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What is a thermobaric bomb?

Russia has been accused of planning to use thermobaric weapons – sometimes known as vacuum bombs – in its 2022 attack of Ukraine. This article examines what these weapons are.

Oksana Markarova, Ukraine’s ambassador to the United States, has accused Russia of using a vacuum bomb during its invasion. However, there has been no official confirmation of this claim. There have also been reported sightings of thermobaric rocket launchers in Ukraine in late February 2022. Given Russian forces are trying to take control of the capital, Kyiv, and other large cities in the east of the country, it is suspected their use is likely in Ukraine.

Of current concern in Ukraine is if the Russian army deploy their TOS-1 (Russian: тяжёлая огнемётная система [ТОС-1], Heavy Flamethrower System) weapon systems to deliver thermobaric attacks. The TOS-1 is a Soviet 220 mm 30-barrel (TOS-1M) or 24-barrel (TOS-1A) multiple thermobaric weapon launcher mounted on a T-72 tank chassis. The devastation this would cause if fired into cities would be immense.

The concern about the use of such weapons largely stems from the fact that they are far more lethal than other conventional explosive weapons of similar calibre, and cause horrific injuries.

This is in large part because they withdraw oxygen from the air around them to create an explosion with a longer blast wave that burns at a much higher temperature. Developed in the mid-20th century, vacuum bombs have been used by global powers such as the US in Iraq and Afghanistan.

Historic Use by States

A thermobaric TOS-1 system was test fired in Panjshir Valley during the Soviet War in Afghanistan[2] and Russia’s MiG-27 attack aircraft also used ODAB-500S/P fuel-air bombs[3] against Mujahideen forces (in Afghanistan).

In the mid 1990s Russia used a series of thermobaric weapons dropped by planes against Chechen separatists such as the TOS-1 Buratino against the capital of Grozny[4], to international condemnation. 1993 also saw unconfirmed reports that Russian military forces used ground-delivered thermobaric weapons in the storming of the Russian parliament during the 1993 Russian constitutional crisis and also during the Battle for Grozny (in the first and second Chechen wars) to attack dug-in Chechen fighters[5]. The use of both TOS-1 heavy MLRS and “RPO-A Shmel” shoulder-fired rocket systems in the Chechen wars was also reported[6].

The US military also used thermobaric weapons in Afghanistan. The SMAW-NE was used by the US Marines during the First Battle of Fallujah and Second Battle of Fallujah[7], and 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[8]

The US and Russia have both tested larger thermobaric bombs.

Introduced in 2003, the GBU-43/B Massive Ordnance Air Blast[9] (nicknamed the Mother of All Bombs) has yet to see use against a target outside a test range – developed for use in mountainous regions of Afghanistan where it was believed al-Qaeda leader Osama bin Laden was hiding. The US has also said a variety of thermobaric weapons have been ‘‘highly effective in Iraq[10]’’. In 2007, Russia followed suit and tested their own giant thermobaric weapon calling it the ‘Father of All Bombs[11]’.

It is theorized 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 TBG-7V thermobaric rocket from the RPG-7 or rockets from either the RShG-1 or the RShG-2 is claimed to have been used[12] by the Spetsnaz during the initial storming of the school.

A government strike in 2012 in Azaz, Syria that killed more than 40 civilians was linked to an ODAB-500 PM, a 500kg fuel-air explosive bomb of Russian origin[15], and on September 29, 2013, a thermobaric weapon was dropped on a secondary school in Raqqa, killing at least fourteen people. The trauma suffered (flash burns and blast wave injuries) suggested a volumetric weapon[16]. Reports by the rebel fighters of the FSA claim the Syrian Air Force used such weapons against residential area targets occupied by the rebel fighters, in the Battle for Aleppo[17]. A panel of UN human rights Investigators also reported that the Syrian government used thermobaric bombs against the rebellious town of Qusayr in March 2013[18].

How does a thermobaric bomb work

A vacuum bomb, also called an aerosol bomb or fuel air explosive, consists of a fuel container with two separate explosive charges. Rather than using a mix of fuel and oxygen within the bomb itself, thermobaric weapons consist largely of fuel – carrying more energy and having the potential to cause great destruction when launched in field operations to destroy bunkers, tunnels and foxholes.

The weapon can be launched either as a rocket or dropped from aircraft. When it strikes its target, the primary explosive charge breaks opens the container and fuel mixture is widely scattered as a cloud.

