Scarred by the ravages of Hurricane Katrina and subsequent repairs, Endeavour's external fuel tank went through years of stress before ever encountering the trials of a space shuttle launch.

Lighter shades of foam on Endeavour's fuel tank indicate fresh repairs to damage from Hurricane Katrina. Credit: Stephen Clark/Spaceflight Now

When Hurricane Katrina struck the Gulf Coast, the bullet-shaped external tank was inside an assembly building at the Lockheed Martin Corp. facility in New Orleans where every shuttle fuel tank is assembled.

Numbered ET-122, the fuel tank is now on the verge of launch with the shuttle Endeavour. Its familiar butterscotch-colored foam is a little darker shade than normal, with lighter smudges indicating fresh repairs to damage suffered when Hurricane Katrina barreled through New Orleans in August 2005.

"It looks a little more suntanned than other tanks because the UV makes the orange foam insulation look a little darker," said Mike Moses, the shuttle integration manager at the Kennedy Space Center. "It's a darker tank and it has some repair spots on it, but it's a good tank and we're looking forward to flying it."

The tank was positioned vertically inside the Michoud assembly building, which lost part of its roof when the deadly hurricane raced onshore. Concrete particles and rain water fell on the tank, leaving behind pitted foam and minor structural damage.

The external tank foam keeps ice from forming when the reservoir is filled with a half-million gallons of liquid oxygen and liquid hydrogen propellants as cold as -423 degrees Fahrenheit.

Engineers fear ice could fall off the tank during the vibrations of launch, striking the sensitive heat shield tiles covering the bottom side of the shuttle orbiter. But foam poses its own risk, triggering the Columbia accident in 2003 when a piece of insulation struck the shuttle's wing and led to the ship's destruction during re-entry.

Wary of any foam damage in the wake of the Columbia disaster, NASA and Lockheed Martin initially moved ET-122 off the production floor because the hurricane damage looked rough.

"The first glance in the Vertical Assembly Building was the damage was to the point where we wanted to put it aside and work on it at a later date," said Jeff Pilet, Lockheed Martin's chief engineer for the external tank.

Once the factory was back up and running, Lockheed Martin continued building external tanks that would fly on the final shuttle missions. Eventually, their attention returned to ET-122.

A more detailed assessment of the tank showed it could be repaired, and officials decided to make it flight-worthy again in hopes of one day launching it with the space shuttle.

But with the fuel tank assembly line working furiously to churn out equipment for a busy shuttle launch manifest over the last five years, it took a while to refurbish ET-122 for its turn to fly.

Photo of Endeavour's external tank inside the test and checkout building at the Michoud Assembly Facility in New Orleans. Credit: Lockheed Martin

"We had other tanks in the flow we needed to move ahead of this to keep them going," Pilet said.

Originally delivered to NASA in 2002, the tank was returned to Lockheed Martin after the Columbia accident for modifications ordered in the wake of the shuttle disaster. The tank was receiving those alterations when Katrina struck.

"We had areas where we obviously had damage to address," Pilet said. "Wherever we had evidence of damage, we did visual and tactile inspections hands-on."

Engineers used thermal scans and X-rays to look for signs of deeper damage.

"All of the damaged foam we detected was removed and repaired," Pilet said.

Luckily for NASA and Lockheed Martin, the tank's metallic shell was not seriously damaged. The only structural repair required was on the back side of the intertank, where the liquid hydrogen and liquid oxygen propellant tanks meet.

Most of the foam damage was also on the back side of the tank, away from the shuttle orbiter's sensitive heat shield. The lightweight orange foam absorbed the bulk of the damage from falling concrete chips, according to engineers.

"On the back side of the intertank, we did have one area in between two stringers where we had a dent in the skin," Pilet said. "We a doubler repair in. It's basically a piece of aluminum that sandwiches in to carry the loads."

The damage area was about 4 inches wide and 14 inches long.

"The fact that the primer wasn't scratched was a very big indicator to us that there was no impact directly to the metal by any of the chips," Pilet said.

Lockheed Martin says there were a total of 103 repairs made to the external tank, including a 20-foot-long strip of foam that was completely removed and replaced due to a large number of small scrapes.

Once the tank was back in flight condition, engineers retested all of its systems and foam to make sure it was safe.

"The takeaway from all this is we have high confidence with the repairs we made to to the (liquid oxygen) tank and that the (foam) is going to remain adhered and not generate debris," Pilet said.

Ken Wilson, NASA's external tank manager, said he is just as confident in the tank's safety as the contractor.

ET-122 "came through with flying colors," Moses said.

Credit: Lockheed Martin

Officials added an insignia to a fuel tank access door commemorating its unique history. It shows a space shuttle launching through the eye of a hurricane.

Because Endeavour's fuel tank is the oldest ever flown, it doesn't have a safety upgrade NASA added to newer external tanks. Support brackets on ET-122's liquid oxygen feedline are made of foam-covered aluminum, while newer tanks use less foam over titanium brackets.

NASA changed the bracket design after foam fell from the brackets on earlier flights in a phenomenon known as cryo pumping. But analysis shows any foam that does come from the brackets will fall away once Endeavour reaches the thin layers of the upper atmosphere, where the threat of heat shield damage diminishes.

"When you have a super cold substrate it will pull down air and liquify it into a cavity," Pilet said. "When you're in flight, that liquid changes state back to air. When that happens, you have a rapid change in volume and the (foam) can come off."

The good news, Pilet said, is engineers understand the physics of cryo pumping and know they always occur late in the launch sequence.

Due to the tank's age, engineers replaced sensors and other sensitive components. Although external tank foam is only certified for six years, tests show the material is robust enough to survive longer.

"Other than these areas, would expect the performance in general should be just like what we would see on a normal tank," Pilet said.