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From: (Dani Eder)
Subject: Re: STS ET Layout, why?
Date: May 15 1996

Jacob M McGuire <> writes:

>  When designing rockets, it would seem to make sense to put the tank
>containing the denser fuel at the "bottom" of the stack, so that the
>higher loads needed to support it are only carried by part of the stack,
>rather than the entire thing.  So why is the LOX tank in the ET at the

>  Is it some kind of aerodynamic stability thing?

>  Is it because the ET has to take the thrust loads of the SRBs anyway,
>so it has to be that strong already?

No, it's a steering thing.  The three engines in the Orbiter provide
the thrust after the solids separate.  You want all three engines
thrusting at the center of mass of the vehicle.  That way if one 
engine stops running, the remaining two won't be trying to spin
the vehicle.  The center of mass of the Orbiter/ET combination
is close to the oxygen tank when full, because the oxygen is 6/7 of
the fuel weight, and the ET weighs 6x as much as the orbiter.
As the tank empties, the center of mass of the Orbiter/ET moves
downward and toward the orbiter (the empty ET only weighs 1/4 as
much as the orbiter).  So by placing the oxygen tank forward,
the engines don't have to swing very much during the flight to
keep thrusting through the center of mass.  Making fuel lines
and support structure deal with large angle changes is hard.

The second factor is related to the inlet pressures of the SSME
pumps.  The engine pumps spit out fuel at a high enough pressure
to force it into the combustion chamber against the back pressure
in the chamber itself (3000 psi).  A pump like they use needs a
certain pressure in the fuel coming in or the pump will start
sucking vacuum (cavitation).  For the SSME pumps it is about 50 psi.
That pressure comes partly from the height of the column of
fuel from the tank to the engines, and partly from the pressure
in the tank itself.  By putting the oxygen tank up front, you
get more column pressure, and so the tank walls can withstand
less pressure, and so be lighter.  Since oxygen is the heavier
fuel component, you save more by making that tank lighter than
by putting the hydrogen up front.  Note also that oxygen being
a much heavier fuel (1.14 g/cc vs 0.07 g/cc for hydrogen), gets
more pressure rise per foot of column height.

Dani Eder

From: (Henry Spencer)
Subject: Re: Shuttle ET venting question?
Date: Mon, 22 Mar 1999 17:48:02 GMT

In article <>,
Cary Martynuik  <> wrote:
>What is the reason for the venting of the ET? I can see how it would
>aid in helping the ET to break up as it de-orbits by putting it into a
>tumble, but is this a primary reason for venting, or are there others?

If you just let pressure build up inside, it will rupture in some fairly
unpredictable way, and the escaping gas may well change the tank's path in
some random way.  The emphasis throughout is on predictability.  (Tumbling
the tank is another example of this:  the tank would begin breaking up
sooner if it was stable in a single attitude, because the reentry heat
would be concentrated on one area... but such a stable attitude might have
aerodynamic lift, which would change the trajectory unpredictably.)
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: Re: Fill 'er up
Date: Sun, 28 May 2000 20:35:08 GMT

In article <>,
Brett O'Callaghan  <> wrote:
> there a cross-section anywhere that shows the internal structure of
>the ET?  I'm guessing it's not exactly a big, empty tank with not much
>else in it....

Pretty close, actually.  The LOX tank and LH2 tank have a few bits of
internal hardware -- slosh baffles in the LOX tank, the siphon in the LH2
tank -- and there are some bits of plumbing and structure here and there,
like a beam crossing the intertank ring between the two tanks, but that's
just about it.  Remember, the thing was designed to be thrown away, in a
system whose designers had high hopes for reusability of everything else,
so there was some effort made to minimize the complexity of the ET, with
an eye on holding down its cost.
Microsoft shouldn't be broken up.       |  Henry Spencer
It should be shut down.  -- Phil Agre   |      (aka

From: Bruce Dunn <>
Subject: Re: New LOX/Methane tanks
Date: Tue, 05 Dec 2000 00:50:29 GMT

> I've been trying to find information on existing LOX/LCH4 lower stages without
> much success.

A useful comparison can be made with the lastest Super Light Weight
Shuttle tank, which has a mass of about 26.5 metric tons, including
about 2 tons of insulation, largely for the LH2 tank.  It holds 543
cubic meters of LO2, and 1465 cubic meters of LH2, for an overall
specific tank mass of 13.2 kg/m^3.

Your design holds about 625 cubic meters of LOX and 498 cubic meters of
LCH4, for an overall specific tank mass of 9.8 kg/m^3.  This might not
be unreasonable, assuming savings in insulation over the Shuttle tank,
and the adoption of a common bulkhead design (the Shuttle ET intertank
has a mass of something like 5 metric tons).

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