This cloud is able to penetrate building openings or defences that are not totally sealed. A second charge then detonates that dust cloud causing a huge fireball, a huge blast wave and a vacuum which sucks up all surrounding oxygen. Thermobaric weapons are used for a variety of purposes and can come in a range of sizes. As above, large air-launched versions have been made, specifically to target armed actors hiding in caves and tunnels – the use of this weapon in enclosed spaces is devastating.

A key element in the development of this weapon was based on the observations of the impact of ‘peak pressure’ by conventional weapons. For many years, manufacturers of traditional explosive munitions sought a weapon design that aimed to increase the peak pressure – the so-called blast wave. This was driven by the belief that the higher the pressure went in a blast, the more damage it could cause. And in large part, this worked.

However, munition scientists also realised that another way to deliver explosive forces was to try to impart energy direct to a target in order to destroy it. In this framing, high peak pressure did not necessarily equate to high energy. They realised that if they did not always aim for peak pressure and, instead, designed a munition with a long rather than a short pulse, they could impart much more energy. The result? A fuel air mix that detonated slowly. This allowed the blast wave to “couple” with a target like a building and, when that happened, they found it had the power to push the building over.

What was not expected was what came next – something the scientists found really interesting but the rest of us might find terrifying. The long pulse ‘bounced’ around the place, literally. The scientists found that those areas where a long ‘positive pulse’ added to a reflected ‘positive pulse’, but where they also found areas where the long ‘negative pulse’ added to a reflected ‘negative pulse’. What happened in between these long positive pulses and long negative pulses was of particular note – because it could cause a vacuum effect that was as devastating as the initial blast.

Fuel-Air Explosives (FAE)

Fuel-air explosion hazards are of considerable concern in the agricultural, refining and chemical processing industry. There have been a number of catastrophic explosion accidents, with significant consequences in terms of death, injury, property damage, business interruption, loss of goodwill and environmental impact, following such events.

One such example was the devastation the Flixborough Nyproplant, an explosion at a chemical plant close to the village of Flixborough, North Lincolnshire, England on Saturday, 1 June 1974. When 36 tonnes of cyclohexane vapour escaped during a maintenance task, mixed with air, and found an ignition source, the ensuing blast killed 28 people and seriously injured 36 out of a total of 72 people on site at the time. Properties up to four miles away were damaged in the blast.

A fuel-air explosion differs from that created by a conventional explosive in that the fuel elements do not carry their own oxygen. For an explosion to occur, fuel must be mixed with ambient atmospheric air, and when mixing is complete, be initiated by a delayed ignition source. Delayed ignition is required in order that the fuel can mix appropriately with the air. Fuels involved take the form of gases, vapours, aerosolised liquids or particulate dust clouds.

The energy release in a fuel-air explosion greatly exceeds the energy of detonation of condensed explosives such as RDX/TNT/PETN, with heats of explosion (Q) of some open literature fuels being identified in Table 1. As can be seen, the energy created by an aluminium dust cloud is almost four times that produced by TNT.

FuelQ (kJ/kg)Ease of initiation
TNT (solid explosive)4200Secondary explosive
Ethyne5000Easy
Aluminium (dust cloud)16000Difficult
Ethylene oxide (vapour)21000Easy
Gasoline (aerosol)37000Moderate
Propane38000Moderate
Table 1: Heats of explosion of some unclassified fuels

For vapour cloud explosions there is a minimum ratio of fuel vapour to air below which ignition will not occur. Similarly, there is also a maximum ratio of fuel vapour to air above which ignition will not occur. These are termed upper and lower explosion limits. Almost all organic material in the form of a dust cloud will ignite at temperatures below 500 oC .

For those who have watched the film Sleepy Hollow with Johnny Depp, the windmill explosion scene is a theatrical demonstration of flour mixing with air and subsequently detonating. Dusts are quantities of particulate solids – small enough that they remain suspended in the air for significant periods of time when airborne. There is a direct correlation between particle size and explosive hazard – the smaller the particle the more reactive the dust.

The full heat of explosion of a fuel is only realised when it is mixed with its chemical equivalent (stoichiometric) quantity of air. However, a fuel/air explosion is very inefficient in terms of energy (little more than 40%). This is because energy is absorbed by nitrogen in the air, which dilutes the system, and imparting motion to the explosion gases reduces the energy available to the blast wave. The nine-fold advantage of propane over TNT is therefore reduced to four-fold in reality.

