive blast wave and a vacuum which sucks up all surrounding oxygen.
Other variants are the Slurry Explosive Warhead, which consists of a combustible liquid mixed with a high explosive or solid explosive compound, and the Reactive Surround Warhead, which is a thin-walled container filled with combustible aluminium and nitrocellulose.
The energy produced by the charge both scatters and ignites the fuel as it expands. The shock wave expands from the point of detonation with the thermobaric mixture combusting behind it, maintaining the pressure of the shock wave. The pressure created is uniform and may last for up to one second, creating an effect of significant lethality. For comparison, the pressure of a fuel air explosive weapon may produce a blast overpressure of 2,000kilopascals (kPa) (290psi), whereas a conventional high explosive at the same distance from the blast would produce an overpressure of 200kPa. A blast overpressure of 1,380kPa in the open is uniformly fatal, according to a 2005 paper published by Toney Baskin and John Holcomb.
The result of a thermobaric blast is a very high peak temperature of 1,900 degrees celsius or higher, a long peak blast wave, and an extensive area of damage. The weapon can destroy reinforced buildings, and equipment and kill or injure people. 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, covered slit trenches, bunkers). It is capable of dealing large scale destruction in a wide area.
A thermobaric bomb’s effective yield depends on a 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. 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.
Thermobaric weapons have been around for decades, and reportedly used to clear helicopter landing zones by US forces in Vietnam. The US also used them in Afghanistan. First, in 2001 to try to destroy al-Qaeda forces hiding in the caves of the Tora Bora mountains, and in 2017, against Islamic State forces.
In 2003, the US tested a 9,800kg bomb, nicknamed the “Mother of all bombs”. Four years later, Russia developed a similar device, the Father of all bombs”. This created an explosion equivalent to a 44-tonne conventional bomb – making it the biggest non-nuclear explosive device in the world.
Russia has used thermobaric weapons – also known as vacuum bombs – in Ukraine, says the UK’s Ministry of Defence (MoD). The Soviet Union was an enthusiastic adopter of thermobaric weapons and has developed them in multiple guises from shoulder-launched rockets, to multiple rocket launchers, and air-dropped bombs. They have also been deployed in combat from Afghanistan, to Chechnya, throughout the war in Ukraine and in Syria too.
Bumblebee is the original Russian thermobaric weapon and is known by its formal name of RPO-Shmel. Its development in the 1980s eventually fed into the design of what became the TOS-1. The RPO-A Schmel is a shoulder fired weapon designed to deliver the effects of a 152mm howitzer round during close combat. The 93mm rocket carries a 2.1kg thermobaric warhead initially filled with a semi-liquid paste made up of RDX, aluminium and isopropyl nitrate. It is attributed with a maximum range of 1,000m and a sighting range of 600m.
The ODAB-500PMV is an air-dropped FAE with a weight of 525kg, 193kg of which is the explosive content. It can be carried by fixed- or rotary-wing aircraft and deployed from heights of 200-12,000m and 220-5,000m respectively. A Su-34 ground attack aircraft can carry as many as four ODAB-500PMVs, leaving hardpoints for air-to-air missiles and other munitions. Some Syrian Su-22Ms have been observed with six ODABs fitted to their under-wing pylons.
Thermobaric weapons are controversial because they are much more devastating than conventional explosives of similar size, and have a terrible impact on anyone caught in their blast radius. As per various news sources, these weapons have been deployed in such large numbers by the Russians that they have been causing massive civilian casualties in Ukrainian cities.
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.
There are no international laws specifically banning their use, but if a country uses them to target civilian populations in built-up areas, schools or hospitals, then it could be convicted of a war crime under the Hague Conventions of 1899 and 1907.
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