The Thermobaric Weapon, also known as a Fuel-Air Munition or Vacuum Bomb, is a category of powerful explosive device. It's explosion happens in stages. First it release a cloud of powdered explosive, flammable vapour or flammable metal (such as a mixture of finely powdered aluminum and ammonium perchlorate), then that cloud is ignited. Effectively, it turns the air in the area of the cloud into a huge bomb. The resulting explosion has fragmentation, concussion, and incendiary elements. The concussion force plays out over a longer duration than the equivalent mass in most other explosives, which makes it very potent at knocking over structures. Also, the temperature spike of the resulting fireball is much higher than most non-nuclear explosives can generate.
Thermobaric Weaponry is particularly potent inside defensive structures such as bunkers and caves. The walls of such structures provide shelter from conventional explosives, but provide an afterburn affect when hit by fuel-air bombs that is similar to the firing of a piston. The stronger the walls, the more devastating the bomb is to those inside the building. Even when the walls of the structure are strong enough to withstand the shockwave a thermobaric blast consumes a significant proportion of the oxygen in its area of effect leading to an anoxic environment and a sudden pressure drop, followed by a secondary blast as the vacuum is filled.
The developement of thermobaric weapons was retarded to some degree by difficulties in dispersing the fuel widely enough for complete combustion - the German Flakwaffe in WW2 experimented with coal dust clouds to attack Allied bomber streams, but were unable to solve the dispersion problem.
"Natural" fuel-air explosions can occur very easily in any environment containing large quantities of flammable dust - flour and cotton mills, coal processing facilities and instant custard factories are common venues, and anywhere handling volatile flammable liquids is also susceptible. A common feature of these is the chain-explosion where a small initial blast stirs up and ignites more fuel, leading to a secondary explosion … and so on, until the fuel source is exhausted.
Game and Story Use
- Because of the high temperatures, a large Thermobaric Weapon might be used in desperation to stop a Zombie Apocalypse or burn-out a virulent airborne disease.
- In an investigation game, a clue might be that the Big Bad has ordered or manufactured a supply of powdered aluminum. Turns out he's planning to make a very big bomb.
- Fuel-Air Bombs may be used in situations where a government wants to apply force similar in power to a tactical nuke, but not have the nasty side-effects and lingering radioactive contamination.
- However, one of the reasons no-one developed fuel-air bombs at operational level or above during the cold war was that the detonation can be hard to tell from a nuclear blast and might lead to a reply in kind before the target realises his mistake.
- Actually, flour is highly flammable. Flour mills or bakeries or any place that might have flour dust in the air pose a risk of incendiary explosions. This makes thermobaric weapons possible in a low-tech setting.
- Likewise cotton mills, coal mines or anywhere else with a lot of flammable dust in the air.
- This troper knew one player who was fond of chucking bags of flour at a target by trebuchet, and then firing flaming arrows at it.
- Note that as a GM you don't need to allow this - as the Flakwaffe discovered, getting the right dispersion is hard and (quite apart from the rule zero of all RPGs) you would be entirely on-reservation in stating that throwing a bag of flour from a catapult just makes a mess rather than a thermobaric blast.
- In a modern campaign the PCs could end up fighting in an explosive dust atmosphere quite easily - and a place where even nylon clothing can get you blown up, let alone the muzzle flash of a firearm.