The means necessary to obtain the spark
used in igniting the explosion of the 1/4 scale center wing tank:
a 200 Volt Power Supply, two 7,700 micro-farad capacitors
charged to 150 VDC,
and an exposed brake light filament.|
***Correction: above shows
two 7,700 uF capacitors. This is 10
times the capacitance used by Krok. However, the initial
instantaneous current through the filament would be the same. A
duplicate test has been planned with the proper capacitance to see the
difference, if any.***
Dr J.E. Shepherd, J.C. Krok and J.J. Lee of the Explosion Dynamics Laboratory, California Institute of Technology
conducted a 1/4 scale center wing tank explosion test. Their techniques have been questioned regarding the use of Hydrogen
and Propane to assist in the explosion, and recently, their ignition source. Critics contend that Jet A1 fuel, used by itself, would
not have caused the explosion in the above experiment, and that the "spark" used was extremely large--unlikely to occur within
a center wing tank.
Independent aviation experts contend that Jet A fuel is very safe, and difficult to ignite, without misting the fuel as
in a jet engine. Cal-Tech was contracted to show that such an explosion was possible without misting. However, the scientists
used a mixture of Hydrogen
and Propane gas as a vapor simulant because, as they contend, it was difficult to mimic the fuel tank conditions at the
time of the crash.
The spark used by the Cal-Tech scientists to ignite the Hydrogen-Propane vapors was inconsistent with NTSB public statements. At the NTSB Public Hearings in
Baltimore in December of 1997, NTSB officials stated that only "1/4 millijoule" of energy was necessary to cause the fuel tank explosion. The NTSB went further to explain how small
1/4 millijoule was by stating that it was equivolant to dropping a dime from a height of about an inch. However, the
Cal-Tech test created a spark 63,000 times stronger, like dropping a roll of dimes from a second story window.
The following excerpt is taken from the Jet A Explosions - Field Test
Plan 1/4 Scale Experiments
Exhibit No. 20E; Docket No. SA-516, National Transportation Safety
Board, Washington, D.C.
The entire report can be found at:
6.8 Ignition System
Ignition will be carried out by rapidly heating the filament of a type
1156 taillight bulb (12 VDC) with the discharge from a fireset
containing a 1400 uF capacitor charged to 150 VDC. The glass bulb is
deliberately broken and the base of the lamp mounted into a standard
holder connected with the stiff wires through an insulation
feedthrough to the firing line. The function and timing of this
igniter has been determined in separate laboratory ignition tests. A
backup igniter is provided in case the primary igniter fails.
To calculate the energy of the above described ignition system, the reader is refered to Griffiths'
"Introduction to Electrodynamics 2nd edition" (p. 107), where energy (W) is given as:
W = (1/2)CV2
where C is capacitance in Farads and V is voltage. Therefore the energy used in the ignition source of
the Cal-Tech 1/4 scale CWT explosion test was:
Energy = (1/2)(1.4E-3)(150)2 = 15.75 Joules
= 15,750 millijoules
This energy is much larger than the 1/4 millijoule stated at the Baltimore hearings, but was nonetheless used to ignite
a Hydrogen-Propane mixture within a mock center wing tank. The Cal-Tech explosion test only re-confirmed the volatility of
Hydrogen and Propane, and did not address the theory that Jet A vapors could explode in the presence of a
1/4 millijoule spark.
Images of the spark here.