NASA awaits final test data before decision on repair
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 15, 2007
As expected, the Endeavour astronauts will delay their fourth spacewalk, from Friday to Saturday, to protect against the possibility of an unprecedented heat shield repair. Mission Management Team Chairman John Shannon said late today a decision one way or the other will be made Thursday, after a final round of tests in a high-temperature furnace to verify computer models accurately reflect the re-entry environment. Barring an unexpected surprise, Shannon said he remains "cautiously optimistic" a repair spacewalk will not be needed. And he made it clear the risks associated with a repair spacewalk are high enough to rule out attempting a fix if it is not absolutely necessary.
"We're pondering for 24 hours, but my personal feeling was that the data at the 80 to 90 percent level was that we could (return to Earth) as is and that the EVA carries sufficient risk to it that we would not just go do it (to add) additional margin," he said.
Two heat-shield tiles on the shuttle's belly were damaged when a chunk of foam debris, possibly including ice, slammed into the orbiter 58 seconds after launch last Wednesday. The impact gouged out an irregular pit in the tile measuring roughly 2.3 inches by 3.5 inches across and nearly penetrating the full 1.12-inch thickness of the tile. A small, 1-inch by 0.2-inch gash at the bottom of the pit exposes an underlying support pad just above the shuttle's aluminum skin.
The shuttle Columbia was destroyed in 2003 when it re-entered the atmosphere with a gaping 4- to 6-inch hole in the leading edge of its left wing. NASA managers, and even commander Scott Kelly, say they do not view Endeavour's gouge as a Columbia-class problem. Shannon says the issue is whether re-entry heating might cause damage to the shuttle's aluminum skin in the immediate area that would require time-consuming post-landing repairs.
Complex computational fluid dynamics modeling indicates the skin under the gouge would only get about 40 degrees hotter than normal during an "as is" re-entry and never exceed a 350-degree safety threshold. Overnight, engineers subjected a precise model of the damage, using tiles similar to, and gouged out, like those on Endeavour, to re-entry temperatures and flow fields in an arc jet facility at the Johnson Space Center in Houston.
The hellish wind in the furnace caused the aft face of the gouge to lengthen, creating a sort of groove in the tile behind the original pit. But Shannon said the groove did not bore into the denser lower layers of the tile and "we did not violate any temperature constraints on the underside of that aluminum sheet."
As a final test, engineers will roast another set of tile samples to the arc jet facility overnight. These tiles feature more idealized damage with straight lines and precise angles in a bid to duplicate the results from the computational fluid dynamics models. If the more idealized test matches up with the computer models, engineers will have high confidence the predictions of a safe "as is" re-entry are valid.
"The primary point of debate (today) was did we need to run the simplified model in the arc jet facility?" Shannon said. "Because a lot of folks thought that we had two fairly independent analyses, one was the thermal analysis done on computer, one is the arc jet that said you don't have a problem here. But just to tie the two together and make sure we didn't make some significant error in either of those, we're going to run this simplified model in the arc jet. That was the primary point of discussion. People thought we had enough data, some people wanted to go get some more data. So we're going to go get some more data."
But Shannon made it clear he views the risk of a spacewalk repair as significant and that such a repair could only be justified if it was necessary to prevent serious damage during re-entry. And based on the testing to date, that does not appear to be the case.
"The way I would summarize it is, if we were in a critical situation I think we could pull it off, I think we would have the rationale to put all those different things together and come off with a very successful EVA," he said. "But you have to recognize there is some additional risk in doing that task.
"The other side of the equation is on the thermal assessment side. We have almost finished the thermal analysis. I told you yesterday, if you remember, we did computational fluid dynamics to understand the flow within that cavity. That was done out at Ames Research Center and Langley Research Center is backing us up and doing an independent review of that computational fluid dynamics. And that work is still ongoing, it was not completed yet, we expect to have that tomorrow morning. And we're still performing a peer review where someone goes in who's totally independent, who checks all the math, who checks all the physics to make sure we did everything exactly right. That is 90 percent done, but it was not completely done.
