Space shuttle external tank fixes appear sound
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 5, 2006
After a full day of image analysis and inspections, NASA engineers are increasingly optimistic that major changes to the foam insulation on the shuttle Discovery's external fuel tank worked as required to minimize the release of potentially catastrophic debris during the ship's Fourth of July climb to space.
If they're right, and if preliminary indications are confirmed during continued observations and around-the-clock analysis, NASA will move a major step closer to putting its painful post-Columbia return-to-flight effort behind it, shifting the focus instead to resuming assembly of the international space station.
"We have in hand all the data we're going to get from the external tank and the performance was very good," said John Shannon, chairman of NASA's Mission Management Team. "And we got some good data, too, which was really important. We really want to be able to verify what kind of redesigns we were doing."
Shannon said the most significant change made to the external tank since Discovery last flew a year ago - removal of a long foam wind deflector - worked well. Known as a protuberance air-load - PAL - ramp, the wind deflector was in place to smooth the flow of turbulent air over two pressurization lines and a cable tray during the shuttle's climb out of the dense lower atmosphere.
During Discovery's launching last year, a one-pound chunk of foam broke away from the tank's hydrogen PAL ramp and while it didn't hit the orbiter, it was a major surprise and source of concern. Much of the 11 months preparing Discovery for its current mission was focused on proving the PAL ramps could be safely removed and that the pressurization lines and cable tray could, in fact, stand up to the aerodynamic rigors of launch on their own.
Computer modeling and wind tunnel tests convinced agency managers the ramps were not needed and Discovery's launching Tuesday appeared to verify those conclusions. While a final answer will depend on analysis of data captured by sensors mounted in the cable tray - work that is not yet done - Shannon was clearly optimistic.
"The only thing we have to do is go make sure all the aerodynamic modeling that we did is corroborated by the sensors that we flew on this flight," he said. "This was one data point and it was a really good data point, but we want to make sure we are not on the edge of anything. We don't think we are, but we need to go make sure."
During Discovery's launch last year, foam debris also broke away from an area where two struts attach the nose of the shuttle to the tank. Engineers came up with a fix and it, too, appeared to work. Photographs of the tank after it was jettisoned in space showed no foam losses in the area.
Even the ice-frost ramps on the tank appeared to work well. The ice-frost ramps, hand-sculpted foam insulation applied to 34 brackets holding the cable tray and pressurization lines in place, generated controversy before launch when two top NASA managers voted to delay the flight until they could be redesigned. The insulation in question was officially deemed an unacceptable risk, in large part because engineers lacked a solid understanding of foam failure mechanisms.
Shannon said the IFRs on Discovery's tank performed well, with only minimal foam shedding in one ramp extension that was added after the PAL ramps were removed.
Just in front of one ice-frost ramp, however, an eight-by-10-inch sheet of foam apparently peeled away in a half-dozen pieces about two minutes and 50 seconds after launch. The debris posed no impact threat to Discovery because the releases happened after the shuttle was beyond the aerodynamic danger zone.
"It's all about the mass," Shannon said. "If it was a very thin sheet, which is what we think it was, then it won't have enough kinetic energy to damage the vehicle. And this one came off in a time when aerodynamically, we don't have a concern."
Engineers have long suspected that changes in the tank's temperature as supercold propellants are drained by the shuttle's main engines plays a role in foam failure, especially in areas where foam is applied on top of other foam.
Shannon said the location of the missing sheet may help engineers verify that theory or at least gain insights into how the changing thermal environment of the tank plays a role in foam loss as propellant levels change.
"The important part is to understand the physics of why it's coming off," he said. "I think that liquid level is going to make a big difference."
Shannon cautioned that analysis of the tank is not yet complete. Engineers have yet to assess recorded cable tray data about the actual "loads" experienced in the absence of the PAL ramps, and they have yet to recover recorded imagery shot by cameras mounted in the shuttle's solid-fuel boosters. But so far, so good.
The next major question mark is the condition of the shuttle and its heat shield. The astronauts spent the day Wednesday using a sophisticated laser sensor to map out every square inch of the reinforced carbon carbon wing leading edge panels and the shuttle's RCC nose cape, which experience the most extreme heating during re-entry. Shannon said a preliminary look at the data showed no major problems. But the analysis is not yet complete.
"Today, I have the tank imagery that we showed you and the performance was really good," he said. "Tomorrow, we'll have our initial assessment of the wing leading edge and nose cap data that the crew took today and we'll have our early assessment of the underside of the vehicle, the tile of the vehicle, when we do the rotational pitch maneuver as we approach station."
Shannon was referring to a 360-degree pitch-around maneuver commander Steve Lindsey will perform when Discovery is 600 feet directly below the space station during the final stages of rendezvous Thursday. As the shuttle's underside rotates into view, station commander Pavel Vinogradov and flight engineer Jeff Williams will use 400mm and 800mm lenses to photograph the heat shield with one- to three-inch resolution.
So far, engineers have only noticed one protruding "gap filler," a heat-resistant spacer between tiles near the rear leading edge of the shuttle left wing. Two protruding gap fillers had to be removed from Discovery's belly last year during an impromptu spacewalk repair job, but the gap filler seen sticking up today is in a benign location and likely will not need removal.
NASA replaced some 5,000 gap fillers in more critical parts of the shuttle's underside and the rendezvous pitch maneuver during final approach to the station Thursday will give engineers a good look at those areas.
Asked if the data, observations and analyses so far indicated NASA had turned the corner in its external tank redesign work, Shannon said "that's the feeling we have right now. But we'll see what we find tomorrow."
Shuttle program manager Wayne Hale, meanwhile, gave engineers permission to press ahead with work to attach the next external tank to a set of boosters at the Kennedy Space Center. Assuming no major surprises with Discovery, NASA hopes to launch the shuttle Atlantis at the end of August on the next space station assembly mission.
The commander of that flight, veteran astronaut Brent Jett, told CBS News today he was encouraged by the preliminary assessment of Discovery and its tank.
"Seeing the successful launch, getting back to space, is great for the country and to have it happen on Independence Day made it really great," said Jett, who would serve as commander of a rescue flight should one be needed. "The vehicle looks pretty clean so far. So now we can focus on getting ready for (the next mission) STS-115 in just a little over a month and a half.
Asked how difficult NASA might find it to pull off such a fast turnaround, Jett agreed "it'll be real tough for the program, we haven't done that in a while."
"We used to turn missions around pretty quick, but I think with the extra scrutiny and the extra analysis we put into each mission now, it's going to be a tough job. They've got the plans in place, they think they can do it. Our FRR (flight readiness review) happens, I think, about two weeks after landing. So it's going to be tight."