Spacewalk strategy revised for next shuttle flight
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: April 17, 2006
Because of an over-loaded crew timeline and new heat-shield inspection requirements, shuttle flight planners have decided to eliminate one of three previously planned spacewalks from Discovery's upcoming mission - a spacewalk devoted to testing heat-shield repair techniques - in order to give the crew more time off in orbit, officials say. If the astronauts can conserve enough power, however, and if Discovery doesn't tarry on the launch pad, the mission will be extended one day and the spacewalk will be put back in the flight plan.
The goals of Discovery's mission are to carry supplies and equipment to the international space station; to repair a robot arm carrier on the station's main solar array truss; and to perform a variety of other tasks to ready the complex for resumption of assembly later this year.
Three spacewalks, carried out by Piers Sellers and Michael Fossum, were planned:
- EVA-1: Preliminary repair work on the robot arm mobile base station; and tests to determine the stability of the shuttle's robot arm when it is equipped with a 50-foot extension. The extension is used to inspect the shuttle's tiles, nose and wing leading edges for signs of impact damage. NASA planners want to know if the arm-boom combination can carry an astronaut on the end for possible repair work in the future.
- EVA-2: Station robot arm mobile base station repair. The mobile base station moves along rails on the main solar array truss to position the lab's robot arm for assembly work as required. Two ribbon-like power/data cables unreel or wind up in front and behind the cart as it moves along. One of the two cables was inadvertently cut by an on-board safety system last year. During Discovery's mission, Fossum and Sellers will replace the cut cable and its reel and make sure the cables cannot be inadvertently severed in the future. They also will move a new pump module to the station.
- EVA-3: Tests of repair techniques that could be used someday to fix relatively minor damage to the shuttle's tiles, reinforced carbon carbon wing leading edge panels, and RCC nose cap.
The flight plan calls for the crew to carry out an extensive heat-shield inspection, as usual, on the second day of the mission using the robot arm boom extension. The station crew will examine the ship with high-powered cameras during final approach and additional remote inspections will be done the next day. Similar inspections were carried out during the first post-Columbia mission last year.
In recent weeks, however, NASA added additional post-launch heat-shield inspection work to Discovery's flight the day before undocking from the station and also shortly after it departs. Those additional inspections, designed to look for micrometeoroid or space debris impacts, reduced the crew's off-duty time to a single half day over the duration of the mission.
Here is the flight plan with all three spacewalks:
Concerned about allowing the crew just a half-shift for rest and relaxation in a busy flight, NASA managers considered simply extending the mission one day up front. But in the end, they decided to replace the third spacewalk with crew off-duty time on flight day nine.
NASA is holding open the possibility of reinserting the tile-repair demonstration spacewalk on flight day 10 if the astronauts can conserve enough electrical power for a one-day mission extension.
They also will need a bit of help before launch.
The shuttle's on-board supplies of hydrogen and oxygen, used to generate electricity, diminish with every day the ship stays on the ground after loading. To have a chance for a mission extension, Discovery needs to get off the pad within three days of its eventual target launch date. Otherwise, the flight will be put on hold for 72 hours so the hydrogen and oxygen supplies can be topped off.
In more "routine" launch campaigns, the shuttle can make three attempts over four to five days before a stand down to reload ground tanks and on-board fuel cell supplies.