Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 103

existed. A hydrogen gas bubble developing from a nuclear reactor acci-
dent is a highly unusual event and is an example of the particular environ-
ment that is required for hydrogen to explode.
At Wright-Patterson Air Force Base, armor-piercing incendiary and
fragment simulator bullets have been fired into aluminum storage tanks
containing both kerosene and liquid hydrogen. The test results indicated
that the liquid hydrogen was safer than conventional aviation kerosene.
Other tests have involved simulated lightning strikes, with a 6-million
volt generator that fired electrical arcs into the liquid hydrogen contain-
ers. None of these tests caused the liquid hydrogen to explode. Fires did
occur from the simulated lightning strikes, but the fires were less severe
even though the total heat content of the hydrogen was twice that of kero-
sene. These tests indicated that liquid hydrogen would be safer than fossil
fuels in combat where a fuel tank could be penetrated.
Hydrogen does have a wider range of flammability when compared
to gasoline. A mixture as low as 4% hydrogen in air, or as high as 74% will
burn, while the fuel to air ratios for gasoline range from 1 to 7.6%. It also
takes very little energy to ignite a hydrogen flame, about 20 micro-joules,
compared to gasoline which requires 240 micro-joules. However, these
characteristics are reduced by the fact that as the lightest of all elements,
hydrogen has a very small specific gravity. The diffusion rate of a gas is
inversely proportional to the square root of its specific gravity so the pe-
riod of time in which hydrogen and oxygen are in a combustible mixture
is much shorter than other hydrocarbon fuels. The lighter the element is,
the more rapidly it disperses when it is released in the atmosphere.
In a crash or accident where hydrogen is released, it rapidly dispers-
es up and away from the ground and any combustible material within the
area. Gasoline and other hydrocarbon fuels are heavier since the hydrogen
is bonded to carbon which is a much heavier element. When hydrocar-
bon fuels vaporize, their gases tend to sink rather than rise into the atmo-
sphere. This allows burning gasoline to cover objects and burn them. In
most accidents, hydrogen would be a more desirable fuel.
In 1977, two fully loaded Boeing 747 commercial aircraft crashed
into each other on a foggy runway in the Canary Islands. This accident
was then the worst in aviation history and took 583 lives. An inquiry con-
cluded most of the deaths in the Canary Islands accident resulted from
the aviation fuel fire that lasted for more than 10 hours. G. Daniel Brewer,
who was the hydrogen program manager for Lockheed, stated that if both
aircraft had been using liquid hydrogen as fuel instead of kerosene, hun-