Stuf4 wrote:

http://quest.arc.nasa.gov/people/jou...s87-12-23.html

Excerpt:
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STS-87 is Home! The Post-Flight Inspection Begins
by Greg Katnik
December 23, l997

STS-87 rolled to a stop...the extent of damage at the conclusion of
this mission was not "normal." ... During the STS-87 mission, there
was a change made on the external tank. ... Foam cause damage to a
ceramic tile?! That seems unlikely, however when that foam is combined
with a flight velocity between speeds of MACH two to MACH four, it
becomes a projectile with incredible damage potential. The big
question? At what phase of the flight did it happen and what changes
need to be made to correct this for future missions? ... It was
determined that during the ascent, the foam separation from the
external tank was carried by the aerodynamic flow and pelted the nose
of the orbiter and cascaded aft from that point. Once again, this foam
was carried in a relative air-stream between MACH two and MACH 4!



A simple trajectory design change can blow the foam debris away from the
Orbiter, instead of blowing the blizzard of debris towards the Orbiter.
From Mach 2 to 4 the Shuttle flys a +2 degree angle of attack. Changing the
angle of attack to -2 degrees in this region would rotate the relative wind
on the Orbiter by 4 degrees. Essentially, blowing the foam away from the
bottom of the Orbiter instead of towards it.

Craig Fink