|Removing the Gap Fillers: A Spacewalking First||
Discovery astronaut Steve Robinson is set for a spacewalking first on Wednesday when he goes underneath Space Shuttle Discovery to work on its heat shield. His task: to ensure a safe re-entry by removing a pair of "gap fillers" protruding from Discovery's tiles.
Image left: A photo from the International Space Station shows the location of two protruding gap fillers on Discovery's underbelly, with closeups inset. Click for High Resolution Credit: NASA.
The ceramic coated-fabric gap fillers are used to fill very small spaces and provide a cushion between the Shuttle's protective tiles. There are thousands of gap fillers on the bottom of each Shuttle. But on Discovery, two of those fillers are sticking out from between the tiles. This could potentially affect the aerodynamic flow during re-entry, causing turbulence and unwanted heating in a scenario known as "tripping the boundary layer." Engineering analysis has determined neither of the gap fillers that are sticking out is needed for landing.
Deputy Shuttle Program Manager Wayne Hale said he asked the experts if they had enough data "to be 100 percent confident the vehicle could fly safely during entry." Citing the "large uncertainty" about aerodynamics at the altitudes and speeds of a Shuttle re-entry, Hale said "in the end it came down to be a really simple decision ... the remedy is easy and we ought to go exercise that remedy."
"That remedy" is where Robinson comes in. Riding the International Space Station's robotic arm, he'll carefully move under the Shuttle and remove the gap fillers, using very simple techniques.
Image right: Astronaut Steve Robinson will use the Station's robot arm to get "underneath" the Shuttle and remove the gap fillers.For more details, see this message uplinked to the crew. Photo credit: NASA.
"There won't be any yanking going on," Robinson says. "It will be a gentle pull with my hand. If that doesn't work, I have some forceps. I will give it a slightly more than gentle pull. If that doesn't work, I saw it off."
During their training, Robinson and his fellow spacewalker Soichi Noguchi spent dozens of hours practicing techniques similar to what Robinson will use to remove the gap fillers, and engineers on the ground have been working on the specific procedures.
"It should be a simple task," says Cindy Begley, NASA's lead spacewalk official for the 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."
Begley says the team is taking steps to prevent accidental damage, by taking some tools off of Robinson and putting his safety tethers behind him.
Image left: Engineers on the ground practice removing a gap filler with forceps. Robinson will try to pull the material out by hand, but can also use forceps or cut the gap filler if necessary. Click for High Resolution Image. Photo credit: NASA.
View Video of Training:
+ Video 1 (371 Kb) | + Video 2 (134 Kb) | + Video 3 (292 Kb)
"Like most kinds of repairs," says Robinson, "it's conceptually very simple, but it has to be done very carefully."
The entire operation reflects NASA's renewed focus on safety. Thanks to new cameras, inspection tools and procedures, NASA is able to examine the Shuttle's health as never before. In the past, the problem likely would have gone undiscovered until landing. And even though Discovery could potentially land safely as-is, why not use the tools available to be sure?
Hale sums up the new mindset this way: "If we cannot prove that it's safe, then we don't want to go there."