But what are the effects in real life?

The initial detonation pressures and velocities in fuel-air explosions are much less than those for an equivalent weight of conventional explosive. Therefore, fuel-air explosions have little comparable brisance or shattering effect. Notwithstanding this, they do have other advantages in the propagation of the air blast. When a condensed explosive detonates as a ‘point source’, the blast overpressure falls rapidly with increasing distance from the charge. In contrast, a fuel-air explosion is a large cloud, which cannot be regarded as a point source and therefore the decay in peak overpressure falls much less rapidly with distance from the edge of the cloud. Furthermore, the duration of the positive phase of the blast wave is greater than that from condensed explosive so the impulse will be even greater. FAE therefore possess the potential to do more harm on the surrounding environment, the positive phase duration being an important parameter in the response of structures to a blast.

Although on first sight the lower overpressure to that of a conventional solid explosive would seem to be a disadvantage, civilian accidents, trials and operational use have proved the effectiveness of FAE against personnel, soft skinned vehicles, parked aircraft and structures – a maximum overpressure of 0.9 bar is all that is required to sink a ship and this value was obtained from the sinking of the ‘mothballed’ destroyer USS McNulty in 1973 with ethylene and propylene oxide.

The following data has been published in open source (Table 2):

Breakage of:Overpressure (bar)Significance to Belligerent
Windows0.03 – 0.07Penetration injuries
Corrugated steel0.07 – 0.17Security force checkpoints
Wood panels0.07 – 0.14Blast inside buildings
Concrete walls0.13 – 0.21Blast outside buildings
Metal sheeting (5 mm)>0.3Destroys parked aircraft and B vehicles. >0.4 disables shipping
Brick walls0.48 – 0.55Destruction of buildings
Reinforced structures0.7 – 0.9Sinks shipping, brings down bridges, cracks underwater tunnels
Table 2: Fuel-air explosion overpressure damage

In terms of belligerent intent, FAE have been used by both terrorists and criminals to varying degrees of success.

Notable examples are as follows, success being dependent very much upon delayed initiation: the 1983 Beirut bombing, where butane and PETN were used; the 1993 World Trade Centre bombing where an urea nitrate main charge (enhanced with aluminium, magnesium and iron oxide) sat surrounded by bottled hydrogen to enhance the fireball and afterburn of the solid metal particles; 1994 Jerusalem, propane and detonating cord (PETN); Columbine High School 1999, where propane bombs were planted in the school dining hall; the Ghriba synagogue bombing in 2002, with propane cylinders, PETN and delay detonators; Glasgow International Airport 2007, propane; Time Square 2010, propane, gasoline and urea with an attempted delay in design; Brindisi school bombing 2012, propane; Notre Dame 2016, propane; Rotterdam 2017, butane; Melbourne 2018, propane.

The trend above demonstrates the ease with which propane can be obtained, but also the limited success rate due to explosive limits and severity index. Very few examples show effective dispersal mechanisms followed by a secondary initiation mechanism. It has been more practical to expel cylinder contents into the lower floors of structures and ‘marry up’ the fuel-air cloud with an ignition source in another room.

Thermobaric Compositions

Russia inherited a major lead in volumetric explosive technology during the 1980s through the development of thermobaric weapons, or metal-rich explosives, which allow two casualty producing elements to be harnessed simultaneously – heat (thermo) and over pressure (baric). 

The thermobaric explosive was first weaponised by Russia as a replacement for the LPO-50 backpack flame-thrower and then RPO-A, an 11kg shoulder launched missile with a 2 kg charge, was fielded in Afghanistan to clear the Mujahideen from mountain caves and the Karez irrigation tunnel system.

Two-phase initiation with FAE adds complexity and the attraction of a thermobaric system is that it requires no separate secondary means of initiation. As such, it can be used reliably as a stand-alone or projected IED.

