Newsday July 29, 2005
NASA's foam conundrum
Repeat of shedding damage has engineers stumped, as outcome could dictate space program's future
By Bryn Nelson
The unexpected loss of foam insulation from the shuttle Discovery's external fuel tank has proven a big embarrassment for NASA officials who had expressed confidence in a redesign intended to prevent a repeat of the Columbia catastrophe.
Now that videos have documented an eerily similar foam shedding -- albeit one that apparently left no significant damage -- one question looms largest: How did this happen?
NASA officials are scrambling to find an answer. But the inherent design requirements of the shuttle and the simple laws of physics have conspired to create an engineering headache whose resolution may direct the future of the human space flight program.
Harry Lambright, a space policy expert at the Maxwell School of Syracuse University, said the question is now one of policy: how much will it cost to find a resolution, namely bringing the risk to future flights below an acceptable threshold?
Spending significantly more money on a shuttle program slated to retire in five years versus speeding up the arrival of the next generation will be no easy trade-off, especially with NASA still trying to recover from the Columbia disaster.
"You perform, and that makes people feel you can do it, you can do more," Lambright said. "You can't recover technically and politically until you've probably launched a couple of these shuttles cleanly. That's the real problem. You need a success, and you've got to repeat it."
John Pike, a veteran space policy analyst and director of GlobalSecurity.org, said NASA has gone through contortions to convince itself that it can solve the foam shedding problem while acknowledging that it may never fully understand the underlying phenomenon.
Nevertheless, he sees a relatively short downtime for NASA, given the looming retirement of the fleet, the scarcity of money for another extensive overhaul, and the need for NASA's human space program to stay in the public eye to ensure its continued support.
"I think there is an enormous pressure to get the shuttle flying again and keep it flying," he said.
During a briefing yesterday, deputy shuttle program director Wayne Hale instead characterized the big question as: "Do you make dumb decisions or inappropriate choices" because of scheduling pressures?
Hale defended NASA's "new culture," asserting the agency's decision-makers would not allow themselves to make an inappropriate decision in the face of such pressures.
Compounding the agency's task are the intricacies of shuttle flight itself. At launch time, the bright orange, 15-story external fuel tank contains 535,000 gallons of super-cooled liquid hydrogen and oxygen -- the fuel needed to reach orbit. The liquid oxygen in the tank's upper compartment is chilled to minus 298 degrees, while the hydrogen in the lower compartment reaches minus 423 degrees.
The one-inch coating of orange insulation prevents condensation from forming and freezing on the aluminum skin of the tank, reducing the risk of ice striking the shuttle. But it creates a potential problem of its own in the form of potential foam shedding.
In the wake of the Columbia disaster, much of the tank's redesign focused on a V-shaped bipod attaching the tank to the shuttle. Investigators fingered this region as the origin of the 1.67-pound foam piece that hit Columbia's left wing during take-off, which allowed superheated gases to penetrate the vehicle's thermal protection system on its re-entry.
But on the Discovery mission, problems have reared up in completely different areas, including one known as the protuberance air load ramp, or PAL ramp, which consists of a thick hand-sprayed layer of foam and is designed to reduce unsteady airflow beneath the tank's cable trays and pressurization lines. NASA engineers had discussed removing the ramp due to foam shedding concerns but decided against it because of its role in mitigating turbulent airflow.
Nevertheless, the PAL ramp's integrity was tested with two "non-destructive" techniques, and officials agreed on a fly "as is" determination for the ramps on the first flight's fuel tank.
Now that this confidence has been seriously undermined, new design alternatives could include automating the foam application process, reducing the ramp size by two-thirds or replacing it altogether with a metallic fence-like structure on the backside of the cable tray designed to similarly prevent unsteady airflow.
NASA also revealed a second debris shedding event from an adjacent spot 20 seconds after the loss of the 0.9 pound PAL ramp piece, though tests suggest the three smaller pieces did not significantly damage the orbiter.
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