Newsgroups: sci.space.shuttle
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Loading of external tank
Date: Wed, 14 Jun 2000 13:21:43 GMT

In article <3947462B.A20C5973@videotron.ca>,
JF Mezei  <jfmezei.spamnot@videotron.ca> wrote:
>When the external tank needs to be loaded with liquified gases (O2 and H2),
>how is that done ?

There may be some small variations for the shuttle, but the normal
procedure for filling rocket tanks with cryogenic fuels is to start with a
slow flow to chill the tank down.  Most of the initial flow boils off; it
has to be kept slow so that the volume of boiloff gas is within the limits
of the vent system.  Once the tank is cold enough that the stuff is
staying liquid, you crank up the flow rate to finish the filling promptly.
And then you maintain a trickle flow to keep it topped up until launch.

Sometimes it is necessary to pressurize the tanks slightly during this
process, for various reasons including structural strength -- there's a
lot of thermal contraction during chilldown and filling, and that can put
extra stresses on the tank, since it doesn't happen evenly over the whole
tank.  For example, the original shuttle ET had a design requirement to
*not* require pressurization for strength while on the ground, but they
blew it slightly:  the LOX tank in fact had to be pressurized during the
filling process (although not after it finished), because at one stage
early in filling, the thermal stresses were excessive.

Fill pressurization is typically done with helium, by the way.  Whatever
gas is used has to be compatible with the cryogenic fluid (you can't
pressurize LH2 tanks with air!), and even if something else is used for
in-flight pressurization, helium is often the most convenient thing to use
on the ground.

Cooling the tanks by letting propellant boil off may seem untidy, but
anything else usually ends up being far more complicated.  The hardware
needed for the venting has to be there anyway, to handle boiloff after
filling.  The only real complication is that the fuel pumping system has
to be able to handle initial slow flow -- sometimes there are separate
pumps, a small one for chilldown and a big one for fill -- and that's
relatively minor.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)