The damage was caused by a softball-size piece of insulation, possibly mixed with ice, that broke away from a propellant feedline bracket on the side of Endeavour's external tank 58 seconds after blastoff Wednesday. The debris hit an aft strut at a relative velocity of 205 mph. Surprisingly, the debris bounced off the strut and a large piece hit the belly of the orbiter at a relative velocity of about 150 mph. Shannon said engineers were surprised by the ricochet, believing a chunk of foam or ice would break up into numerous small pieces rather than bounce off a strut as the debris in question actually did.
Regardless of the underlying physics, the debris gouged out an irregular cone-like pit that tapers to a gash measuring 1-inch long and 0.2 inches wide at the base of the 1.12-inch thick tile. The small gash reveals so-called filler bar material, a strip of heat-resistant material that underlies the boundary between adjacent tiles. That was good news, Shannon said, because no bare aluminum is visible.
Laser scans of the gouge were used to fabricate precise three-dimensional maps of the damage as well as precise three-dimensional mockups using tiles identical to those on Endeavour. Shannon said engineers are running complex computational fluid dynamics calculations to model the flow of re-entry heat inside the damage zone. Those models will be verified by subjecting the deliberately damaged tiles to an arc jet furnace capable of duplicating re-entry heating and flow fields.
"That takes time, and I was not comfortable, as the MMT chairman, in waiting for the final answer from that team before we got started on any type of repair options," Shannon said. "So we kicked off a Team 4, which is a standing team of operators, spacewalk experts, crew members, engineering team members, they are standing by on every flight to work any problems like this and they're meeting right now to go discuss different types of repair options.
"We have the emittance wash, which is the black, it's almost like paint or shoe polish that keeps heat from building up in a cavity. We have the gun that squirts the caulk-like material that's very heat resistant into a cavity. And we have the overlay, which is a 15-by-24 (inch) sheet of silica carbide that you auger in, they almost look like drywall screws but they're better, and you auger that in to cover up any kind of a damage area.
"That team is meeting, they're going to look at different options," Shannon said. "Pretty much everybody at this point, even without doing the analysis, is very comfortable that if we just come in and we fill up this little area right here, that will provide us with some additional margin. The analysis is still in work to say whether that margin is even required or not."