From: Doug Jones (djones@xcor.com)
Date: Tue May 28 2002 - 22:30:07 MDT
spike66 wrote:
>
> Charlie Stross wrote:
>
> >> I understood that they are pressurized but not to 1 atmosphere.
> >> The pressure in the tank would be lower at cruise altitude than
> >> on the deck. spike
> >>
> > Pedantic point: the cabin isn't pressurized to 1 atmosphere, either!
> > Ever wondered why you need to swallow while the airliner's
> > descending?
> >
> No Charlie, I didn't wonder, I realize the cabin pressure
> is below 1 atmosphere in flight. What puzzles me is that a
> fuel tank could explode if the pressure is below 1 atmosphere
> and way rich in fuel, even given an ignition source. Kerosine
> is much less explosive than gasoline, and we all know
> what happens when we have insufficient compression in
> one of the cylinders in a car engine, this with an
> ideal fuel-air mixture. Mike what is the typical pressure
> in a jet's tank at altitude? spike
Kerosene simply has a lower vapor pressure than gasoline, so at sea
level on a not-too-hot day, too little vapor is generated to produce a
flammable fuel-air mixture; it is too lean. This makes it much safer
for handling on the ground under most circumstances.
At higher altitude (lower total pressure), if the fuel tank has been
heated up by running the air conditioning system on the ground (there
are components of that near the center tank on a 747) the vapor pressure
is higher, *and* the air pressure is lower. Bad news- there is more
fuel, less air, and the mixture rises above the lower explosive limit.
All it takes is one spark.
Nitrogen gas can be generated on board and used for fuel tank purging;
the USAF already does so on some aircraft, in some cases as a side
effect of the on-board oxygen separator. There were calls for requiring
such systems after TWA 800, they will probably be renewed.
Doug Jones
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