QUESTION:
How wide is the gap between each tile and its neighbour, and, when the
orbiter is gliding down through the atmosphere, how significant is the
aerodynamic drag, if any, from all those thousands of g's.

ANSWER from Roger Koss on January 29, 1998:
I went to the Shuttle Reference Manual on-line at the KSC
Website, and this is what I found:
"Where surface pressure gradients would cause cross flow of boundary
layer air within the intertile gaps, tile gap fillers are provided to
minimize heating. The tile gap filler materials consist of white AB312
fibers or a black-pigmented AB312 cloth cover containing alumina fibers.
These materials are used around the leading edge of the forward fuselage
nose cap, windshields and side hatch, wing, trailing edge of elevons,
vertical stabilizer, rudder/speed brake, body flap and heat shield of
the shuttle's main engines."
I guess what this is saying is that, in effect, there are no gaps
between the tiles which are exposed to the airflow.
FYI, the URL for the Shuttle Reference Manual on-line is:

http://www.ksc.nasa.gov/shuttle/technology/sts-newsref/stsref-toc.html

In print, this is a Technical Reference Manual which completely fills a
3 inch notebook binder. It was originally intended as a reference for
the news media, and probably contains more information about the Space
Shuttle then you would ever WANT to know !