Two issues
ago , I wrote about how hard things hit is much
more dependent upon how fast they’re going than on how big
they are, with the impetus for the piece being a newspaper
story implying a Cessna could be a terrorist threat.
One of my readers then e-mailed me asking about a Cessna
loaded with explosives. I replied that the explosives had to
get deep within a building to have any effect on its
structure, and that a Cessna simply couldn’t do the job. It
seems as though the issue started people thinking because
another reader asked what I thought about the official
explanation for the crash of TWA
Flight 800, to which I
replied that I agreed with it.
The two questions have made it clear to me that while that
piece provided answers which would allow one to internalize
the gestalt behind the slogan "speed kills", it raised other
questions about explosives and explosions.
Thus, I have been led to research the
issue, so as to obtain the detailed knowledge needed to write
this piece, a serendipitously satisfying salutary side effect of my
predilection to pontificate in print.
I’ve always known (well I’ve known for as long as I can
remember knowing anything) that gasoline is highly explosive,
and that liquid fuels in general contain potential energy in
extremely concentrated form. The interesting things I’ve
learned in my research have to do with the differences between
fuels, explosives, and fire.
My tendency towards being sesquipedalian in my writing
would often lead me to use "conflagration" in lieu of "fire",
but I discovered a fuel-air explosion was not actually an
explosion, but a "deflagration".
Huh? Well this was a case where something I always knew,
just wasn’t so. That is, I had at one time learned that an
explosion was defined as fast moving fire.
Gasoline explosion , yes. Dynamite, TNT, Plastique, Semtex,
RDX, etc., no.
That is, the explosion of a fuel-air mixture actually
(usually) deflagrates, while a true [high] explosive
detonates.
The difference between the two is that the latter
propagates supersonically , and is why the "knock" effect of
using low-octane fuel in a high compression gasoline engine is
referred to as detonation.
Before I explain the implications of the differences
between detonation and deflagration, a digression into an
explanation of octane is in order.
Octane rating , the definition of a gasoline’s detonation resistance, is so named because Octane, like Butane, Methane,
Propane, etc., is a flammable hydrocarbon, and in the early
days of high-compression gasoline engines, the octane rating
referred to the detonation resistance of a gasoline-antiknock
agent mix as a percentage compared to that of a pure
Octane-Gasoline mix. Thus 87-octane gasoline is 87% as
resistant to detonation as a pure 100% gasoline-octane
mixture.
Diesel engines utilize such high compression that the heat
of compression detonates the fuel-air mixture, while gasoline
engines rely on a spark plug to ignite the mixture at just
before the point of maximum compression. The gasoline
deflagration then proceeds, in a small fraction of a second,
throughout the mixture from the spark plug to the top of the
piston. If the engine compresses the mixture to a heat above
the auto-ignition temperature of the fuel-air mixture, it
detonates rather than burns, and is why one way of curing
knock in older engines (whose combustion chambers may have
become smaller because of carbon buildup) is via switching to
a higher-octane fuel.
It’s also why using higher than recommended octane
gasoline, especially in a new engine, is a waste of money.
Pre-ignition on the other hand, is the setting off of the
mixture in a gasoline engine too early in the cycle, usually
by a glowing piece of carbon left in the combustion chamber.
The mixture still deflagrates rather than detonates, but does
so at the wrong time.
OK, back to the implications of the differences between
detonation and deflagration.
Objects (including a wave of combustion products) which
move in air (at sea level) at velocities in excess of 1090
feet per second or 761 miles per
hour are supersonic;
moving faster than sound.
The crack of a bullwhip and the thunderclap following a
lightning strike are sonic booms; pressure waves piling up and
crashing into each other. This overpressure is what makes a
high explosive detonation such as that of a stick of dynamite
so much more destructive than that of a low explosive
deflagration, such as that of a gasoline-air mixture.
This, however, is not to dismiss the
destructive power of a fuel-air deflagration. That
is, while a stick of dynamite (a
little less than a half pound of the stuff) has an energy
content of about 2000 BTUs, a like amount of jet fuel has an
energy content of 10,000 BTUs.
With jet fuel weighing about 6.7 pounds per gallon, a
gallon of it sloshing around in TWA 800’s center fuel tank had
the energy equivalent of about 67 sticks of dynamite.
Before you go nuts worrying about the gasoline in your car,
consider that a Big Mac, at 2,200 BTUs contains more energy
than a stick of dynamite, and a pound of wood contains just a
bit less energy than two sticks of dynamite.
What’s going on here?
Well neither a Big Mac in your body nor the wood in a
fireplace or stove burns at a rate anything approaching
deflagration.
A gasoline-air mixture deflagrates at about one foot per
second. A pound of wood might take from minutes to hours to be
consumed. You could take half a day to burn a Big Mac.
Power is how fast energy is released, and is why
the supersonic detonation of a stick of dynamite is more
powerful than the subsonic deflagration of a half-pound of jet
fuel or gasoline in air.
I mean, think about it, even a "tame" dynamite, detonating
at a velocity of about Mach 5, in traveling about 5000 times
as fast as a low-explosive gasoline-air deflagration, has its
shock wave hit objects about 25 million times as
hard as the shock wave from the gasoline-air deflagration.
(Remember? All that stuff from two issues ago about a faster
object hitting harder than a slower one in proportion to the
square of the velocity difference?) You go five
thousand times as fast and you hit 25 million times as
hard.
Earlier on in this piece I said fuel-air mixtures
usually deflagrate rather than detonate. Pentane-air
mixtures detonate with about as high a velocity as a "tame"
dynamite ("tame", because there are dynamites with three times
the detonation velocity of a "tame" dynamite), and whaddaya
know, JPL conducted experiments in 1980 which had gasoline-air
mixtures detonating nearly 6000 times as fast as a
gasoline-air deflagration, or 20% faster than my "tame"
dynamite baseline.
Ouch! No wonder detonation in internal combustion engines
has been known to trash pistons.
Keep in mind that while detonation is supersonic combustion
and deflagration is subsonic combustion, NFPA 69 defines an
explosion as a situation where such combustion ruptures
the container in which it occurs, due to the overpressure
caused by the constrained combustion. That is, that even
"mere" deflagration in the center tank of TWA 800 could raise
the pressure in the tank to the point of blowing it apart.
Think about this. If you can blow the top off a pot which
takes 5 to 10 minutes to come to a boil, or pop the safety of
a steam boiler, by an overpressure of a few pounds, what
happens to the temperature (and thus the pressure) in a sealed
tank where even a deflagration can release the energy of tens
to hundreds of gallons of vaporized jet fuel in a few
seconds?
