WASHINGTON --- Seven days into the STS-114 mission of space shuttle Discovery, astronaut Steve Robinson and crewmates completed a first-of-its-kind heat-shield repair quickly and as planned on the shuttle’s underbelly during the August 3 spacewalk from the International Space Station.
Orbiting 357 kilometers above Massachusetts in the United States, Robinson rode the station’s Canadarm 2 robot arm to two places where pieces of ceramic-coated gap filler fabric protruded from between Discovery’s heat-shield tiles. With gloved fingers, he gently tugged the protrusions until they came out.
After he removed the second protrusion, Robinson said, "It looks like this big patient is cured."
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 the fillers were sticking out from between the tiles.
The fear was that the protruding strips of fabric could interfere with a phenomenon called a boundary layer transition, during which the flow of air over the shuttle’s surface on reentry changes from laminar (smooth) to turbulent. The smooth layer of air has an insulating effect on the heat-shielding tiles; turbulence raises the air temperature.
The engineering team did not know if the protruding gap fillers would cause such turbulence, increasing the shuttle surface temperature by 10-30 percent and possibly causing damage to the heat tiles on Discovery’s reentry to Earth’s atmosphere.
“This particular subject is not well understood because nobody else flies in these machines,” said Wayne Hale, deputy manager of the space shuttle program and chairman of the NASA mission management team, during an August 2 press conference at Johnson Space Center in Houston, Texas.
“Nobody else flies mach 22 [22 times the speed of sound] at 216,000 feet [65.8 kilometers],” he added, “ ... the only data we’ve got comes from the shuttle and that’s all there is in the world.”
Hale said Steve Poulos, orbiter project manager for the space shuttle program, has been asked to provide a mitigation plan for such gap filler problems in future vehicles. A set of preliminary proposed process changes should be ready within two weeks.
“This is the new shuttle program, the new NASA,” Hale said. “If we cannot prove it’s safe, then we don’t want to go there. This exceeded our threshold and we needed to take action.”
The only other safety issue still pending for Discovery involves a thermal blanket just under Commander Eileen Collins’s crew cabin window on the orbiter’s nose.
The blanket, made of woven ceramic fabric between the fabric layers, may have been punctured during launch by flying debris, causing a 20-centimeter section of the blanket to puff out slightly from the shuttle surface.
NASA considers this a potential “debris problem, not a reentry heating problem,” said Paul Hill, STS-114 shuttle flight director, during an August 3 press conference, also in Houston. Sections of the blanket could potentially hit some critical places aft of the window.
The engineers, he said, “are evaluating a wide range of options in what we consider to be the unlikely case that we would have to go out and do something about it.”
Those options include cutting the blanket and pulling it off the shuttle, or shredding the blanket so that, if comes off the shuttle, it comes off in much smaller, harmless pieces.
Hill said a final decision about the thermal blanket will be made August 4.
Only one spacewalker could work on the underside of Discovery, but the repair efforts took teamwork.
Fellow spacewalker Japan Aerospace and Exploration Agency astronaut Soichi Noguchi provided communications and visual support to Robinson and flight controllers. Australian mission specialist Andy Thomas was the spacewalk coordinator. Pilot Jim Kelly and mission specialist Wendy Lawrence operated the station’s robot arm.
Before the arm could return Robinson to the payload bay, he took pictures of the heat shield for analysis by engineers and of the space shuttle docked to the space station. He said the curved lines of the shuttle contrasted beautifully with the linear space station structure.
“You’re going to be glad I had a camera,” Robinson told his crewmates and NASA mission control.
Earlier in the spacewalk, Noguchi and Robinson attached an external stowage platform and a materials-exposure experiment onto the station.
Inside the space station, the astronauts continue to stow cargo and supplies.
This was the third STS-114 spacewalk and the 61st spacewalk dedicated to space station assembly and maintenance.
NASA Astronauts Fix Space Shuttle Problem; Unprecedented On-Orbit Repair Readies Discovery for Return to Earth