A while back - March 04 -- I noted the risk from terrorists using thermobaric or fuel-air explosives. This type of blast is much more effective at destroying buildings from the inside than normal (condensed) explosives. One factor is the greater energy release from explosive mix that takes oxygen from the air, but the other is the sustained impulse that a fuel-air blast produces. Many structures rely on gravity for their structural strength - arches are a good example - or have very limited ability to withstand a horizontal load. A fuel-air blast has long enough duration to cause such structures to lose their integrity, and basically they just fall apart.Terrorists have known about these weapons for some time; five years ago the IRA were reported to be collaborating with FARC in Colombia to develop a fuel-air device, with every possibility that the recipe would be shared with other groups.Last month the reporting restrictions were lifted on the trial in Britain of terror suspect Dhiren Barot. His key plan was reportedly called the Gas Limos Project in which limousines packed with cylinders of propane and explosives were to be placed in car parks underneath crowded buildings. Barot has pleaded guilty to the charges against him.Whats interesting here is the use of gas cylinders as well as normal explosives. The 1993 plot to blow up the World Trade Center using 1300 lbs of a urea/nitric acid composition failed. A fuel-air device might be much more effective. (And indeed, out there on the fringe there really are people who think the WTC was destroyed by such devices ) However it has to be said that engineering an effective fuel-air blast is a major technical challenge. Simply sticking some explosives around a propane tank might get you an impressive fireball but it would not necessarily generate much of a blast - this requires the gas and air to be thoroughly mixed in exactly the correct ratio over a large volume and then ignited correctly.Meanwhile fiction struggles to catch up -- BBC TV series Spooks recently featured an episode with terrorists attempting to use a thermobaric bomb in London.The technology may be new, but the general idea goes way back. A group of religious fanatics aimed to destroy the British Houses of Parliament and wipe out the entire government on November 5th, 1605. The blast was to be provided by 36 barrels of gunpowder stowed in the cellars below the House. The plot was betrayed, but date is marked annually with bonfires and fireworks in England - Remember, remember the 5th of November--David Hambling
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