Debate still rages about shuttle fuel tank foam risks
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: June 16, 2006
NASA managers met at the Kennedy Space Center today to formally review the shuttle Discovery's flight processing and its readiness for launch as early as July 1. One major item on the agenda is a discussion of the threat posed by foam making up so-called ice-frost ramps on the shuttle's external tank.
The engineering community believes it should be classified as "probable/catastrophic," meaning that over the remaining 17-flight life of the shuttle program, it's probable that foam will break away from the ice-frost ramps with catastrophic results.
Other engineers disagree, arguing a designation of probable/catastrophic overstates the actual threat. All agree the ice-frost ramps should be modified to minimize foam shedding.
Shuttle program manager Wayne Hale believes it's acceptable to launch a few missions while the ramps are redesigned, arguing the threat on any given launch is comparable to what the program already accepts for other critical systems.
However it plays out, a probable/catastrophic classification is problematic in the post-Columbia environment, especially when measured against the recommendations of the Columbia Accident Investigation Board.
Lindsey introduced his crew to the flight readiness review participants early today before flying back to the Johnson Space Center for final preparations. The astronauts just finished three days of launch site training, including a dress-rehearsal countdown Thursday.
Speaking to reporters before departing, Lindsey said the debate over how to technically classify the ice-frost ramps in a risk matrix would not change his belief that Discovery is ready for flight.
"Yesterday they had a meeting about it and I'm sure they're going to talk about it today," he said. "I don't know exactly where it's going to be classified. I think one of the things, though, that the managers especially wanted to do was make sure that everybody in the program recognizes this is our top risk. And I think they wanted to put it up there (in the risk matrix) to annunciate that hey, this thing is something we have to fix and this thing is something we really have to watch. They're kind of (breaking it out) to separate it from all the other risks that are out there to make sure everybody's aware of that."
The risk matrix is simply a box three rectangles across and four down. The vertical axis lists the likelihood of an event - improbable, remote, infrequent and probable - and the horizontal axis lists the severity of a failure, from marginal to critical and finally, catastrophic. The matrix covers the range from "improbable/marginal" to "probable/catastrophic."
"In terms of the actual risk of flying with the ice-frost ramp, just because you put it in that category doesn't change the risk at all," Lindsey said. "Technically, it's the same situation we had before."
Even so, he said, "there's a significant amount of debate in the community as to where that (risk) really should be. There's a group that thinks it should be up there (probable/catastrophic), there's a group that thinks it should be one block down, a little less amount of risk. For us, with the ice frost ramps, we've flown with them, we've seen pieces come of off them but not large pieces that would cause damage to the orbiter."
In a worst-case scenario, however, catastrophic damage could, in fact, occur. So why not fix the problem before Discovery flies?
In Hale's view, NASA already is accepting a fair amount of risk based on an earlier decision to remove so-called protuberance air-load - PAL - ramps from the tank in the wake of a major foam shedding incident during Discovery's last flight a year ago.
The PAL ramps were in place to shield external pressurization lines and a critical cable tray from aerodynamic buffeting as the shuttle climbs out of the dense lower atmosphere. The pressurization lines are held in place by 34 brackets that are covered in foam insulation - ice-frost ramps - to prevent potentially dangerous ice formation before launch.
Wind tunnel testing and computer modeling have convinced managers the tank and its external lines and fittings are tough enough to stand up to the stresses of launch without the PAL ramps. But NASA does not yet have a redesign in place for the ice-frost ramps, which now represent the most potentially dangerous concentrations of foam on the tank.
Engineers had hoped to implement a redesign before Discovery's flight, but wind tunnel testing showed the proposed change fared worse than the old design. As a result, NASA Administrator Michael Griffin, Hale and other senior agency managers agreed to stick with the old design for the next few shuttle flights while another redesign is perfected.
After an extensive debris verification review May 31, Hale said he agreed the ice-frost ramp foam should be listed in the risk matrix as probable/catastrophic. But he said he believes NASA is justified in pressing ahead with near-term shuttle flights while engineers devise a bracket redesign that eventually will allow them to remove the ice/frost ramp foam altogether. While the IFR foam represents a clear long-term threat, the risk on any given flight is in line with dangers posed by other systems.
Lindsey agreed with that assessment and said he agreed with Hale that removing the PAL ramps - the biggest change ever made to the tank's aerodynamics - was enough for one mission.
"There's this thing in the flight test world and the engineering world called the law of unintended consequences," Lindsey said. "When you make a change, you often influence other things. Every time we make a big change, we're taking that risk and you can't drive it to zero.
"I'm pretty content we've done everything we can in the wind tunnel with these ice-frost ramps as well as the PAL ramp changes. ... We can't know more about these ice-frost ramps and instrument them and get data on how to change them unless we go to flight test. And I think that's where we are right now."
The flight readiness review is not expected to be wrapped up until midday Saturday. A news conference is planned to announce the results and, presumably, an official launch date. The current target is July 1 at 3:48:15 p.m.
"They have a long day ahead of them," Lindsey told reporters today. "We, like you, are looking forward to hearing the results of that review."