The fuel consists of a monopropellant and energetic particles (particles with high heats of combustion such as beryllium, boron, lithium, aluminium, magnesium, zinc or sulphur). In terms of functionality, a bursting charge is still required but in this instance disperses an explosive slurry, which then explodes to achieve the desired blast effect. The mechanism works as follows (and can be considered two-stage):

  • Firstly, anaerobic action (without air oxygen) occurs inside a conventional high explosive core in microseconds;
  • Then aerobic delayed burning action of the fuel-rich outer charge, which depends mainly on the consumption of the surrounding air and which results in an intense fireball (5000 oC+) and high blast overpressure. The fuel can rapidly diffuse within structures, consuming the oxygen and creating conditions that make human survivability extremely unlikely (heat, flame and barotrauma to the lungs) 

Are thermobaric bombs illegal?

Unlike landmines or cluster munitions, no international conventions or laws specifically bans their use, though AOAV believes there should be, given their likely use in towns and cities.

If they are used against civilian populations in populated areas, this could be construed as a war crime under the Hague Conventions of 1899 and 1907. And the International Criminal Court prosecutor Karim Khan has said his court will investigate possible war crimes in Ukraine.

Further, in an analysis of legal standards for protecting civilians in war by the United Nations Institute for Disarmament Research, author Maya Brehm notes that thermobaric weapons may fall under the scope of the Convention on Certain Conventional Weapons regarding incendiary weapons, stating “thermobaric weapons generate high temperatures that can start fires, and can cause particularly cruel wounds to people within a wide area[36].” Brehm also notes that thermobaric weapons might already fall under other rules governing enhanced explosives instead[37]. Yet with both the United States and Russia maintaining some thermobaric weapons in military inventory, the success of a treaty banning thermobarics seems unlikely.

The Arms Control Association says there are no treaties that prevent the development of thermobaric weapons, although they could conceivably fall under a protocol of the Convention on Conventional Weapons, which prohibits incendiary weapons[38].

As it said: ’Currently there is no international treaty that specifically regulates the use of nanotechnology for military purposes or otherwise. A preventive arms control treaty to regulate or ban the use of nanotechnology for military purposes is unlikely to materialize because international arms control treaties tend to be reactive to technological developments and are limited in scope, prohibiting or regulating only specific weapons defined by their design, intent, and characteristics[39]’’.

However, the use of nanotechnology is already restricted to the extent that it is used to develop or enhance weapons that are prohibited by existing arms control treaties, such as biological weapons, chemical weapons, non-detectable fragments, blinding laser weapons, anti-personnel mines, explosive remnants of war,  and, most recently, cluster munitions. Nanotechnology, if used as an enabling technology for weapons development in these areas, would be regulated by the relevant treaty.

Depending on which approach is taken, the legality of a military application of nanotechnology may well be considered differently. This is particularly so ‘when the application of nanotechnology is designed to enhance penetration capabilities of a weapon, such as thermobaric explosives, to destroy targets inside hardened and deeply buried structures or buildings, yet potentially involving hazardous health and environmental impacts[40].’ For example, the deployment of nano-energetic thermobaric explosives could well be justified on the grounds that targeting terrorists or insurgents inside hardened compounds outweighs considerations of severe suffering from the primary blast or thermal damage for combatants or civilians taking a direct part in hostilities.

AOAV thanks Brimstone Consultancy Limited, and AOAV volunteer researcher Nadine Easby for their generous help on this paper.

References

Academic Dictionaries and Encyclopedias. 2022. Thermobaric weapon. [online] Available at: <https://en-academic.com/dic.nsf/enwiki/19316> [Accessed 1 January 2022].

AOAV. 2014. Azaz, Syria – AOAV. [online] Available at: <https://aoav.org.uk/2014/azaz-syria/> [Accessed 1 January 2022].

Baker, C., 2016. U.S Army Joint Global Response. Joint Global Response Force Research, p.53.

Blank, S., 2005. Eurasia Daily Monitor. [online] The Jamestown Foundation. Available at: <https://jamestown.org/program/russia-will-sell-eritrea-anti-tank-missiles-to-use-against-ethiopias-russian-made-weapons/> [Accessed 1 January 2022].

Brehm, M., 2012. Protecting Civilians from the Effects of Explosive Weapons: An Analysis of International Legal and Policy Standards. [online] UNIDIR. Available at: <https://unidir.org/files/publications/pdfs/protecting-civilians-from-the-effects-of-explosive-weapons-en-293.pd> [Accessed 1 January 2022].

