(Roy Smith) writes:
I assume NASA didn't just flip a coin to decide which goes on top, so
there must be more to it that that. What's the real answer?

I think it's due to center-of-mass considerations. The loaded LOX tank is
heavier than the loaded LH2 tank, and you need to be able to be sure
that the center of mass of the combined Shuttle/ET is located far
enough forward that the thrust line from the SSMEs can pass through it.

--
Richard W Kaszeta

http://www.kaszeta.org/rich