In article ,
Pat Flannery wrote:
...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.

I thought the gimbaling would probably be the main problem... the thing
is already fairly unstable without getting it completely unstable by
sticking the LH2 on top and having it constantly trying to go out of
control.

As I understand it, aerodynamic stability did not end up being an
overwhelming concern, because the SRBs are such a large part of the
stack's mass for most of the atmospheric flight. (Their nozzle gimbals
also handle most of the control then.) By the time they depart, the air
is thinning out fast and the importance of the aerodynamics is dwindling.

The main issue was reducing movement of the center of mass and keeping it
within SSME gimbaling range, so that the off-center SSMEs can always have
their thrust vector pointed toward it. The SRBs and the ET LOX tank
totally dominate the stack's center of mass for most of the flight, so
their centers *had* to be in roughly the same direction, as seen from the
SSMEs.

Was there any particular reason the LOX ended up on top in the Atlas?
Gimbaling limits again?

Gimbal range wasn't an issue with the engines directly under the tanks.
Reducing aerodynamic instability, to go easier on the control system,
might have been. I don't think I've ever seen a discussion of exactly why
Atlas has the LOX on top.

I note that both Jupiter and Thor had it on the bottom.

Designers with different priorities, probably. The Atlas's balloon tanks
minimize the structural-mass penalty of putting the heavy LOX high up, so
the more conventional structures might be expected to encourage putting it
on the bottom. Max Hunter, who was chief engineer for Thor, in later
years was big on saving structural mass by putting the LOX right above the
engines, so that may have been the issue for Thor.
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