Pilot Mark Kelly, Lisa Nowak and Stephanie Wilson are gearing up carry out so-called focused inspections of Discovery's heat shield to double check several areas of interest that were noticed during earlier inspections.

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 occaionally 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 thruysters 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 cabon 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.

"Then as we looked at it a little further," Poulos said in his pre-flight briefing, "we realized from our debris transport work there was a potential for some gap fillers to liberate and potentially impact the RCC, RCC being less compliant from an impact perspective than our tile. So we generated a priority 2A zone. We have removed and replaced all of those gap fillers as well.

"So on the bottom of the vehicle, our real focus was to mitigate the thermal concern for RCC, mitigate the impact concern for RCC. The residual or remaining items, priority 2B and 3 - the risk associated with liberation and impact relative to the tile - is a very small risk.

"From a boundary layer transition perspective, we have done a significant amount of analysis and in most instances at Mach 21 (21 times the speed of sound) or less, in terms of our prediction for boundary layer transition, we will be able to clear the vehicle. If we have a gap filler that is extended above the outer mold line to a significant extent ... then we might have to consider doing something about that. But at the end of the day, we have the analytical tools in place to go off and assess if ... it is a concern or not."

Additional coverage for subscribers:
VIDEO: DAY 3 MANAGEMENT TEAM BRIEFING DIAL-UP | BROADBAND
VIDEO: POST-DOCKING MISSION STATUS DIAL-UP | BROADBAND
VIDEO: SHUTTLE CREW FLOATS INTO THE STATION PLAY
VIDEO: DISCOVERY DOCKS TO THE SPACE STATION PLAY
VIDEO: WATCH THE ENTIRE PIROUETTE BACKFLIP MANEUVER PLAY
VIDEO: FLIGHT DIRECTOR EXPLAINS RENDEZVOUS AND DOCKING PLAY

VIDEO: INITIAL TANK ASSESSMENT DIAL-UP | BROADBAND
VIDEO: DAY 2 MISSION STATUS BRIEFING DIAL-UP | BROADBAND
VIDEO: FLIGHT DIRECTOR EXPLAINS INSPECTIONS WITH BOOM PLAY
VIDEO: FLIGHT DIRECTOR EXPLAINS ROBOT ARM INSPECTIONS PLAY

VIDEO: THE FULL LAUNCH EXPERIENCE PLAY
VIDEO: RIDE ALONG DURING LAUNCH VIA CREW MODULE CAMERA PLAY
VIDEO: AMAZING FOOTAGE FROM WB-57 HIGH-ALTITUDE AIRCRAFT PLAY

VIDEO: SHUTTLE LANDING FACILITY TOWER PLAY
VIDEO: PLAYALINDA BEACH TRACKER PLAY
VIDEO: BEACH MOUND TRACKER PLAY
VIDEO: PATRICK AFB TRACKER PLAY
VIDEO: UCS 23 TRACKER PLAY
VIDEO: UCS 11 TRACKER PLAY
VIDEO: CS 6 TRACKER PLAY
VIDEO: CS 2 TRACKER PLAY
VIDEO: CS 1 TRACKER PLAY
VIDEO: VEHICLE ASSEMBLY BUILDING ROOF PLAY
VIDEO: LAUNCH PAD FRONT CAMERA PLAY
VIDEO: COMPLEX 39 PRESS SITE PLAY
VIDEO: LAUNCH PAD SIDE VIEW PLAY
MORE: STS-121 VIDEO COVERAGE
SUBSCRIBE NOW