The Economist explains

What are thermobaric weapons, and does Russia have them in Ukraine?

Also known as “vacuum bombs”, they are fearsomely destructive when used indiscriminately

MOSCOW, RUSSIA - MAY 7, 2021: TOS-1A Solntsepyok multiple rocket launchers move along Moscow's Red Square during a dress rehearsal of a military parade marking the 76th anniversary of the victory over Nazi Germany in World War II. Gavriil Grigorov/TASS (Photo by Gavriil Grigorov\TASS via Getty Images)

SINCE RUSSIA invaded Ukraine on February 24th, Ukrainian officials and human-rights organisations have raised the alarm over the possible use of thermobaric weapons or “vacuum bombs”, with horrifying accounts of how they suck the air out of victims’ lungs. On March 9th Britain’s defence ministry said that Russia had confirmed it is using the weapons in Ukraine. How does a thermobaric weapon work, and what role might they play in the war?

Russia is not unique in having access to such weapons, and neither are they new. The technology was developed by the US Air Force in the 1960s for clearing minefields in Vietnam. In its earliest incarnation, a 340kg bomb scattered three smaller submunitions which released an aerosol cloud of ethylene oxide, a flammable, colourless gas. When detonated, this created a powerful blast wave over a large area. This was known as a fuel-air explosive. Later developments, in both the West and Russia, produced solid fuel-air explosives, or thermobaric designs. These typically involved a core of high explosive surrounded by powdered metal such as aluminium. The explosive fireball ignites the aluminium, but as there is not enough oxygen in the immediate surroundings for the metal powder to burn fully, the outer edge of the fuel-rich cloud continues to burn explosively as it expands.

Although the peak strength of the shock wave (known as overpressure) of a thermobaric blast is typically less than that of a high explosive, it is of longer duration and produces a larger overall impulse (see chart). That makes it much deadlier; it can destroy delicate lung tissue or force air into the bloodstream, causing a massive embolism without leaving external signs of injury. And a thermobaric blast typically has a powerful negative phase of pressure, as the fireball cools and air is sucked towards the point of explosion, hence the grim moniker of “vacuum bomb”. The combination of a greater impulse and the negative phase makes thermobarics particularly effective at demolishing structures. And the longer blast pulse can bend around corners, reaching otherwise protected positions such as trenches or bunkers.

Russian forces use the terms “fuel-air explosive” and “thermobaric” interchangeably, while America and Britain refer to them as “enhanced blast” weapons. Western armies have plenty of enhanced-blast weapons, including a thermobaric version of the air-to-ground Hellfire missile, and a portable rocket-launcher known as an “anti-structure munition” optimised to destroy bunkers. America used “bunker busting” bombs to attack Taliban caves in Afghanistan in 2002. But Russia has shown a particular interest in large-scale applications. The Russian TOS-1 “Buratino”, which has been deployed in Ukraine, is a tracked launch vehicle capable of firing 24 thermobaric rockets, each weighing as much as 217kg. It is officially described as a “heavy flamethrower” rather than an artillery piece, because it takes on the traditional flamethrower role of attacking fortifications. One salvo covers an area of 80,000 square metres, or more than 11 football pitches.

Russia used such weapons against civilian targets in Grozny, Chechnya’s capital, in 1999 and in Syria, in 2018. It is impossible to use the TOS-1 in a populated area in accordance with humanitarian law, which demands that weapons be discriminate and their effects proportional to military objectives. Marc Garlasco of PAX, a Dutch anti-war campaign group, calls the TOS-1 “a war crime on tracks”. And something even deadlier may be in the wings. In 2007 the Russian army released a video of a test of a seven-tonne air-dropped bomb dubbed “the father of all bombs”. This thermobaric device had an effect reportedly equivalent to 40 tonnes of TNT; Russian officials compared its effects to those of a small nuclear weapon.

Thermobaric weapons are not necessarily more abhorrent or indiscriminate than the conventional sort, unlike anti-personnel mines or cluster bombs. But they can be used in indiscriminate and highly destructive ways. If Vladimir Putin’s forces use them in Ukraine, it could have devastating effects.

Our recent coverage of the Ukraine crisis can be found here.

More from The Economist explains:
The war in Ukraine, explained in maps
Why Z is for Putin
What is the Wagner Group, Russia’s mercenary organisation?

More from The Economist explains

What evidence reveals about the Gaza hospital blast’s source

The damage points to a malfunctioning rocket, not an air strike

What is Palestinian Islamic Jihad?

Israel blames the group for a deadly explosion at a hospital in Gaza

What is Hizbullah?

The Iran-backed militia has long resented Israel