NASA Sets Spacewalk to Repair Discovery's Heat Shield
Engineers and astronauts test a gap-filler removal technique using forceps at NASA's Johnson Space Center.
HOUSTON - The crew of the space shuttle Discovery will perform an unprecedented on-orbit repair Wednesday, sending an astronaut under the orbiter's belly to remove a two strips of material jutting out from its tile-covered heat shield, mission managers said Monday.
The decision caps three days of scrutiny by imaging specialists, shuttle tile engineers, aerodynamicists and spacewalk planners to determine exactly how to deal with the two bits of ceramic fiber cloth - known as gap-fillers - sticking out from between heat-resistant tiles under the forward section of the Discovery orbiter.
"In the end it came down to be a really simple decision," said Wayne Hale, NASA's deputy shuttle program manager, during a briefing here at NASA's Johnson Space Center (JSC). "We came to the conclusion that we don't know enough to really feel good about this, so therefore the remedy is easy and we ought to go exercise the remedy."
That remedy is an unprecedented spacewalk to send one astronaut, attached to a robotic arm, under the shuttle to pluck out the strips by hand, or cut them off if they prove too stubborn, Hale said. The extravehicular activity (EVA), currently expected to take about 90 minutes, will be folded into the timeline for the third and final spacewalk of Discovery's STS-114 mission, an early morning extravehicular activity set for Aug. 3, he added.
"I think it's a fairly simple task," Cindy Begley, lead EVA officer for Discovery's flight, said of the repair plan earlier today. "It's just making sure we're not going to hit the vehicle, and we're doing that."
Hale said that while the gap-filler repair is slated for an Aug. 3 spacewalk, an extra day added to the STS-114 mission on Sunday allows room to push the EVA back a day should the shuttle crew and spacewalk planners request it.
"We would take the extra transfer day that we've added...and move it in front of the EVA instead of in back of it if they request," Hale said.
The two gap-fillers jutting from Discovery's belly were not caused by impacts or foam debris from the shuttle's external tank, and likely shook loose from the thin glue mount that connects them to the orbiter's undercarriage, shuttle officials said. The ceramic material is used to keep the orbiter's heat-resistant tiles from clattering against one another during launch, as well as to fill in excess space between individual tiles, they added.
If the filler material sticks out from between tiles during reentry, they can disrupt the aerodynamic flow around the orbiter during reentry, causing higher than normal local heating on the order of hundreds of degrees Fahrenheit, said Chuck Campbell, a NASA subsystem engineer who studies the heating issues associated with shuttle reentry.
After a thorough study, aerodynamic heating specialists were unable to say that the two protruding gap-fillers seen on Discovery would not pose a danger to its crew during the orbiter's planned Aug. 8 reentry through the Earth's atmosphere.
"Given that large degree of uncertainty, life could be normal during reentry or some bad things could happen," Hale said. "To set our minds at rest...the EVA team has been off working hard for the last three days."
During a morning status report, Begley said the EVA team planned to use the robotic arm aboard the International Space Station (ISS) to maneuver STS-114 astronaut Stephen Robinson underneath Discovery with his tools and tethers strapped close behind him to ensure he did not accidentally strike the vital heat-resistant tiles. Using his own, spacesuit-clad he fingers, he would then pluck the gap-fillers one at a time, she added.
Robinson will also carry a saw tool, scissors and forceps should the gap-fillers prove too tough to yank out by hand, Begley has said.
The gap-filler removal will likely take place after Robinson and his spacewalking partner Soichi Noguchi, of the Japanese Aerospace Exploration Agency (JAXA), install the External Stowage Platform 2 - a spare parts fixture for the ISS - to the station's Quest module, shuttle officials said.
Since the repair is a solo task, Noguchi will be free to retrieve a faulty rotary motor from the ISS and perform other tasks while Robinson removes and gap-filler. The two astronauts have already tested out potential heat tile and reinforced carbon carbon panel repairs in their first spacewalk on Saturday. They also retrieved the tools Robinson will use in the gap-filler removal during an early morning spacewalk today.
Meanwhile, shuttle engineers are already looking at how to redesign gap-fillers and their installation processes to prevent such protrusions in future orbiter flights, NASA officials said.
Shuttle astronauts have repaired their own orbiters via spacewalks three times in the past, once to replace a broken television camera and twice to properly stow a Ku-band antenna, Hale said.
"We're committed to safety," Hale said, adding that Discovery's flight is paving new ground for NASA's shuttle program. "We're building up a new database and we're going to be smarter and safer in the future because we have this new knowledge."
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