By KEAY DAVIDSON
San Francisco Chronicle
July 29, 2005
In the wake of the shuttle Columbia tragedy in 2003, NASA officials redesigned parts of the external tank to minimize the risk of falling foam or ice on future flights.
The redesign targets included the upper part of the PAL ramp - PAL stands for protuberance air load - which runs vertically up the side of the external tank.
The agency even considered "elimination of the (PAL) ramps," according to a public NASA document from August 2004.
But Tuesday, as Discovery launched into orbit, becoming the first shuttle flight in more than two years, its external fuel tank lost a huge piece of foam in the vicinity of the PAL ramp. NASA acknowledged that failure Wednesday; Thursday, officials also admitted that a few smaller pieces of foam had been lost from a nearby location on the external tank.
In early 2003, falling foam damaged a wing on the shuttle Columbia during launch; later, the ship and its seven crew members burned up during re-entry to Earth's atmosphere.
The 2004 NASA document said the agency "has re-evaluated the existing design of the tank's protuberance air load ramps - known as PAL ramps - because the ramps, which consist of thick, manually sprayed layers of foam, could, if liberated, become a source of debris."
The PAL ramps "are designed to prevent unsteady air flow" - in effect, violent atmospheric turbulence, which can damage or destroy aircraft components - from occurring on sensitive components of the external tank, notes the NASA document, a "NASA Facts" informational memo issued by NASA's Marshall Space Flight Center in Huntsville, Ala.
Among "three redesign options" under consideration were "elimination of the ramps," shrinking them to one-third their present size, or guarding the sensitive components by adding a protective "fence" that would lessen the "unsteady air flow," says the document.
But in the end, as NASA struggled to get the shuttle fleet back into space, the agency elected not to make a significant redesign of the PAL ramp - - at least for the near future.
That was a big mistake, as even NASA now admits.
The PAL ramp is "the big offender we've got to solve before the next flight," deputy shuttle program manager Wayne Hale acknowledged during a news conference Thursday.
During the redesign of the external tank, NASA officials "talked about the PAL ramp at great length because it was a manually applied (spray-on) piece of foam," Hale explained. Because it's applied manually, by imperfect humans, the spraying process might introduce "the kinds of variations" in foam thickness that can make it vulnerable in certain spots, he said.
He did not explain in detail what those vulnerabilities could have been. However, in the past, NASA experts have worried that air may be trapped inside "air pockets" within the foam. After launch, when the shuttle soars to tens of miles above Earth en route to orbit, the air pressure plunges close to zero - while the air pressure within the air pocket remains equal to sea level air pressure. Result: The air pocket bursts, blasting foam away from the external tank.
Hence the necessity of applying the foam as smoothly and evenly as possible, to prevent air pockets from forming in the first place.
Based on this week's events, Hale admitted, "clearly what we decided for the first flight, we didn't do it right."
Hale also reported that even more foam flew off the external tank than NASA realized Wednesday, when it reported the loss of a piece almost 3 feet long adjacent to the PAL ramp.
Now, it turns out, near the PAL ramp, three smaller pieces of foam also separated from the external tank. Those pieces might have been up to 7 inches long.
The big chunk and the three smaller chunks broke off about 20 seconds apart. It's unclear whether they broke off for similar reasons, Hale said.
"Whether they're related, (caused by) some common phenomenon - we don't know the answer to that," Hale said. "Yet the smart folks (at NASA) are off working on that. It will take a little while, but we will get to the root of the problem, and we will fix it."
NASA experts are still trying to determine whether any pieces of falling foam actually hit the shuttle. Based on supercomputer simulations at NASA-Ames, most experts believe that at worst, a small piece of foam hit a shuttle wing with about one-tenth as much force as needed to damage it, Hale said. However, not all experts agree on whether that actually happened.
Fortunately, the shuttle appears to have sustained no damage whatsoever that appears threatening, he stressed.
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