Thermobaric bombs, al Qaeda’s new weapon of destruction, raises the stakes in the war on terror.
Investigators now believe the bombing on Sep. 21 that killed dozens and left massive damage at the Islamabad Marriott, including a gaping hole in the ground in front of the building, was a crude form of a device that intensifies and enhances an explosive, a thermobaric bomb.
The bomb was delivered in a truck that contained what investigators believe was aluminum powder in addition to grenades and artillery shells. The aluminum power is believed to have been responsible for the acceleration and expansion of the impact of the bomb.
While barriers around the hotel kept the truck bomb at some distance from the structure, the devastation indicated that there had to be something capable of raising the devastation level considerably
Aluminum powder has long been used to boost the power of explosives. Blast weapons like the 15,000-pound BLU-82 Daisy Cutter and the 21,600-pound “Mother of All Bombs” use it to increase their destructive force.
Devices with a high proportion of metal powder to explosive are termed “thermobaric.” When the explosive goes off, the metal powder at the leading edge of the fireball burns as it contacts the air. With a crude device, the powder simply burns and adds to the fireball. In more advanced weapons, the burning metal produces a sub-sonic shockwave (known as deflagration); the most advanced produce a detonation (supersonic shockwave) of tremendous destructive power.
Normal, condensed explosives produce a very short pressure pulse. A “volumetric” one, from a detonating fireball, produces an extended blast pulse that is far more damaging to buildings. The Marriott attack left a large crater, indicating that much of the blast came from a point source. The metal powder seems to have contributed only to the incendiary effects. According to the Guardian, “the temperature had reached 400C, investigators said, which made the hotel’s sprinkler system and the fire service useless.”
By all accounts, there was a long delay before the device went off, with the truck burning sometime before the explosion.This was not a device built by master bombmakers. And the hotel’s security barrier performed a vital function of keeping the bomb away from the building. Distance is life in these situations. (Compare the Marriott blast with the Oklahoma City bombing; there was a lot of structural damage to the Alfred P. Murrah Federal Building, because McVeigh was able to drive right up to it.) However, a thermobaric blast would extend the radius of effect of such a truck bomb significantly.
The blast and fire damage at the Islamabad Marriott were severe enough as it was. But a similar device with enhanced engineering could have leveled the building and caused far worse casualties. Terrorists showed that one of the most secure buildings in Islamabad was still vulnerable to attack, but there was far less damage than there might have been.
Thermobaric weapons distinguish themselves from conventional explosive weapons by using atmospheric oxygen, instead of carrying an oxidizer in their explosives. They are also called high-impulse thermobaric weapons (HITs), fuel-air explosives (FAE or FAX) or sometimes fuel-air munitions, heat and pressure weapons, or vacuum bombs. They produce more explosive energy for a given size than do other conventional explosives, but have the disadvantage of being less predictable in their effect
The effects produced by FAEs (a long-duration high pressure and heat impulse) are often likened to the effects produced by low-yield nuclear weapons, but without the problems of radiation. However, this is inexact; for all current and foreseen sub-kiloton-yield nuclear weapon designs, prompt radiation effects predominate, producing some secondary heating; very little of the nominal yield is actually delivered as blast. The resulting injury dealt by either weapon on a targeted population is nonetheless great.