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NASA Authorizes Spacewalk To Fix Shuttle Problem

02 August 2005

Astronaut to remove protruding "gap fillers" from Discovery's heat shield

By Cheryl Pellerin
Washington File Staff Writer

Washington – NASA has authorized a repair task for the space shuttle during an August 3 spacewalk that experts say should ensure a safe landing for the Discovery crew.

Mission managers decided August 1 to remove two sections of ceramic-coated fabric called gap fillers that are protruding from between heat-shielding tiles on the shuttle’s underbelly near the craft’s nose.

The protruding sections are roughly 2.5-centimeters high and less than 15 centimeters long – about the length of a tile, said Chuck Campbell, NASA subsystem engineer for orbiter entry thermodynamics, during an August 1 press conference from the Johnson Space Center in Houston.

During the spacewalk, mission specialist Steve Robinson will venture under the shuttle on the tip of the International Space Station's robotic arm, locate the gap fillers and gently tug on them with his fingers or with a pair or forceps until the fabric comes loose.

“Like most kinds of repairs, it is conceptually very simple but it has to be done very, very carefully,” Robinson said during an August 2 news conference aboard the space station with the shuttle and station crew members and U.S., Japanese and Russian reporters.

Robinson’s crewmates will be able to watch the operation using a camera on the shuttle’s Orbiter Boom Sensor System, and they will be able to communicate with him by radio.

“The tiles, as we all know, are fragile and an EVA [extravehicular activity] crew member out there is a pretty large mass,” he added, “but we predict that it won’t be too complex.”

If pulling on the fabric sections doesn’t work, Robinson will use a modified hacksaw-like tool to cut them off at or near the surface of the tiles.

The idea, Robinson said, “is to pull out this thin gap filler either by hand or with a pair of forceps, and we’ll use the hacksaw only if necessary.”

The engineering team has been working on the gap-filler problem for three days, said Wayne Hale, deputy manager of the space shuttle program and chairman of the NASA mission management team.

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; 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,” Hale said. “Nobody else flies mach 22 [22 times the speed of sound] at 216,000 feet [65.8 kilometers] ... 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.”

Also on August 2, President Bush called the space station and shuttle crews to discuss the mission.

“I just wanted to tell you all how proud the American people are of our astronauts,” he said. “I want to thank you for being risk-takers for the sake of exploration. I want to welcome our Japanese and Australian and Russian friends.”

Before signing off he added, “a lot of Americans will be praying for a safe return. ...Thanks for being such great examples of courage for a lot of our fellow citizens.”

Over the past four days, Hale said, “the space station has been resupplied, rejuvenated and almost rebuilt in one half of one shuttle flight.”

The astronauts have been working each day to transfer supplies to the station and to load the shuttle with unneeded equipment and other materials that have been taking up room on the space station.

On July 31, during their second spacewalk, Japan Aerospace and Exploration Agency astronaut Soichi Noguchi and Robinson replaced a failed space station control moment gyroscope, which helps keep the station properly oriented in orbit.

“Even as important as putting that new control moment gyro on the station is bringing the failed one back,” Hale said. “Understanding why it failed is absolutely critical to the long-term success of the International Space Station.”

Crew members are reviewing the revised timeline and preparing equipment for the excursion. Spacewalkers Robinson and Noguchi will also install an external stowage platform on the station. The gap-filler repair will be the final task of the August 3 EVA.

Information about the Discovery mission is available on NASA’s “Return to Flight” Web site.

(The Washington File is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)