Final decision: No repair needed on tile damage
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 16, 2007
NASA's Mission Management Team today ruled out an unprecedented heat shield repair spacewalk and cleared the shuttle Endeavour for re-entry and landing next week "as is" based on computer modeling and tests in a high-temperature furnace that show a small gouge in the ship's belly will not cause serious damage during the fiery plunge to Earth.
"The MMT made two significant decisions tonight," Shannon said. "The first was a unanimous recommendation that the damage we saw after reviewing all the engineering tests and analysis was not a threat to crew safety, this was not something that the astronauts are in danger about. We had thought that for several days, but we were waiting for the final analysis to be complete.
"We did all the things that we said we were going to do over the last few days. We had engineering analyses, we had computational fluid dynamics of the cavity from both Ames Research Center and the Langley Research Center, they were both in agreement. We did the thermal analysis and that continued to show good margins and we also did two arc jet tests where we put a re-entry heating profile on the damage sites.
"We went through all of that data and it was unanimous that we were not in a loss of crew/vehicle case," Shannon said. "The discussion then centered on whether we should use as is and return Endeavour in its current condition or if the uncertainties in the analysis could potentially cause some underlying tile damage or structural damage that we would have to deal with at the Kennedy Space Center. So we had that debate. And it was not unanimous, but it was pretty overwhelming to go with the use-as-is condition, in other words not to do the tile repair."
Commander Scott Kelly, pilot Charles Hobaugh, Tracy Caldwell, flight engineer Rick Mastracchio, Canadian flier Dafydd Williams, Al Drew and educator-astronaut Barbara Morgan were informed of the decision at 9 p.m. as they were preparing for sleep.
"Just wanted to pass along the MMT just broke out of their meeting," astronaut Shane Kimbrough radioed from mission control. "The MMT has made a decision to fly the TPS (thermal protection system) as is, no EVA repairs will be required. MMT is still looking at the cut glove issue and any future EVAs on the flight will be per the nominal content. So just wanted to wish you a good night, another great day in space and looking forward to tomorrow."
"Please pass along our thanks for all the hard work," Kelly replied.
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 crossing the boundary between two tiles, measuring roughly two inches by three 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 exposed an underlying support pad just above the shuttle's aluminum skin.
The shuttle Columbia was destroyed Feb. 1, 2003, when it re-entered the atmosphere with a gaping 4- to 6-inch hole in the leading edge of its left wing. The wing melted from the inside out, the shuttle broke apart above Texas and all seven crew members were killed. NASA managers have said all week that they do not view Endeavour's gouge as a Columbia-class problem. Shannon said the issue was simply 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.
Testing and computer models predict the underlying aluminum skin of the shuttle's right wing will never get hotter than 350 degrees Fahrenheit, NASA's safety limit. At worst, Shannon said, a few downstream tiles could be damaged, along with the two that were gouged out by the impact during launch.
Playing it safe, Shannon earlier asked a team of astronauts, engineers, flight controllers and managers to study repair options to determine the best approach if the heat shield had to be fixed. To protect their options, a station assembly spacewalk planned for Friday - what was to have been the crew's fourth and final EVA - was delayed 24 hours to Saturday.
But Shannon said late Thursday that a final round of tests agreed with computational fluid dynamics modeling and an independent review of the work turned up no major surprises. The MMT then voted and cleared Endeavour's crew for a standard station assembly spacewalk Saturday and an "as is" landing next week.
But the vote was not unanimous. While representatives of more than 30 organizations voted to proceed with landing as is, Johnson Space Center engineering argued it would be "prudent" to carry out a repair even though the data indicated Endeavour could safely return in its damaged state.
"It was almost unanimous to use as is," Shannon said. "The one dissenting organization was the Johnson Space Center engineering group, who took a look at the potential benefits of doing a repair and said that they could not see a reason why that would cause additional damage to the orbiter and thought that that was something we should think about as a program."
But Shannon said sister organizations at the Marshall Space Flight Center and the Kennedy Space Center, along with the astronaut office at Johnson "all of the safety organizations I have on my panels, they were all in agreement that the use-as-is was the appropriate way to go."
