Recent videos of Russian rocket attacks in Ukraine show dramatic visible shockwaves emanating from the explosions. These videos are identified as strikes by TOS-1A multiple rocket launchers firing thermobaric rounds, described as a “truly a weapon from hell.” The rockets are known for their explosive power, and the unusual visible shockwaves are a feature of their unique design – and may help investigators looking into violations of international humanitarian law find where the weapons are used.
The TOS-1A delivers heavier firepower over a shorter range than other Russian multiple launchers, firing a salvo of 24 rockets, each weighing 217 kilos to a range of eight kilometres. The thermobaric warheads are distinct from normal high explosives because the blast comes from a rapidly expanding fireball rather than a single point like normal high explosive. Russia classifies the weapon as a ‘heavy flamethrower’ rather than artillery: it is less effective against infantry in the open because it does not produce shrapnel, but is brought up to engage strongpoints and fortifications from close range. It is indiscriminately destructive, leading Marc Garlasco, of Netherlands-based group PAX
Every explosion, even toy balloon popping, produces a shockwave. This is similar to a sound wave but travels at supersonic speed. As the shockwave spreads out, it slows down, and soon decays into a sound wave. Like soundwaves, shockwaves are usually as invisible, but according to Prof. A Michael Birk of Queens’s University, Canada, we may be able to see them via to the formation of an effect known as a condensation cloud.
Prof Birk has led research into Boiling Liquid Expanding Vapor Explosions or BLEVEs (pronounced ‘blevvies’) which occur when vessel holding a pressurized liquefied gas fail catastrophically and the expanding gas cloud detonates, and which produce a two-phase explosion.
“You have a large volume at high pressure gas that suddenly expands out into the environment. This expansion pushes on the ambient surrounding air and this starts a shock wave (a hemisphere or sphere) that runs out at supersonic speed.,” Prof Birk told Forbes. “The expansion causes the pressure to drop in the initial volume and it actually overshoots and the pressure drops below ambient pressure.”
During this below-ambient or negative-pressure phase, if the air is humid the reduced pressure causes moisture to condense out of the air, creating an instant cloud of fog. The condensation cloud may only last a fraction of a second before pressure is restored and it evaporates again, just long enough for it to be visible. What we see on these videos are transient condensation clouds just behind the shockwave.
Condensation clouds are sometimes seen in large conventional explosions. For example, in the 2020 Beirut port blast, when over five hundred tons of ammonium nitrate detonated, video briefly shows a giant, rapidly-expanding white shell behind the shockwave.
The effect is most obvious in film from early nuclear warhead tests, like the Baker test during Operation Crossroads in 1946, in which a 25-kiloton atomic bomb was detonated underwater beneath a flotilla of 68 target ships. A white cloud, known as a Wilson Cloud in nuclear weapons research, obscured the scene for two seconds after the initial shockwaves before dispersing to show the column of water and wreckage thrown high into the air.
The TOS-1 rockets are packed with thermobaric explosive, a mixture of liquid isopropyl nitrate and powdered magnesium. The magnesium powder burns on contact with air, producing an expanding fireball similar to a BLEVE and producing the same powerful shockwave and trailing negative-pressure phase. This is why thermobarics are sometimes misleadingly called vacuum bombs.
While thermobaric blasts do not necessarily produce greater pressure than other explosives, the shockwave lasts longer and is far more effective at bringing down buildings. Unlike other explosions, a thermobaric blast wave ‘flows’ around corners and can kill personnel inside trenches or bunkers who are protected from shrapnel. Hence the use of thermobarics in place of a flamethrower to attack fortified positions.
This type of weapon is especially dangerous for civilians, and thermobarics are widely criticized for their indiscriminate destruction, especially in urban areas. In August a team of medical and physics researchers from Saint Louis University, Missouri calculated that in addition to the immediate deaths, one salvo of TOS-1 rockets in an urban area would likely inflict more than 300 cases of traumatic brain injury, some with long-lasting effects.
“The TOS-1 violates the principle of distinction under international law,” Garlasco told Forbes. “The requirement for a weapon to distinguish between a military object and a civilian object. In the real world that means if you hit a tank or command post in a town it is impossible not to also envelop civilian homes in such strikes.”
Garlasco is training war crimes investigators for Ukraine, and says that videos may help them identify locations where crimes have been committed.
“Videos of TOS-1 use are of great value to war crimes investigators as they help us confirm weapon use in populated areas in addition to any forensic evidence gathered at the scene,” says Garlasco.
Russia has made free use of thermobarics, cluster bombs and anti-personnel mines in this conflict, apparently confident that nobody will ever be held to account for breaking international law. That may prove to be another serious miscalculation. The explosion videos on social media may help ensure that justice is done, however long it takes.
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