Crew Admits Repair ‘Misgivings’
As they prepared for a spacewalk to repair damage to the underside of the space shuttle Discovery, crew members said Tuesday that they were at first alarmed at the prospect of tinkering with the delicate heat-resistant tiles that protected the craft.
They finally came around to agreeing to perform the task after realizing the danger of not doing anything about two protruding pieces of ceramic-coated fabric known as “gap fillers” outweighed the relatively simple task of pulling them out.
“I think a number of us did have misgivings,” astronaut Andrew Thomas told reporters in a news conference from space.
Paul Hill, the lead flight director, said the crew was “definitely apprehensive” when first told about the repair plan.
But after looking at the alternatives and seeing how seemingly simple the task would be, the astronauts were reassured. “What seems like a scary proposition ... is something within our experience,” Hill said.
Thomas agreed: “The removal of the material should be pretty straightforward and pretty easy.”
Astronaut Stephen Robinson was to make the repair during a previously planned spacewalk set for 1:14 a.m. PDT today.
Plans called for him to be carried to the underside of the orbiter on the robotic arm of the International Space Station, to which Discovery is docked.
“We predict it won’t be too complex,” Robinson said. “It’s very simple, but it has to be done very, very carefully.”
The crew took time out from preparations for the repair job to accept a phone call from President Bush, who complimented them on their courage.
“I want to tell you all how proud the American people are,” Bush said. “Thank you for being risk-takers for the sake of exploration. You’ve got a strong supporter for your mission here in the White House.”
Discovery is scheduled to land before dawn Monday, providing all the safety issues are resolved.
Dings to the tiles covering the bottom of Discovery have been cleared. On Tuesday, N. Wayne Hale Jr., deputy shuttle program manager, said the reinforced carbon panels that covered the nose cone and leading edges of the wings were also found to be in good shape.
The last two items are the faulty gap fillers and a small, stitched blanket under the commander’s window that is sticking out after apparently being hit by debris, Hale said.
Engineers are studying what would happen if the blanket tears loose during reentry.
Gap fillers are credit-card-thin spacers placed between the thousands of silica tiles that cover the underside of the spacecraft. They keep the tiles from rubbing together, or “chattering,” during liftoff.
The chattering is caused by the disruption of the airflow between the craft and the external fuel tank that sits alongside it during launch.
The spacers are unnecessary during the landing because the fuel tank, having dropped off during liftoff, is no longer interfering with the smooth flow over the spacecraft’s hull.
Protruding gap fillers have been noted after landing on past shuttle flights.
Several factors contributed to NASA’s decision to do something about the protrusions on Discovery.
The first is that it can. With all the new cameras and sensors installed on Discovery after the accident that destroyed the shuttle Columbia in 2003, engineers have received more data about the health of the craft than ever before.
The protrusions were spotted on the third day of Discovery’s mission, when close-up pictures of its underside were taken by the space station crew.
That initiated a lengthy debate inside the engineering community about whether to leave them in place or undertake the delicate task of repairing or removing them.
“The reaction [was] we’re nuts to put a crew member close to the tiles,” Hill said. “Most of the folks in that room really did not want to do this.”
Engineers feared that an astronaut working so close to the tiles could gouge one or even pull it out by accident.
But tests in the laboratory showed that it would take more than clumsiness to seriously damage a tile.
Another concern was for the astronaut’s safety. Almost every activity on a mission is rehearsed again and again, particularly when it comes to spacewalks.
NASA has never sent an astronaut to the underside of a vehicle, which is out of sight of the shuttle crew.
An astronaut would also be dangling on the end of a robotic arm that, should it malfunction and lock up, could block the orbiter from leaving the space station.
The final factor in the decision to go ahead with the spacewalk was the uncertainty among engineers about what would happen if Discovery tried to land with the gap fillers as is.
Even though each is sticking out about an inch, analyses showed that was enough to disrupt the flow of air over the tiles behind the spacers, increasing the localized heating by as much as several hundred degrees.
“We never had protrusions that stuck out this far forward, this far out,” Hill said.
The tiles are rated to survive heat above 2,300 degrees, but it was impossible to predict how hot the tiles near the protrusions might get.
There is little reference material to work with because no other flying vehicle comes close to the speed of the shuttle.
The offending gap fillers are near the main landing gear doors, close to the nose, which receives some of the hottest temperatures outside the wings’ leading edges, which are able to tolerate temperatures above 3,000 degrees.
If the worst were to happen and Robinson gouged a tile, Discovery is carrying a special emittance wash system that could be used to repair it.