August 10, 2006

scientists comment on liquid explosives terrorism threat

Following news of an alleged threat to detonate liquid explosives on UK domestic flights, scientists give their comment on the feasability of such a plot.

Professor Peter Zimmerman, Chair of Science and Security at Kings College London, said:

“Many kinds of explosive can be used to destroy an airplane in flight, because the air pressure in the cabin will add to the destructive power of the explosive. An airliner is a very fast-flying big balloon, and — speaking very figuratively — if the cabin is ruptured and the fuselage skin torn by an explosion at cruising altitude, the aerodynamic force on the rip and the air trying to escape the cabin can greatly multiply the destructive power of a bomb.

“We do not know enough to speculate about what types of devices the terrorists would have sought to bring on board aircraft, but from the new prohibition on liquids in the cabin it’s possible to guess that the explosive was thought to be a liquid or gel. Most liquid explosives are unstable and dangerous to handle; the best-known example is nitroglycerine. However there are extremely safe commercial explosives which come in two liquids which are mixed just before they are to be detonated. Each liquid alone is non-explosive; only the mixture can be made to explode with a detonator. Explosive detonators are small, simple, and if lightly camouflaged might very well escape detection by x-ray scanners.

“Almost all potent explosives come from just a few chemical families, and detectors for them exist. Those machines are reasonably inexpensive and take very little time to use on every piece of carry-on luggage. They should be used at every boarding gate for every aircraft leaving the UK for any destination, but particularly to the United States and other allies.

“Explosives are very important commercial products. While they should only be sold to and used by licensed experts, high explosives are required in the construction and mining industries and in some important manufacturing processes. We will have to continue to live with them as items of commerce in our society.”

Graham Shaw, Consultant at Marine and Underwater Security Limited, said:

“There are a number of ways in which the pressure cabin of an aircraft could be disrupted and these do not necessarily have to be what is normally understood as explosive devices.

“By way of explanation there different forms of explosives:

One sort provides a highly disruptive but short pulse.
Another one provides a much lower order but considerably longer pushing or lifting pulse
The third type can involve rapid burning which can cause secondary damage. Rapid burning would cause large amounts of heat and also deplete oxygen in an area.
“Once the fuselage is disrupted – depending on height – there may be a rapid decompression of the carriage. This may not kill people but the pilot would have to take rapid action and rapidly dive to get back into an atmosphere with plenty of oxygen.

“A mix of two or more liquids or liquid and solid can in certain circumstances provide rapid gas expansion (like a large soda siphon spartlet) and this can provide a major ‘nudge’ to the surrounding container and effectively misshape it or disrupt it. An analogy is when Romans split rocks in UK in winter – they made a hole and filled with water – when ice forms that would cause an expansion event that would split the rock.

“Rapid expansion devices (as could be caused by mixing liquids together or a liquid and a powder) are used in mining and could be used to disrupt aircraft fuselage.”

Dr Clifford Jones, University of Aberdeen, said:

“An ‘explosive’ is a chemical substance which reacts so rapidly that the post-reaction gas travels with sufficient speed to create an overpressure, that is, it causes the local pressure to be exceed atmospheric. This clearly has destructive potential. In any combustion process, the more intimately mixed the fuel and oxidant before burning the more rapid combustion propagation, and the ultimate in such mixing is for the fuel and oxidant to be in the same molecule.

“This is so of ‘high explosives’ such as TNT. High explosives may be solid or liquid. Sometimes an explosive is two-component, the fuel and oxidant being initially physically separated but coming together rapidly enough once reacting for the combustion to create an overpressure. A home-made bomb might be so constructed, with ammonium nitrate as oxidant and something as innocuous as sawdust as the fuel. Overpressures can also arise when a liquid fuel and a liquid oxidant are so reacted, perhaps hydrogen peroxide and acetone. For an explosive to raise the local pressure by one hundredth can break windows: if it raises it by one tenth structural damage to buildings and death or injury to persons will result.

“Organic peroxides are capable of explosive decomposition and might therefore be described as liquefied explosives. Such materials are used at refineries and petrochemical plants.”

Dr Frank Barnaby, Oxford Research Group, said:

“We don’t have adequate information to make a guess on what the device was. However to say that is just a liquid does not make sense as you would need a detonator which would have shown up on current security checks.”