External fuel tank foam losses not in danger zone
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 4, 2006
The shuttle Discovery's external tank lost only small pieces of foam insulation during launch today, and those were well after the period when aerodynamic effects can lead to dangerous impacts with the orbiter, officials said late today.
But it will take several days to complete a detailed analysis of still photos, videotape, film, radar data and wing leading edge sensor data and to confirm Discovery came through its ground-shaking July Fourth launching in good health. But the preliminary reports were positive.
"I think the tank performed very, very well indeed," shuttle program manager Wayne Hale told reporters after an initial look at ascent video. "I'm very pleased. As opposed to where we were last year, we saw nothing that gives us any kind of concern about the health of the crew or the vehicle or any pause to think that we wouldn't be safe to fly the next tank."
For foam insulation to cause damage to the shuttle's heat shield, engineers believe it must be released in the dense, lower atmosphere, which causes the lightweight foam to quickly decelerate. The shuttle then can run into it at a high relative velocity, damaging heat shield tiles or critical wing leading edge panels.
During Columbia's ill-fated launch in January 2003, a 1.67-pound chunk of foam broke away from a so-called bipod ramp on the external tank and slammed into a wing leading edge panel at a relative velocity of more than 530 mph, blasting a catastrophic hole in the wing.
During Discovery's launching last July on the first post-Columbia flight, a one-pound piece of foam fell away from a long wind deflector. It missed the shuttle, but had it been released earlier it might have caused serious damage.
NASA ultimately removed the wind deflectors, but left in place so-called ice-frost ramp foam covering 34 brackets that hold external pressurization lines and a cable tray to the tank. That foam is considered high risk, primarily because engineers do not yet have a good understanding of what can cause the foam to break away. They are especially eager to find out how much foam might have been released from Discovery's tank and, just as important, when it was released.
Engineers now believe foam debris poses a serious threat only in the first two minutes and 15 seconds of flight. After that, the shuttle is above the sensible atmosphere and debris released at that point or beyond would not decelerate enough to cause a dangerously high relative velocity impact.
NASA has spent the past three years working to minimize foam shedding and to ensure that any insulation that is released is smaller than the critical threshold of .25 pounds that could cause catastrophic damage. But until now, a major unknown has been the timing of foam releases.
"We put the tank under a microscope this time," Hale said. "We have lots and lots of data coming in, imagery coming in. As you know, we're very concerned about foam coming off the tank. We've worked very hard to eliminate, in so far as possible, the major losses of foam off the tank. The tank performed very well.
"In particular, I'm pleased that it performed well in the time period early in the flight where we're traveling at supersonic speeds in the lower atmosphere."
Discovery's flight, he said, will provide valuable data to engineers struggling to develop a better understanding of the foam and how it behaves during launch.
"Because we have the ET under a microscope, we're looking at every little thing," he said. "And I want you to know it did not perform flawlessly in the sense that we still lost some foam off the tank. And we told you that as going to happen, we knew that was going to happen and the really good news is it happened late. That helps us a tremendous amount.
"Previously, when we were looking at the photographs that were taken at external tank separation (from the shuttle), when we're trying to build these statistical models which go into our risk analysis, we knew that pieces came off the tank but we didn't know when they came off the tank.
"And so, we had to calculate our worst-case probabilities based on those pieces coming off at the worst time. Based on what we saw today, we are losing some pieces off the tank, that's number one. Number two is, with one possible exception we are pretty clear that they are below the allowable masses, or below the mass size that we worry about. And number three, in every case they happen considerably later than the time that we worry about and that is to say pretty near the vacuum of space. So from that standpoint, we got an excellent report on the tank."
In addition, video from the external tank camera showed the pressurization lines and cable tray stood up to the aerodynamic rigors of launch without the protection of the protuberance air-load - PAL - ramp wind deflectors that were removed after Discovery's last flight.
"We saw no vibration or any indication of abnormal loads on the cable tray or the pressurization line protuberances, which as you know were of some interest to us because we did remove the PAL ramp, the wind deflector as it were," Hale said. "So that all performed in an outstanding manner."
The astronauts went to bed around 10 p.m. after preparing still more pictures of the tank for overnight downlink to mission control. Still photos and video shot by cameras in an umbilical cavity in the belly of the shuttle should provide sharp views of the tank as it separated from the shuttle to help engineers map out exactly where foam losses occurred.
"I just wanted to tell you that for both NASA and the entire country, this was a spectacular Fourth of July present for us to have such a beautiful launch," mission control radioed the crew at bed time. "We feel the vehicle is in great shape."