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In a message dated 9/26/01 10:46:04 PM, Jim McKibbin wrote:

<<If you are talking about cryogenic propellants then
the pressure in the vapor space is determined by the fluid inside the tank
which is at saturated (boiling) conditions. The pressure is determined by the
temperature of the propellant which is at equilibrium with its vapor and
contained within a pressure vessel. The pressure will go up and down as the
temperature changes and is independent of ambient pressure assuming that the
vent is closed.>>

Forgive this brief aside into the non-airplane world.  Rocket liquid oxygen
tanks (and liquid hydrogen, for that matter) are always vented to atmosphere
until just before liftoff.  The tank pressure is ambient and the liquid
TEMPERATURE is what varies as ambient pressure varies, since the tanks are
far from perfectly insulated.  Just before liftoff the vents are snapped shut
and the tanks pressurized to flight pressure with nitrogen or helium gas.  
(This pressure may be 50 psi, it may be 500 psi.)  This time is held to a
minimum since the cryogenic fluid warms up and it becomes less dense.  
Surprisingly, this is a significant factor in vehicle design, as the density
of liquid oxygen various a large amount with a small temperature change --
even ten degrees may be too much -- so this happens about 3 minutes before
liftoff.

Sorry for the digression.  I'll behave in the future...

- Rob Wolf
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