U.S. Space agency technicians continue their slow, meticulous examination of the space shuttle fleet to eliminate potentially threatening problems that doomed the 2003 flight of Columbia and which appeared on the last mission in July. NASA still hopes to return shuttles to flight next May, but the schedule depends on whether it can minimize the shedding of hard insulating foam from the orbiter's external fuel tank during launch.
Processing a space shuttle for flight has always been methodical. Its complex technology assures that. But now work has slowed to a crawl as NASA tries to understand why foam keeps breaking off the huge external fuel tank during launch, continuing to threaten the shuttle's fragile heat shield of ceramic tiles and reinforced carbon panels.
A briefcase-sized chunk punctured the carbon panels on Columbia's wing, causing the orbiter to burn up in the searing heat of re-entry. A subsequent two-and-a-half-year effort to secure the foam with new spraying procedures failed to prevent a piece from separating on the July flight of Discovery, the first mission since the Columbia accident. Shuttle program manager Wayne Hale says the July incident sent the space agency on a more intense forensic hunt for the cause using sophisticated imaging tools such as x-rays, ultrasound, and radiography to see inside the foam.
"I would just like to point out the great change in attitude that has come through on the shuttle program," said Mr. Hale. "We are looking at things in much more depth, trying to make sure that we have identified all our problems early and solved them completely so that we can fly safely."
The painstaking search has yielded an unprecedented view of nine hairline cracks in the foam of one external tank, two of them on the surface. The NASA engineer supervising the project, John Chapman, says the minute fractures might have been caused by the expansion and contraction of the tank as it was test loaded and drained of its hydrogen and oxygen fuel, which must be kept frigid to maintain its liquid state. The foam acts as insulation to prevent the cold fuel from causing ice buildup on the tank's surface. Mr. Chapman says technicians are trying to determine whether the tiny cracks are related to the foam loss of the earlier missions.
"It's obvious that no crack is inconsequential in the business that we are in, but we are still examining that to find out what causes these cracks, are they indeed detrimental -- we don't know that they are detrimental right now, and how can we possibly prevent them?" he asked.
In the region of the fuel tank where foam broke away in July, the material protects fuel lines and electrical fixtures from being ripped off by aerodynamic forces during launch. Shuttle Manager Hale says NASA is testing improved methods of spraying the foam on the tank manually and robotically to provide a more uniform layer. He hopes that the foam can eventually be eliminated because the pipes and fixtures have been strengthened over the years.
"In the long run we have decided we would like to remove this fairly large piece of foam, just eliminate the hazard that it might cause," said Mr. Hale. "We think we have a very strong case to be ready to take that off by the third flight tank and some folks believe we can accelerate that, potentially even remove it for the return to flight tank."
In addition to the foam investigation, the agency's work is also burdened by a new quirk that occurred during the July mission. Two thin pieces of ceramic-coated fabric that fill the narrow spaces between the shuttle's thermal surface tiles protruded a couple of centimeters. This might seem benign, but in the supersonic speed environment of spacecraft re-entry, engineers say the protrusions would disturb the smooth flow of air around the shuttle's belly, raising the already superheated temperatures.
To avoid the problem on the July mission, two astronauts went on a spacewalk to pluck the gap fillers out. On the ground, technicians have begun tugging at thousands of them from nose to tail on the three remaining shuttles to see if they remain fixed. NASA official Steve Poulos oversees the scrupulous effort.
"At the end of the day [ultimately], we will have checked every gap filler on the vehicle," said Mr. Poulos. "We'll just work our way aft [toward the tail]. We may or may not get 100 percent, every single gap filler, evaluated or removed and replaced when we are ready to go fly, but from a risk perspective, I see it as very low."
It is not zero risk, however, so the task proceeds.
Shuttle officials say it is not this effort, but the work on reducing foam shedding that will determine when the next shuttle launches to resume International Space Station construction. The deliberate pace is a reaction to criticism from Columbia flight accident investigators, who found that NASA had downplayed potential safety problems to meet a rigid flight schedule.
Wayne Hale puts it this way.
"The technical progress is going to drive the schedule, not the other way around," he noted.