Shannon says tile damage not a threat to astronauts
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 13, 2007
The foam-damaged heat-shield tiles on the belly of the shuttle Endeavour do not represent a Columbia-class threat of catastrophic failure during re-entry, the chairman of NASA's Mission Management Team told reporters today. John Shannon said the issue is more a matter of whether post-landing repairs might be needed that could delay Endeavour's next flight or whether it might make more sense to stage a relatively simple spacewalk repair job to give the shuttle additional margin during re-entry.
"We're not talking about catastrophic damage," Shannon said. "But if I have to pull off five or six tiles (after landing) and put a doubler on some structure, replace a rib or anything like that, that's going to increase my turnaround time between (flights) and I'd like to avoid that if possible, if I have an EVA that I think is easy to execute. Now all of that assumes we come back and show that we would have localized heating that could cause some damage underneath and we haven't done that yet."
Asked a second time about the threat represented by the tile damage, Shannon said "this is not a catastrophic loss-of-orbiter case at all. This is a case where you want to do the prudent thing for the vehicle."
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.
Another bit of good news was the location of the gouge. As luck would have it, the pit is directly under the intersection of two internal ribs on the other side of the shuttle's aluminum outer skin. The structural components would help dissipate any heat that managed to soak into the skin under the gash.
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."
Regardless of which repair option might be selected, Shannon said, "we could execute it if required and we have the team working really hard to understand whether it will be required."
A decision one way or the other is expected by late Tuesday or Wednesday.
"If you ask me whether I think it will or not, I'd probably lean toward it's not going to be absolutely required to do," he said. "The initial simplified cavity model that they ran today passed with no structural damage, no reduction of structural margins at all. But again, it's a simplified model. Since the flow is a little tricky, we want to make sure that we get it exactly right and we'll run that, make sure we understand it and if we don't have to do anything, well we had good practice in pulling together these procedures. And if we do have to do it, I feel fully confident we could execute that with a minimal impact to the mission."
Based on foam loss seen during the first post-Columbia mission, the external tank brackets in question are in the process of being redesigned. The new design will not be ready until three missions from now. But based on the experience with Endeavour, engineers have reviewed other missions and Shannon said they now believe they might have spotted losses during two other flights. But he said the foam in question is small and it's not at all clear it represents the sort of problem that could ground the fleet.
"To me, it's quite a bit of a stretch on the bracket to say wow, you had this, how do you keep flying? Well, this is kind of a pain to have to go and run all these assessments, it's kind of a pain if I have to get somebody under the vehicle to put a little squirt of goo (in a gouge). But to put this in a little bit of perspective, it's not very big. But it's something, I don't want to impact the turnaround of the vehicle. So I'm going to go and address it. If I had to go do half an EVA of repair on every flight from now until 12 flights from now, we'd go do that. We're set up to understand exactly what's on the vehicle and we have the repair capability. But we'd like not to do that, we'd like to devote the entire mission to just doing the ISS assembly or the ISS logistics. But we have it, we're going to go address it."
If a spacewalk repair is required, it will be carried out by Rick Mastracchio and Dave WIlliams. Williams and station astronaut Clay Anderson already are scheduled for spacewalks Wednesday and Friday. If a repair is ordered, it likely would take the place of the Friday excursion, which includes a variety of relatively low-priority tasks. To reach the damage site, the astronauts would ride on a 50-foot-long boom attached to the shuttle's robot arm.
Mastracchio and Williams "completed a standard flow of tile repair classes and their last class was literally about three or four weeks before flight," said spacewalk officer Paul Boehm. "We are actually very fortunate in this situation that Rick has actually performed some of these things in a vacuum (chamber) to see how the material reacts in that. So Rick and Dave would be our two candidates for doing that particular type of EVA."
As for the risk involved in working under the space shuttle, Boehm said "one of the concerns we have, obviously, is they're wearing a 300-pound EMU (spacesuit) with about 150 pounds of tools on them. ... Obviously, any of those things dinging into the tile underneath potentially could damage the tile. So we take extra care to make sure all our safety tethers and tools and everything are safely away from the orbiter when we go in for those type of repairs."