Cumming-Bruce, N., 2013. U.N. Panel Reports Increasing Brutality by Both Sides in Syria (Published 2013). [online] Nytimes.com. Available at: <https://www.nytimes.com/2013/06/05/world/middleeast/un-panel-reports-increasing-brutality-by-both-sides-in-syria.html> [Accessed 1 January 2022]

En.wikipedia.org. 2021. Syrian civil war – Wikipedia. [online] Available at: <https://en.wikipedia.org/wiki/Syrian_civil_war> [Accessed 1 January 2022].

En.wikipedia.org. 2022. Thermobaric weapon – Wikipedia. [online] Available at: <https://en.wikipedia.org/wiki/Thermobaric_weapon> [Accessed 1 January 2022].

Episkopos, M., 2021. Russia’s Father of All Bombs Is No Metaphor. [online] The National Interest. Available at: <https://nationalinterest.org/blog/reboot/russias-father-all-bombs-no-metaphor-193326> [Accessed 1 January 2022].

En.wikipedia.org. n.d. Mk 153 Shoulder-Launched Multipurpose Assault Weapon – Wikipedia. [online] Available at: <https://en.wikipedia.org/wiki/Mk_153_Shoulder-Launched_Multipurpose_Assault_Weapon> [Accessed 1 January 2022].

Human Rights Watch. 2013. Syria: Fuel-Air Bombs Strike School. [online] Available at: <https://www.hrw.org/news/2013/10/01/syria-fuel-air-bombs-strike-school> [Accessed 1 January 2022].

Mcgregor, A., 2006. Hezbollah’s Creative Tactical Use of Anti-Tank Weaponry. [online] The Jamestown Foundation. Available at: <https://jamestown.org/program/hezbollahs-creative-tactical-use-of-anti-tank-weaponry/> [Accessed 1 January 2022].

Nasu, H., 2012. Nanotechnology and challenges to international humanitarian law: a preliminary legal assessment. International Review of the Red Cross, 94(886), pp.653-672.

Nowack, P., 2007. CBC News In Depth: Technology. [online] Cbc.ca. Available at: <https://www.cbc.ca/news2/background/tech/how-it-works/thermobaric-weapons.html> [Accessed 1 January 2022].

Reuters. 2020. U.S. warship in Arabian Sea seizes suspected Iranian weapons. [online] Available at: <https://www.reuters.com/article/uk-usa-iran-missiles-military-idINKBN2072NN> [Accessed 1 January 2022].

Roblin, S., 2020. Why Russia’s TOS-1 MLRS ‘Buratino’ Is No Joke. [online] The National Interest. Available at: <https://nationalinterest.org/blog/buzz/why-russias-tos-1-mlrs-buratino-no-joke-128782> [Accessed 1 January 2022].

Tribess, M., 2022. Afghanistan/US, ‘Mother of all bombs’ | How does law protect in war? – Online casebook. [online] Casebook.icrc.org. Available at: <https://casebook.icrc.org/case-study/afghanistanus-mother-all-bombs> [Accessed 1 January 2022].

Wikiwand. 2018. Battle of al-Bab | Wikiwand. [online] Available at: <https://www.wikiwand.com/en/Battle_of_al-Bab> [Accessed 1 January 2022].


[1](Thermobaric weapon – Wikipedia, n.d.)

[2] ibid.

[3] ibid.

[4] (Roblin, 2020)

[5] ibid.

[6] (Thermobaric weapon – Wikipedia, n.d.)

[7] (Mk 153 Shoulder-Launched Multipurpose Assault Weapon – Wikipedia, n.d.)

[8](Thermobaric weapon – Wikipedia, n.d)

[9] (Tribess, 2022)

[10] (Nowack, 2007)

[11](Episkopos, 2021)

[12](Thermobaric weapon – Wikipedia, n.d)

[13](Mcgregor, 2006)

[14] (Miranda, 2022)

[15] (Azaz, Syria – AOAV, 2014)

[16](Syria: Fuel-Air Bombs Strike School, 2013)

[17](Syrian civil war – Wikipedia, 2021)

[18](Cumming-Bruce, 2013)

[19] (Baker, 2016)

[27] (Battle of al-Bab | Wikiwand, 2018)

[35] (U.S. warship in Arabian Sea seizes suspected Iranian weapons, 2020)

[36] (Brehm, 2012)

[37] ibid.

[38](Nowack, 2007)

[39] (Nasu, 2012)

[40] ibid.

[41] ibid.