July Shuttle Flight Is Still a Go
Saturday, April 29, 2006
NASA officials said yesterday they have decided to go ahead with a July space shuttle launch even though engineers have been unable to correct a potentially critical defect in the insulating foam that cloaks the external fuel tank.
The officials said tests had failed to produce a new design that would stop insulation from breaking away from the tank's "ice frost ramps" -- foam-covered metal brackets that hold pressure lines and electrical cables to its side -- during launch.
"The decision was to fly these ice frost ramps as is, knowing that we can expect to have some small foam loss that could pose a risk," shuttle Project Manager N. Wayne Hale said during a televised news conference.
The "worst case," he said, would occur if the foam "comes off at the maximum mass, which would be on the order of 3 to 3 1/2 ounces, comes off at the worst time and follows the worst possible trajectory to the most vulnerable part of the orbiter.
"It would not be what we would like to have," Hale said. "It would cause what we would call critical damage."
The decision to fly was the latest event in a three-year quest to overcome the multiple shortcomings of the external tank since the shuttle Columbia disintegrated over Texas in February 2003. That disaster was caused by foam debris from the tank that breached the orbiter's heat shielding during launch.
The three remaining shuttles have flown once since then, only to be immediately grounded again when the tank last July lost an unacceptably large piece of insulation from a foam ridge known as the Protuberance Air Load, or PAL, ramp. It serves as a windbreak to protect cables and fuel lines from turbulence during launch.
After months of tests, engineers decided earlier this year to fly the shuttle without the PAL ramp, and, after several delays, have scheduled Discovery for liftoff during a "window" that opens July 1. Hale said yesterday the target date remains unchanged.
After the PAL decision, engineers turned to the 34 ice frost ramps, which protect pipes that pressurize the fuel tanks as they empty during launch and the "cable tray" that contains the tank's electrical lines. The ramps have a history of losing bits of insulation during launch.
Ken Welzyn, chief engineer for the external tank, said experts were focusing on four brackets that begin a launch chilled by the liquid hydrogen fuel beneath them, then warm up as the hydrogen is expended, causing the metal to expand and perhaps crack, tossing off pieces of foam during the critical moments after liftoff.
Engineers have conducted wind-tunnel tests of new ice frost ramp designs over the past few months, with no conclusive evidence that the foam on any of the experimental configurations is markedly better than that on the existing ramps.
Finally, at a regularly scheduled meeting Thursday of the Program Requirements Control Board, which evaluates shuttle preparations, program managers came to a near impasse over whether to fly the July mission with unchanged ice frost ramps or wait until the ramp dilemma is resolved.
One source who participated in the meeting said the decision to fly was made after NASA Administrator Michael D. Griffin interceded, pointing out that the July mission was a test flight, and that one big change -- elimination of the PAL ramp -- was enough for one flight. The source was not authorized to discuss the meeting and did not want to be identified.
Hale repeated this rationale at yesterday's news conference, but Griffin was also present, lending his implicit endorsement to the decision even though he spoke only briefly and only in passing about the external tank.
Hale noted that elimination of the PAL ramp would result in "increased aerodynamic loads" on the external tank, including the ice frost ramps, but the upcoming mission would allow engineers to examine these effects in detail.
This was important, added Associate Administrator of Space Operations William Gerstenmaier, because "we're really pushing the state of the art, as far as our testing. At some point, you really need to go to flight."