In article ,
Pat Flannery wrote:
LOX is on top on the ET also.
Due to it's higher mass than LH2 per volume, the LOX will end up on top
in pretty much any design you use, for balance purposes during ascent.
You want the CG as far forward as possible.

You might like it there, but other considerations can intervene, like the
extra structural weight incurred by putting most of the propellant mass up
high. The LOX was on the bottom in the S-II, the S-IV/S-IVB, and the
Ariane 1/2/3/4 third stage, and still is in the Centaur, the Delta IV
upper stage, and the GSLV third stage. It might have ended up on the
bottom in the ET, too, had it not been for the asymmetric vehicle
configuration -- keeping the ET's changing center of mass within the
limited gimbal range of the SSMEs absolutely dictated putting the LOX tank
as far away from the SSMEs as humanly possible.
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