At the time of Columbia's destruction, NASA engineers believed a phenomenon
known as cryopumping was the most likely explanation for foam shedding. When
the external tank is fueled for launch, air trapped in voids in the
insulation or near the skin of the tank can turn into a liquid. As the
shuttle rockets away, aerodynamic heating can cause that trapped liquid to
turn back into a gas. The pressure generated by that phase transition, it
was believed, could blow overlying pieces of foam away from the tank.

The Columbia Accident Investigation Board concluded such cryopumping alone
could not explain the separation of the suitcase-size chunk of debris that
doomed Columbia. But Readdy said today additional testing shows a different
type of cryopumping can, in fact, cause such shedding.

"We've found out that the bolts and the nuts being applied to actually
construct the different areas of the tank ... before you put the insulation
on, that any kind of gap in there might be an opportunity for liquid
nitrogen or liquid air to form," he said. "And what happens is, during the
ascent environment, when the shock waves form on the external tank,
aerodynamic heating and friction occurs and as a result, even trapped air
kind of expands."

The expansion of that trapped air "imparts a velocity to that particular
piece that causes large pieces to come off and instead of (peeling) away
from the tank, actually being pushed away from the tank due to that gas
pressure behind it," Readdy said. "That is really the root cause we've been
able to discover here.

"And part of the new design is to change the bolt configuration, to actually
close those areas out so that there is no opportunity for the liquid
nitrogen or liquid air to form and close out a whole number of other areas.
The other thing is characterizing the condition of the foam application more
carefully so we have a much more controlled environment, not only in terms
of the humidity that we're able to apply this foam, but the rate at which
the foam must be applied, the surfaces near it, a whole number of other
factors."

http://spaceflightnow.com/shuttle/sts114/040220tank/


Well that may slow things futher and largely explain the slip to next year.
wonder if the existing tanks can be fixed?