"But the analysis that we have shows there would be no damage at all to the underlying structure, which was very good news to us, and we would not violate any of our certification temperature, we would not even reduce our 1.4 factor of safety. So that was all good news, but it was not a 100 percent story. It was more like an 85 percent story."
The risk associated with any spacewalk was made clear today when astronaut Rick Mastracchio had to cut short a space station assembly spacewalk after observing a small tear in the outer Vectran covering of his left spacesuit glove. A spare set of gloves is available if needed and uncertainty about what caused the tear would not stand in the way of a high-priority repair spacewalk.
If a heat shield repair is ordered, it will be carried out by Mastracchio and Canadian astronaut Dave Williams. Mastracchio, anchored to the end of a 50-foot-boom attached to the shuttle's own 50-foot-long robot arm, would be maneuvered to the damage site under the shuttle. Williams would be tethered to the boom to provide assistance as needed.
The repair plan would have Mastracchio dab on a black paint-like material known as emittance wash and then use a grease gun-like device to inject a thick heat-resistant material called STA-54 into the gouge.
Shannon pointed out three areas of concern about a repair spacewalk, saying "none of these are real show stoppers but you kind of have to add them up in your mind."
First, he said, working under the shuttle on the end of a 100-foot boom with poor television coverage in proximity with a critical system has never been attempted before. "That was not a show stopper, but it was something to think about," he said.
Second, astronauts have never used STA-54 in the vacuum of space and third, no one knows if the applicator would work properly or if the spacewalkers might have problems applying the thick putty like material. "It takes a little bit of practice to get it in exactly the right spot." Shannon said engineers today had a long discussion "about could we make the situation worse?"
"In my mind, I thought tomorrow would be a better day to make a decision," he said. "I think most of the data is in place for us to make a decision, but I wanted the team to go off and think about it overnight and at tomorrow's MMT we'll have all of the final assessment complete and the team will have thought about the different data for 24 hours and we'll discuss it some more and decide what the appropriate option is."
Late today, mission control told the astronauts a fourth spacewalk, originally planned for Friday, would be delayed to Saturday. That will give flight controllers time to refine their plans if the MMT does, in fact, decide a tile repair is needed. In that case, the repair work would be carried out Saturday and a fifth spacewalk could be added Monday to complete the delayed station work. That scenario likely would result in a two-day mission extension and a landing back at the Kennedy Space Center on Aug. 24.
If the tile repair is not needed, the astronauts would simply carry out the previously planned space station assembly spacewalk on Saturday and land as currently scheduled on Aug. 22. One wild card in the planning is the status of work to figure out what caused the tear in Mastracchio's glove today. While uncertainty likely would not prevent a repair spacewalk, it's not clear what sort of rationale might be needed to clear the way for a lower-priority station EVA.
"I'd like to give you some big picture words of what our plan forward is," astronaut Shannon Walker called from mission control to kick off a daily planning conference. "All the decisions have not been made yet, the MMT is actually still meeting as we speak, but one thing that has been decided is that EVA-4 will be on Saturday."
"Copy Shannon. And I hope everybody here is ready for this EVA, too, whatever it is," station commander Fyodor Yurchikhin replied.
"Yes, exactly. We do not know what the EVA will be," Walker said. "Obviously, it will either be TPS (thermal protection system) repair or the regular EVA-4. So what we have done is plan your day tomorrow as if it's going to be TPS repair, but stressing that no decision has been made either way."
"We copy," an astronaut said.
"OK, that being said, the plan is on board but don't go off and memorize it tonight because we're still working on it down here and it is not in synch with form 24 at all," Walker said. "But at the moment, you will see TPS-related activities on it."
Shuttle commander Scott Kelly then asked, "any word when the MMT's going to adjourn? Are they supposed to make a decision today on repair, or not repair?"
"Scott, we are hoping they are going to adjourn within the next hour or so, but actually we do not expect a decision today on whether or not we're going to repair or not repair," Walker replied.
"OK, thank you."
"At the moment, they don't have all the data they would like to make their decision," she continued.
"And no indication of which way they're leaning?" Kelly asked.
"Unfortunately, we have no idea which way the wind is blowing at the moment."