'Bullet-shaped' fuel tank brings Nasa closer to shuttle launch

A redesigned space shuttle fuel tank arrived at Nasa's main assembly hangar in what managers said was a first tangible sign that the two-year effort to return the fleet to flight following the Columbia disaster is nearing its end.

"It is the safest tank we've ever made," said Nasa's external tank programme manager Sandy Coleman, who watched as the 47-metre long, "bullet-shaped" vessel was removed from a barge and rolled into the Vehicle Assembly Building at Cape Canaveral's Kennedy Space Centre.

The new tank is key to preventing a repeat of the February 1, 2003, Columbia disaster. Seven astronauts were killed when the ship broke up as it re-entered the Earth's atmosphere because of a hole in Columbia's wing caused by a piece of falling insulation during lift-off.

Among other recommendations, the panel that investigated the accident told Nasa to redesign the tank to prevent foam insulation from falling and striking the orbiter.

A slew of testing, including wind tunnel experiments, structural analyses and computer modelling, give officials confidence that foam chunks no larger than 14 grammes about the size of two marshmallows will fall from the tank during a shuttle launch, doing no harm.

"Actually, we're hoping for about 5 grammes," added Coleman.

The arrival of the new tank is a major milestone in Nasa's effort to return the fleet to flight with the launch of shuttle Discovery in late May or early June.

"We still have a lot of work to do," said Michael Wetmore, director of shuttle processing at the Kennedy Space Centre, "but today we're feeling good."

The External Tank No 120 no longer has two 76-cm long, ramp-shaped wedges of foam that once nestled around metal struts connecting the tank to the orbiter.

The tank, which is jettisoned after 8-1/2 minutes of flight, holds cryogenic propellants that can cause ice buildups on the outside of the tank. The foam was intended to prevent ice from forming so pieces would not break off and hit the shuttle during launch.

On Columbia's doomed flight, it was not ice that proved a hazard, but the foam itself. Investigators determined the tank's left wedge of foam slid off at liftoff and crashed into the shuttle's left wing. The hole it gouged was never detected before the shuttle re-entered the atmosphere for landing and disintegrated during the high-speed glide back to Earth.

Instead of foam wedges, the new tank has electric heaters on the metal skin to prevent ice forming. The tank also carries a video camera for filming during lift-off so ground control teams can spot damage.

Nasa plans to test the new fuel tank once Discovery reaches the launch pad in March. A practice countdown will refresh the shuttle launch team after a 27-month hiatus in flights, test new software and procedures, and check that electrical and plumbing systems needed for launch work smoothly.

The team will be particularly looking for any unexpected changes introduced into the equipment or systems as a result of the post-Columbia modifications, said Michael Kostelnik, Nasa's deputy associate administrator for the shuttle and space station programs.

"It's the unknowns that we have to watch out for," Kostelnik said.