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V 1.41
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Background Attack Aftermath Evidence Misinformation Analysis Memorial

Thermobarics

Theories that Thermobaric Devices Destroyed the Twin Towers

A 'thermobaric' weapon is one that uses atmospheric oxygen, instead of carrying its own oxidizer, to achieve an explosion. Thus, such weapons are often called fuel air explosives (FAE). The most common type of thermobaric weapon uses a primary charge to disperse its fuel into an aerosol, and a secondary charge to ignite the aerosol. 1   The flame front rapidly propagates through the mixture producing a pressure wave and a potentially large area of intense overpressures.

Note that any of a number of fuels can be used in a thermobaric bomb, and not all of them generate visible fireballs. Hydrogen, for example, burns in air with a flame that is not visible in daylight. 2  

The ability of thermobaric devices to generate high overpressures may seem be counterintuitive, since they use only air and a simple fuel such as propane, rather than a high-explosive compound such as TNT. The overpressures result from the speed at which the pre-mixed fuel-air mixture combusts, causing pressures and temperatures to build up over a large area. H. Michael Sweeney explains the process:

The chief difference in METC unit (Multiple Explosives Transitional Container) design over traditional explosive devices moves away from a densely packed explosive core towards a large volume of highly explosive but low-density mass in the form of a gaseous cloud. In the normal bomb all explosive energy comes from a tightly packed core and must drive outward against air pressure and objects it encounters. It rapidly bleeds off energy at the square of the distance as it accumulates a wall of pressure resistance and a mass of heavy debris, which it must continually regather and push along.

The new design starts as a small device but transforms itself through simple means from a dense-core technology to a much larger gaseous-cloud state. Igniting the explosive cloud at any peripheral or central point creates a chain-reaction-like and progressively growing explosive force. As the force of the explosion moves outward, it continues to ignite fresh explosive materials as encountered and gains momentum rather than loosing sic it. Further, because the gaseous cloud is efficiently mixed explosive materials combined with abundant free-air oxygen, ignition is far more complete and productive - leaving little or no chemical residue or traditional flash evidence (other than a burn signature, which any investigator would presume to be from ordinary fire) on immediately encountered objects. The net result is as if a significantly larger central core device had been detonated, with the complete and even combustion making difficult any aftermath analysis as to the true nature of the explosives used. Finally, the shape of the cloud and the ignition point within the cloud, if properly controlled, provides an extremely easy means to create shaped charge effects despite a relatively free-form original cloud shape. 3  

A Thermobarics Demolition Scenario

Clumping in North Tower explosion
Were the Towers blasted apart in successive floor-wide detonations of distributed thermobaric bombs, marched down from the crash zones at quickening rates? Note the clumping in the elongated features of the rubble cloud in this photo taken mid-way through the North Tower's destruction, where the average spacing between clumps is similar to the spacing between floors.

One can imagine a scenario in which a thermobaric devices were installed at each floor in the service core of each Tower. Each device would listen for a radio signal with a particular signature which would trigger a primary charge, dispersing the aerosol throughout its floor. Then, about five seconds later, a secondary charge would be triggered causing an explosion with overpressures sufficient to shatter the perimeter walls.

One advantage this theory has over most other explosives theories is that it avoids the need to install explosives near the Towers' perimeter columns. The thermobaric devices could have been installed entirely in discretely accessed portions of the Towers' cores. The number of devices could also be much smaller -- perhaps just one per floor. The devices could have been encased in impact- and heat-resistant containers similar to those used to protect aircraft voice and data recorders, so as to prevent accidental detonation from the aircraft impacts and fires.

Other advantages of thermobarics include an absence of conventional explosive residues, and much higher energy densities than conventional explosives. For example, whereas TNT yields 4.2 MJ/kg, hydrogen produces 120 MJ/kg (not counting the weight of the oxygen it uses to burn). 4  

Of the possible fuels that could be used in thermobarics, hydrogen has several unique attributes which could have been used to advantage by the planners.

  • The flash produced by hydrogen combustion is not visible to the naked eye in daylight conditions. The use of hydrogen-based thermobarics is thus consistent with the absence of colorful fireworks in the destruction of the Twin Towers.
  • On a weight basis, hydrogen has one of the highest energy densities of any fuel -- several times that of any hydrocarbon. The use of hydrogen would have allowed operatives to install far less material than would be required with other explosives.
  • The combustion of hydrogen in air produces only water vapor, a residue that is consistent with the vast light-colored clouds produced by the Towers' destruction.
  • Hydrogen has a very wide explosive range -- from 4 to 75 percent in air. That compares to 2.1 to 10.1 percent for propane and 0.7 to 5 percent for kerosene. 5   Thus it would be relatively easy to design hydrogen-based thermobarics that would function reliably in a variety of conditions.
  • Hydrogen has a very high vapor pressure compared to other fuels. This would have enabled its rapid dispersal into ambient air by shattering pressure vessels containing it.

Technical Challenges

The use of thermobarics to destroy the Twin Towers would have presented some technical challenges. One would be to assure that ignitions of the aerosols on each floor not proceed downward faster than the descending rubble cloud. If the planners allowed five seconds for the mixing of aerosol on each floor, they would have to start the dispersals about 30 stories below the zones of destruction. Two potential problems would be:

  • Aerosols on lower floors being prematurely ignited by stray sparks
  • Combustion propagating from floors to floors below them

The shut-off of electrical power to the Towers may have largely obviated the first problem. It is interesting that dust jets are seen around the mechanical equipment floors, where sparks would have been more likely.

Engineering of the thermobarics may have addressed the second problem. The isolation of floors by fire doors and elevator-shaft fire dampers, combined with the distribution of aerosols primarily in the tenant spaces, may have been sufficient to prevent flames from propagating from one floor to the next. The floors themselves, even after being shattered by the thermobarics, would provide a barrier to the propagation of flames for at least the eighth of a second or so between the destruction of successive floors.

Both of these problems could have been avoided by designing the thermobaric charges to disperse their contents in a split second, eliminating the interval of several seconds during which sparks or flame propagation could have caused premature ignition.


References

1. Thermobaric weapon, WikiPedia.org,
2. Hydrogen Safety, Humboldt.edu,
3. CIA's METC Explosives, totse.com, [cached]
4. Chemical Potential Energy, HyperTextBook.com,
5. Gases - Explosive and Flammability Concentration Limits, EngineeringToolBox.com,

page last modified: 2011-09-08