Discovery officially cleared of any launch damage
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 9, 2006
After around-the-clock analysis, NASA managers today officially gave the shuttle Discovery's heat shield a clean bill of health, concluding there are no problems with tiles, the ship's nose cap or wing leading edge panels that require any repair work by the astronauts.
It was a long awaited moment for thousands of NASA and contractor engineers and managers who have put in long hours since Discovery's last flight to correct potentially catastrophic problems with foam insulation.
As it turned out, Discovery's external tank lost less foam than any tank before it and the orbiter suffered virtually no impact damage or other serious problems with its fragile heat shield.
Discovery's crew was given the news just before bedtime, after engineers worked through the night to resolve questions about a protruding gap filler and tears in two small insulation blankets.
"So with that, we've closed out all issues on the TPS (thermal protection system) and we deem the TPS as 100 percent cleared for entry," astronaut Lee Archambault radioed from mission control.
"Well, that is great news, that's fantastic," replied Discovery commander Steve Lindsey. "To get all that done by the end of flight day six, when we did focused inspection on flight day four, is just amazing. So our thanks to the imagery team, the MMT and all the engineers working on this to get the vehicle cleared this quickly. That's very impressive."
"Everyone here around the room as you can imagine is most happy," Archambault said. "I think you'll be delighted when you see the images that you guys sent down after you get back. They really were amazing."
Flight controllers were equally wowed by stunning movies shot by cameras mounted in the shuttle's twin solid-fuel boosters. The rocketcam videos made it back to the Johnson Space Center today, providing engineers with never-before-seen views of launch and booster separation. The giant solid-fuel rockets performed normally, officials said today, with no problems of any significance.
John Shannon, chairman of NASA's Mission Management Team, said he remains optimistic NASA will resolve the few problems that have cropped up during Discovery's flight in time to launch the shuttle Atlantis Aug. 28 on a space station assembly mission, the first shuttle construction flight since Columbia went down in 2003.
"This has really been two missions for us," he said. "The first was to go and do ... a lot of things to assess orbiter health and potential repair capabilities. The other piece of it that's just as important is what we have done for the space station.
"We've talked a lot about how wonderful the orbiter looks and how well the external tank performed and how good the boom was. But I don't want to lose sight of the other half of this, which is how much we're doing to get ourselves ready in August for beginning the assembly of the space station again."
The August mission depends on the success of a spacewalk Monday by astronauts Piers Sellers and Mike Fossum to repair a stalled robot arm transporter on the station's solar array truss. The robotic transporter is required to move the station's robot arm from work site to work site on the unfinished truss and until it is fixed, station assembly is on hold.
Last December, a spring-loaded cable cutter inadvertently fired, severing one of two power and data cables leading to the transporter. Sellers and Fossum plan to replace the cable reel mechanism Monday during a tricky make-or-break spacewalk.
"We need that thing to be working, to move big pieces around during the assembly," Sellers said today. "Right now, it's dead on one side. Our job (Monday) is basically to replace the unit that supplies power to the truck so it can be ready for the next stage of assembly. It's important to get that done before the next guys show up."
A third spacewalk is on tap Wednesday to test wing leading edge repair techniques. Engineers briefly worried the astronauts might have to pluck a protruding gap filler from Discovery's belly during the third excursion, but Steve Poulos, manager of the orbiter projects office at the Johnson Space Center, said today a detailed analysis showed Discovery can safely re-enter the atmosphere as is.
"Nobody had any concerns whatsoever we were going to exceed any of their limits," he said. "We had no dissension across the entire community, everybody was 100 percent on board."
Gap fillers are thin, heat-resistant spacers between tiles to prevent chafing during the vibrations and temperature extremes of launch and orbital flight. Some 16,000 gap fillers are in place, including more than 5,000 that were replaced in wake of Discovery's flight last summer on the first post-Columbia mission.
Gap fillers can occasionally shake loose and if they stick up too far into the air stream during re-entry, they can break up the smooth flow across the belly of the shuttle, creating turbulence that can raise downstream temperatures.
During Discovery's approach to the space station earlier this week, engineers saw a gap filler sticking up from tiles near a propellant feedline access door. The high-resolution photography by the station's crew showed the gap filler was damaged and creased.
Poulos said engineers conducted tests showing the gap filler most likely will roll over in the re-entry air stream. But even if it doesn't, testing showed Discovery is in no danger of excessive heating.
During a normal re-entry, the tiles in that area should not experience temperatures higher than 2,300 degrees at their surface, more than 625 degrees at the bondline where they attach to the shuttle's skin and 350 degrees for the aluminum skin itself.
In a worst-case scenario, Poulos said, one in which the protruding access door gap filler disturbed the insulating "boundary layer" just after atmospheric entry at 24.7 times the speed of sound, temperatures on the tile surface would not exceed 2,254 degrees. The bondline temperatures would not rise above 579 degrees and the shuttle's skin down stream of the gap filler would not get hotter than about 344 degrees.
If the gap filler rolls over like engineers expect, tripping the boundary layer around 19.5 times the speed of sound, conditions will be even more benign: 2,254 degrees at the downstream tile surface; 496 degrees at the bondline; and 289 degrees on the skin.
"With all that data, we were able to clear that gap filler as a non constraint for entry," he said.
Engineers worked one other open item overnight, conducting tests and computer analyses to show two small, damaged thermal blankets near the shuttle's nose will not pull off during entry and pose an impact threat to Discovery's cockpit windows or other areas.
Testing showed the blankets would not begin to peel away unless relatively large "loads" were applied - more than five pounds of pull. The testing and computer analysis showed the blankets would experience no more than about 0.9 pounds of loading during entry.
"In my terminology, not only did the team pound these issues flat, they put a dimple in the board when it was all said and done," Poulos said. "So we are absolutely clear and ready to bring this vehicle home whenever the mission is accomplished."
In one minor correction, Shannon said additional analysis showed a piece of foam insulation that peeled off Discovery's external tank during launch weighed just .055 pounds instead of .11 pounds as he earlier reported. Either way, it was well below NASA's safety limit of .25 pounds for potentially serious impact damage.