Conventional explosives, David Hambling explains (in Swarm Troopers), are composed of large molecules that break down and release energy:
Those bonds are unstable, and when they are broken, the explosive detonates with a velocity of more than eight thousand meters a second.
By contrast, thermobarics do not explode at all; technically, they just burn very fast. Some types have their own oxidizer, but some simply react with oxygen in the air. In its simplest form, enhanced blast can be achieved simply by adding finely powdered metal such as aluminum to an explosive charge. More sophisticated versions consist of nothing but powdered metal and oxidizer; the explosive is released into a cloud, which is then set off with devastating effects.
Thermobarics are typically several times as powerful as TNT by weight because the oxidation reaction is more energetic than the breakdown of an explosive molecule. However, what is more surprising is that thermobarics are so far more destructive than condensed explosives with the same power. This is because the blast from an expanding thermobaric fireball goes on for longer than a normal blast. It still only lasts a matter of milliseconds, but the increased duration makes it more effective at bringing down walls.
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Known as the SMAW-NE (for Novel Explosive) , the new warhead contains four pounds of a mixture known as PBXIH-135, which combines a standard plastic explosive (PBX – Plastic Bonded eXplosive) with a precisely calibrated amount of finely powdered aluminum.
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One limitation was that the new SMAW round was far more effective inside a building than in the open air. Marines started using a two-stage approach: firing one of the old high-explosive SMAW rounds to make a hole in a wall, then firing a thermobaric round through the hole into the interior.
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You might survive a blast of forty pounds per square inch from a condensed explosive, but just ten pounds per square inch for a few milliseconds longer from a thermobaric blast will pulverize your lungs. Body armor and sandbags offer no protection from this sort of damage.
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In particular, the technology for producing nanoscale particles of aluminum, and storing them safely, has progressed
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Unclassified results from one Canadian research group suggest that it should be feasible to make warheads around five times as powerful as existing munitions without changing the ingredients.
We’ve had thermobaric rounds for the 40mm grenades for quite a while now.