Bill Readdy, NASA's associate administrator for spaceflight, said recertifying the external tank was at the root of a decision Thursday by senior managers to delay the first post-Columbia mission from September to March 2005.

"Return to flight has always been driven by fixing the tank because the tank was clearly the cause of the (Columbia) accident," Readdy told reporters. "An awful lot of what has been going on has been analysis of debris transport, how things that come off the tank (and are) transported from some area of the tank and then impinge on some other portion of the space shuttle stack.

A large piece of foam insulation broke away from the shuttle Columbia's external tank during launch last year and struck the leading edge of the ship's left wing. The impact blasted a large hole in the reinforced carbon carbon material used to protect the leading edge from entry temperatures of 3,000 degrees. During the descent to Earth 16 days later, hot gas burned its way into the left wing and triggered the shuttle's destruction.

At the time of Columbia's launch, engineers did not believe large pieces of foam from the external tank could strike the orbiter, relying on a debris transport model that indicated such debris would peel away and fall down along the tank without entering the complex airflow between the shuttle and the tank.

They now believe the mechanism responsible for foam shedding causes pieces of insulation to be blown away from the surface of the tank, out into the main airflow where it quickly decelerates. If the debris comes from an area toward the top of the tank, the shuttle can ram into it at a high relative velocity.

Prior to and during Columbia's flight, "we were able to run 400 or so detailed runs of simulations using computational fluid dynamics models," Readdy said. "Since that time, with the improved models and improved computing power we now have, we have been able to run millions of runs, literally, and characterized the debris transport mechanism.

"And what that has done has caused us to look at how the insulation is applied to the tank, where it's applied, where it had been coming off. We were able to review an awful lot of the photography we had of the external tank previously and as a result, levied new requirements on return to flight for the external tank and recertification of the tank for flight. So the external tank has certainly been a pacing item."

At the time of Columbia's destruction, NASA engineers believed a phenomenon known as cryopumping was responsible for foam shedding. When the external tank is fueled for launch, air trapped in voids in the insulation or near the skin of the tank can turn into a liquid. As the shuttle rockets away, aerodynamic heating can cause that trapped liquid to turn back into a gas. The pressure generated by that phase transition, it was believed, could blow overlying pieces of foam away from the tank.

The Columbia Accident Investigation Board concluded such cryopumping alone could not explain the separation of the suitcase-size chunk of debris that doomed Columbia. But Readdy said today additional testing shows a different type of cryopumping can, in fact, cause such shedding.

"We've found out that the bolts and the nuts being applied to actually construct the different areas of the tank ... before you put the insulation on, that any kind of gap in there might be an opportunity for liquid nitrogen or liquid air to form," he said. "And what happens is, during the ascent environment, when the shock waves form on the external tank, aerodynamic heating and friction occurs and as a result, even trapped air kind of expands."

The expansion of that trapped air "imparts a velocity to that particular piece that causes large pieces to come off and instead of (peeling) away from the tank, actually being pushed away from the tank due to that gas pressure behind it," Readdy said. "That is really the root cause we've been able to discover here.

"And part of the new design is to change the bolt configuration, to actually close those areas out so that there is no opportunity for the liquid nitrogen or liquid air to form and close out a whole number of other areas. The other thing is characterizing the condition of the foam application more carefully so we have a much more controlled environment, not only in terms of the humidity that we're able to apply this foam, but the rate at which the foam must be applied, the surfaces near it, a whole number of other factors."

The foam that doomed Columbia tore away from the so-called left bipod ramp, an aerodynamic wedge of insulation covering the fitting used to attach one of the shuttle's two forward attachment struts. The ramps, in place to prevent ice buildups on the attachment fittings, have been eliminated in favor of electric heaters.

Foam shedding is the major driver behind virtually all of the constraints facing the first post-Columbia flight.

Launch windows are severely constrained by requirements for launch and external tank separation to occur in daylight so engineers can visually inspect the tank and the orbiter. A boom-mounted sensor package is being developed so the astronauts can inspect the underside of the shuttle for signs of damage. Repair techniques are being developed to fix any such damage that might occur.

Based on testing in which foam "bullets" are fired at tile and RCC panels, engineers have concluded the largest allowable debris is .04 pounds. Nothing larger will be permitted from a region 75 degrees to either side of the tank's centerline on the side facing the shuttle.

"It does vary somewhat by location," Readdy said. "Obviously, the further toward the nose of the external tank it is, the smaller the allowable it is. ... It also depends on how far around it is on the external tank. But I think the number is .04 pounds. In terms of foam, though, that's still a fairly large piece."

Playing it safe, NASA managers recently decided to process a second shuttle in parallel to carry out a rescue mission in the event of damage that can't be repaired (see the Feb. 19 update for complete details).

Readdy said the first two post-Columbia missions should be viewed as test flights. If foam shedding is, in fact, reduced to the desired level, it's possible at least some of the current constraints, including the requirement for daylight launches, could be relaxed.

"It's a constraint we hope we won't have to live with for the life of the program, that being daylight launch, daylight external tank separation," Readdy said. "Because we're also developing radar techniques that may allow us to detect any kind of debris that might be separated during ascent instead of using visual cameras. ... We're also looking at other sensors we might use to document the condition of the external tank. So perhaps the daylight launch, daylight external tank separation may not be permanent constraints. ... But clearly they are here for the next two flights."