Potential spacewalk repair never attempted before
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: August 1, 2005
NASA engineers are refining plans for an unprecedented but relatively straight forward spacewalk repair job Wednesday to remove two protruding "gap fillers" from the shuttle Discovery's underside heat shield tiles. While no spacewalking astronaut has ever been asked to work under the shuttle, out of direct view, engineers say the gap filler fix is not technically difficult or especially risky.
"It should be a simple task," said Cindy Begley, lead spacewalk officer for Discovery's mission. "It could be just as easy as grabbing it with his fingers and pulling it out. And we hope that's all it's going to be."
A formal decision on whether or not to add the repair task to an already planned spacewalk Wednesday is expected by the end of the day, after NASA's mission management team receives a briefing on the aerodynamic effects of the protruding gap fillers and an assessment of the threat they might pose.
Gap fillers do exactly what their name implies: They fill gaps between adjacent heat-shield tiles to prevent heat intrusion and side-to-side rubbing as the shuttle's aluminum skin flexes. Photographic inspection of Discovery's under side showed that two of the thousands of gap fillers in place had shaken or partially pulled loose during launch or after the shuttle reached orbit. As a result, portions of the two gap fillers in question stick up above the surface of the tiles. One extends about an inch above the tiles while the other protrudes about six-tenths of an inch.
The concern is that the gap fillers could disrupt the smooth flow of supersonic, super-heated air across the belly of the shuttle early in re-entry - the so-called boundary layer - creating turbulent flows that, in turn, could lead to extreme heating that might damage surrounding tiles or even wing leading edge panels. The early creation of turbulent airflow is known as "tripping the boundary layer."
Shuttles have re-entered with extended gap fillers before, suffering tile damage as a result, but this is the first time NASA engineers have had a chance to see one in advance and consider possible fixes. That's because of a post-Columbia safety program that now includes a shuttle pitch-around maneuver during approach to the space station that provides an opportunity to photograph the belly of the orbiter in enormous detail.
Over the weekend, a tiger team of veteran spacewalkers and engineers began "evaluating several methods for removing the gap filler or cutting the gap filler," Begley said "They're evaluating how to get access to the area, which arm to get on and go down there and we have plans in work for a new EVA timeline if we need to do that.
"So we're getting everything staged and ready to go. We're even going to send some information to the crew so they can be looking at that if we end up going in that direction. ... As always, we try to get all our ducks in a row ahead of time."
Assuming the astronauts get the go ahead as expected, the station's robot arm would be "walked off" the Destiny laboratory module in inchworm fashion to reach a mounting point on the station's main solar array truss. Astronaut Stephen Robinson, equipped with forceps, scissors and a hacksaw-like tool, then would lock his boots in a foot restraint and the arm would be maneuvered to carry him to the gap filler work sites.
"The basic part of the task is to get to the worksite and that's something we've never done before, put an EV crew member underneath the vehicle," Begley said. "The task itself, pulling out the gap filler, they're going to have to be very careful of the area not to damage anything while they're there. We're making sure we're taking as many tools off of him as we can and holding the safety tethers back behind him.
"The first attempt is going to be to pull it out," she said. "We don't expect that to take a lot of force. If it seems to be taking a lot of force, then we're going to look at cutting it off. And we have a number of tools (available). We have a forceps you can lock on to the gap filler and gives him a tether point to hang on to it as he's cutting it off to get it out. Those are the things we're looking at. Doing the actual cutting may give us a little bit of debris to look at but otherwise, I think it's a fairly simple task, making sure we're not going to hit the vehicle when we're doing that."
Asked about what worst-case scenario NASA would be protecting against by ordering the repair work, mission operations representative Phil Engelauf said "I'm going to hedge a little bit here because that's going to be the topic of discussion at the (mission management team).
"The aerothermal team is going to come in and present the summation of the analysis they've been working on for a couple of days," Engelauf said. "The preliminary indications are that we're not going to be able to give a very definitive answer and that, in fact, is probably what will drive the discussion one way or the other.
"There are not a lot of vehicles that fly in the flight regime that the shuttle operates in and so our testing and real flight experience with boundary layer transition in the Mach 18 to 20 regions is a pretty thin data base and it's not well supported by analytical models because we've never had anything to validate those models against. The guys have been working furiously to try to understand that.
"Their considerations are bulk local heating as well as control," Engelauf said. "We think we've pretty well cleared the control issues. ... We're really down to local aero heating and there are two different categories of issues. These two pieces of gap filler are located one near the centerline of the vehicle, far forward by the nose landing gear door, and then a second one a little bit farther back but somewhat off to the side of the vehicle.
"For the most forward one, the biggest concern is the far forward early boundary layer transition, early in the flight regime. ... They're worried about transitions above Mach 20, which is earlier than the design point for the vehicle. That would be primarily (a) localized heating situation. But primarily for the tile. With the one over closer to the side, again if you had an asymmetrical transition you could potentially have localized high heating on the RCC (leading edge panels) because of anything wrapping around the vehicle and up over the leading edge of the wing."
Shuttles have returned from space with protruding gap fillers before "without detriment but also somewhat in ignorance because we've never had the opportunity to observe the way we have now. This was not, frankly, one of the things we spent a lot of time working on the past two years."
"There's a fair amount of conservatism prevailing in the community that until we can satisfy ourselves, maybe the better course of action is to go out here and remove these if that's the right thing to do."
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