Discovery astronauts begin mission's first spacewalk
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 30, 2005
Astronauts Stephen Robinson and Soichi Noguchi began a planned six-and-a-half hour spacewalk today, a busy excursion highlighted by long-awaited tests of rudimentary tile and wing leading edge repair techniques that were developed in the wake of the Columbia disaster.
Floating in the shuttle Discovery's airlock, Robinson and Noguchi switched their spacesuits to internal battery power at 5:46 a.m., officially kicking off the 59th spacewalk devoted to station assembly and maintenance. Going into today's excursion, 39 U.S. astronauts, one Canadian, one Frenchman and 10 Russian cosmonauts had logged 348 hours and 15 minutes of spacewalk time servicing and assembling the space station.
Today's spacewalk has five primary goals:
While the repair demonstrations have generated widespread attention, the electronic bypass planned for control moment gyroscope No. 2 is a higher priority item for the engineering community.
The space station uses four CMGs to maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. They are housed in the Z1 truss, which was attached to the Unity module's upward-facing, or zenith hatch - hence the name - during shuttle mission STS-92 in October 2000.
Along with saving fuel, the 800-pound gyros, spinning at 6,600 rpm, allow station crews and flight controllers to reorient the outpost and keep it stable without fuel-consuming, experiment-jarring rocket firings.
But on June 8, 2002, CMG-1 suffered a malfunction and shut down. The station's orientation, or attitude, can be controlled by just two CMGs in a worst-case scenario. And indeed, a second gyro, CMG-2, was knocked off line last year because of trouble with a circuit breaker. The circuit breaker was replaced during a station-based spacewalk, but the new unit malfunctioned in March, taking CMG-2 off line once again.
If all goes well, Robinson will re-route power to CMG-2 today and the astronauts will replace CMG-1 during their second spacewalk Monday.
But gyro problems remain an issue. After CMGs 1 and 2 are spun up and put into operation, CMG-3 will be taken out of the loop until after Discovery departs. While still functional, CMG-3 has been experiencing lubrication-related bearing issues while helping control the orientation of the massive station-shuttle complex. After Discovery departs, the gyro will be returned to service.
Columbia was brought down by a hole in the ship's left wing leading edge caused by the impact of external tank foam insulation during launch 16 days earlier.
NASA originally planned for Robinson and Noguchi to test so-called cure in-place-ablator applicator - CIPAA - backpacks, loaded with a tile repair material known as STA-54, to fill in deliberately damaged tiles in Discovery's cargo bay.
But questions about the reliability of the procedure surfaced last year when engineers noticed the formation of air bubbles in the viscous STA-54 material as the two compounds that made it up were mixed together in the backpack. After extensive troubleshooting, engineers were able to reduce the bubbling but they could not eliminate it. The concern was that bubbles could migrate in weightlessness and form large voids as the material cured. Those voids could weaken the patch and its ability to shield against re-entry heating.
NASA's astronaut office opposed in-flight testing during Discovery's flight and tests were put on hold.
Another promising technique was a so-called overlay tile repair procedure in which damaged tiles would be covered with a thin, flexible sheet of heat-resistant carbon silicon-carbide. The sheet would be mounted atop a gasket and attached with fasteners similar to drywall bolts that would be screwed into surrounding tile.
Both CIPAA and the overlay technique are expected to be tested on a future shuttle flight. Robinson and Noguchi instead will test a tile repair technique known as "emittance wash" in Discovery's cargo bay.
Using a demonstration kit with deliberately damaged tiles, the spacewalkers will paint exposed surfaces with a material that will replace damaged or eroded coating and improve heat rejection.
NASA still has no way to repair the kind of leading edge damage that brought down Columbia, but Robinson and Noguchi will test a rudimentary technique in which a heat-resistant material known as NOAX will be smoothed over small cracks in RCC material.
NOAX, which stands for non-oxide adhesive experimental, will be squirted from a caulk gun-like device and then smoothed out with trowels.
A third repair procedure, aimed at fixing small holes in RCC panels, will be tested next week inside Discovery's crew cabin. It requires a flexible carbon silicon-carbide patch called a "plug" that would be inserted into a hole and held in place from behind by expansion bolts.
Between 20 and 30 different plugs, each with slightly different geometries, would be needed in a real repair kit to ensure a good fit virtually anywhere in the curving leading edge.
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