"Some of the rationale that was summed up by the team members that I think was fairly consistent across the management team was that we have a cavity in the tile that has been analyzed through computational fluid dynamics, through thermal analysis, through two different arc jet tests and we understand that cavity, it does not constitute a risk to the crew, it is not expected to cause any damage to the vehicle structure itself and it is fairly well understood.
"On the other hand, we could potentially do this spacewalk and add some (STA-54 repair) material to the bottom of this cavity ... then we would have a new cavity that we had not analyzed. That was a fairly simple decision, is you have something you know you can live with, why would you take the risk of doing the EVA to change that cavity into what could potentially be an even better situation but also could potentially be a worse situation?"
The MMT decision ended a week of high drama on the high frontier, with engineers working around the clock to assess the threat posed by the dinged tiles and, in parallel, develop repair plans in case they were needed. The astronauts received frequent updates as they pressed through a busy timeline that included three spacewalks to attach a solar array truss segment; replace a stabilizing gyroscope; and prepare a solar array for relocation later this year.
During the third spacewalk Wednesday, Mastracchio noticed a small tear in the Vectran material used in his left glove and flight controllers ordered him back to the space station's airlock as a precaution. This was the second spacewalk in three shuttle missions that has resulted in glove damage and engineers suspect something on the station has an unexpected sharp edge.
Before Saturday's spacewalk can proceed, spacewalk experts at the Johnson Space Center hope to confirm the problem with Mastracchio's glove was not part of a more generic problem that could affect Williams or Anderson. They also are reviewing helmet cam video from Mastracchio in a bid to identify what might have caused the damage.
Here is a revised timeline for Saturday's spacewalk (in EDT and elapsed time; times subject to change):
EDT........HH...MM...EVENT 10:06 AM...00...00...Airlock egress 10:26 AM...00...20...Orbiter boom sensor system (OBSS) boom stand installation 11:26 AM...01...20...Williams: Z1 S-band antenna sub-assembly gimbal locks 11:26 AM...01...20...Anderson: MISSE space exposure experiment package retrieval 12:11 PM...02...05...Wireless instrumentation antenna installation 01:51 PM...03...45...Destiny module debris shield cleanup 02:06 PM...04...00...Unity module debris shield cleanup 03:21 PM...05...15...Other get ahead tasks 03:51 PM...05...45...Cleanup 04:21 PM...06...15...Airlock ingress 04:36 PM...06...30...Airlock repressurization
On Wednesday, Shannon made it clear he viewed a heat shield repair spacewalk as risky and said that such a repair could only be justified if it was necessary to prevent serious damage during re-entry.
The repair plan called for a 50-foot-long extension called the orbiter boom sensor system, or OBSS, to be attached to the end of the shuttle's 50-foot-long robot arm. Mastracchio planned to ride on the end of the boom, his feet anchored in a work platform, while Williams provided assistance as a free floater, tethered to the boom.
Working from Endeavour's aft flight deck, Caldwell was to have moved the astronauts down under the shuttle and back to the damage site, located a few feet aft of the right main landing gear door. The repair planned called for Mastracchio first to dab a black paint-like material known as emittance wash into the gouge, using a simple device that works like a liquid shoe polish applicator.
After the walls of the gouge were coated with emittance wash, he was to use a different type applicator, one that works like a grease gun, to squeeze out and mix a thick, putty like material known as STA-54. Like epoxy, STA-54 mixes as it exits the applicator and it can be difficult to work with. But Mastracchio helped develop some of the necessary procedures and engineers were confident he could successfully fill in a portion of the gouge.
But on Wednesday, Shannon pointed out three areas of concern about the 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 applied STA-54 in the vacuum of space and third, no one knows if the applicator will work properly or if the spacewalkers might have problems getting the thick material into the gash. "It takes a little bit of practice to get it in exactly the right spot."
Late Thursday, Shannon said he would not hesitate to order a spacewalk repair if the data showed repairs were needed to ensure a safe landing. In this case, he said, they simply didn't.
"We believe we're going to have to go replace a couple of tiles that got damaged," he said. "And that is within the normal turnaround flow of a vehicle."
Asked if there was zero chance of a catastrophic loss of vehicle and crew due to the heat shield gouge, Shannon said simply: "Yes."