To date, Marshall engineers have identified several key areas on the tank where foam can shed during launch. Those include:
A "bipod" area where the orbiter attaches to the tank.
A joint or flange area where the liquid oxygen and liquid hydrogen tanks come together.
Several cable and fuel lines that run along the exterior of the tank.
Foam is applied by hand to the cable and fuel line areas at Marshall's Michoud Assembly Facility, the Lockheed Martin-run external tank factory near New Orleans. Columbia accident investigators say the hand-application process is a possible reason the foam shed from the tank. Automated sprays can be better controlled, aerospace experts say.
Changes to fuel lines and foam areas alter the shape of the tank and its flight profile. Engineers have to prove the tank will be stable in flight or it could shake apart, Otte said.
"If we change the mold line of the tank, then we have to go assess what those changes mean aerodynamically," Otte said.
The assessment work is being done primarily at Johnson Space Center with computer simulations through a process called computational fluid dynamics. Basically a computerized wind tunnel simulation, computational fluid dynamics estimates how an object, such as the external tank, will behave in flight.
In the next five years, the external tank may be changed to remove many of the troublesome exterior elements, such as fuel lines and cables, that can lose foam, Otte said.
"We plan to move those inside the tank," he said. "We do have a long-term plan we are putting together that you would begin to notice a difference on the tank."
At present, Otte said, there is no way the shuttle can do without foam, because the propellants must be kept at below minus-300 degrees Fahrenheit. "You've got to have foam, and you've got to have insulation," he said. "What we can do is make it better."