wrote:
A titanium structure would weigh less.

I'm not sure that's correct. The aircraft grade aluminum alloys
available when the shuttle were built had comparable strength (on a
weight basis) to the available titanium alloys.

Further a titanium structured craft could have a less
massive thermal protection system, since things like the wings and
tail
could be allowed more heat.

Somewhat more heat, yes, but titanium is not a cure-all in that regard.

Anyone know .. is it some exotic alloy of aluminum, or just standard
aircraft grade aluminum?

I think it's "standard aircraft grade aluminum," though some of the
shuttle structures are composites of aluminum reinforced with boron
fibers.

Mike Miller, Materials Engineer