Discovery Makes Its Space Deliveries

The Discovery astronauts have successfully installed the 10-ton Leonardo cargo module on the international space station.

Among the goodies awaiting the space station crew were a new stationary bicycle for exercise, an oxygen generator that will eventually allow the space station to support six inhabitants, a machine that cools the station's cabin air and a lab freezer for scientific samples.

Unloading items 220 miles above Earth was even more difficult than moving into a house since at least there's gravity on the ground, Steve Lindsey, Discovery's commander, said in interviews with reporters on the ground.

"It's really kind of a challenge because you're in zero-G ... you've got to go very, very slow because if you go fast, you kind of run into things and bump into other equipment," Lindsey said. "It's kind of an interesting choreography we have to go through."

For the first time in three years, the space station has three crew members — European Space Agency astronaut Thomas Reiter on Thursday joined Pavel Vingogradov and Jeff Williams, who marked their 100th day at the space station Friday.

Discovery's six remaining shuttle crew members awoke Friday to a recording of The Beatles' "Good Day Sunshine," a choice of astronaut Lisa Nowak's family.

Flight controllers also told Discovery's crew that they expected NASA managers on Friday to extend the mission by a day to allow for a third spacewalk. That would bring the mission to 13 days.

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Shuttle Updates From CBS Space Consultant Bill Harwood

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The robotic arm and boom were used two days ago to examine the shuttle's nose cap and wings for damage. Before docking Thursday, Lindsey maneuvered the shuttle into a back flip so that the space station's crew could photograph the shuttle's belly and transmit to the images to engineers in Houston.

CBS News Space Consultant Bill Harwood reports one area of concern involves three gap fillers seen protruding above the surface of surrounding heat-shield tiles on the shuttle's belly. One is toward the edge of the shuttle's left wing, another is near a propellant feedline access door and the third (assuming it actually is a gap filler) is located just behind Discovery's reinforced carbon carbon nose cap.

Gap fillers are thin, ceramic cloth spacers used to smooth the flow of air across the gaps between tiles and to prevent adjacent tiles from rubbing against each other too much during the vibration and stress of launch. The gap fillers are bonded in place, pull tested before launch and their upper surfaces are flush with the surrounding tiles.

Gap fillers occasionally shake loose and extend up into the airflow during re-entry and disrupt the smooth, laminar flow of supersonic air across the belly of the shuttle. Known as "tripping the boundary layer," this phenomenon can create eddies of turbulence that, in turn, result in higher downstream heating.

Boundary layer transitions occur normally when the shuttle's velocity has dropped to around eight to 10 times the speed of sound, starting toward the back of the heat shield and moving forward. But a protruding gap filler in a 1995 shuttle mission tripped the boundary layer at Mach 18, causing significant tile damage during entry.

An early boundary layer transition also can overheat the reinforced carbon carbon panels on the shuttle wing leading edges. Yet another concern is an asymmetric boundary layer transition, changing the aerodynamics and causing the shuttle's flight computers to compensate by firing rocket thrusters or adjusting the ship's elevons.

During Discovery's flight last summer, NASA managers decided to have astronaut Steve Robinson remove two protruding gap fillers during an already planned spacewalk. The removals went smoothly, but NASA engineers decided to change the way gap fillers are attached before launching Discovery on its current mission.

Steve Poulos, manager of the orbiter projects office at the Johnson Space Center, said before launch more than 5,000 gap fillers were replaced. To make sure they were firmly bonded in place, an eight-ounce pull test was replaced with a five-pound test.

NASA did not have time to replace all 16,000 of Discovery's gap fillers. Instead, engineers prioritized the shuttle's underbelly into zones of relative danger, removing and replacing gap fillers in the most critical areas. In general, the most critical zones are toward the front of the shuttle's belly.

"That was a very good effort in a short amount of time to identify the root cause of the problem," Poulos said. "The amount of work done by the technicians, inspectors and engineers at Kennedy to get the 5,000 removed and replaced was just basically awesome."

There were two fundamental concerns: gap fillers that could pull loose and pose an impact threat to the orbiter's heat shield; and gap fillers that could trip the boundary layer and threaten the RCC wing leading edge panels or downstream tiles.

"Priority one is the area that ended up being a concern from a thermal boundary layer transition perspective for the reinforced carbon carbon panels," Poulos said. "That's actually what caused us to make the final decision to remove and replace gap fillers on (Discovery's last flight). Because they were so forward on the vehicle, up close to the nose, the boundary layer trip was actually going to cause an over heating for the RCC panels themselves. By r-and-r'ing all those in (the nose region), we have mitigated that risk in totality. There is no opportunity now for a boundary layer trip to impinge on any of the reinforced carbon carbon."

It is not yet known whether the presumed protruding gap filler seen just behind Discovery's nose cap Thursday extends far enough above the surrounding tiles to represent a boundary layer threat.