NASA's engineers knew that the foam insulation around the space shuttle's giant external fuel tank was a problem. They were particularly worried about any hand-sprayed sections, like the foam that broke off and fatally damaged the Columbia in 2003.

But after spending millions of dollars, reviewing foam losses from dozens of flights and conducting five million computer simulations, engineers decided that a hand-sprayed foam ramp on the tank was an ''allowable,'' a risk that was too unlikely to pose a danger to the shuttle.

''We had enough data that showed we had had very few problems with the PAL ramp,'' said William W. Parsons, manager of the shuttle program.

But Mr. Parsons was speaking to reporters on Wednesday, after announcing that a skateboard-size slab of insulating foam, just shy of a pound, had ripped away from the fuel tank two minutes into the launching of the Discovery on Tuesday.

The falling foam did not hit or damage the Discovery, but its loss has caused the suspension of further launchings and highlighted shortfalls in testing at the National Aeronautics and Space Administration.

Aldo J. Bordano, a retired NASA aerosciences division chief who was on a peer-review panel that studied the agency's analysis of the external tank and foam, said the panel believed that there should have been more physical testing of the external tank's vulnerable components before flying.

''We would have liked more hardware testing,'' Mr. Bordano said yesterday in an interview, ''but that would have added another year to the delay. Ideally, you would want to be able to take a tank and actually put it through some kind of testing like what happens through the ascent phase and see if it hangs together.''

For the moment, he said, everyone should be patient.

''Everyone's surprised right now,'' Mr. Bordano said, ''and they need to put together some facts before conclusions are drawn.''

For several months, officials had been saying there was almost no chance the rigors of liftoff could dislodge foam fragments so big and so potentially catastrophic.

And while the independent monitor of the rebirth of the shuttle program, the Return to Flight Task Group, withheld endorsing the agency's revamped approach to securing foam in its final report in late June, it did not conclude that the shuttle was unsafe as a result.

It did conclude, however, that risk from ejected debris could not be eliminated, stating, ''The hard fact of the matter is that the external tank will always shed debris, perhaps even pieces large enough to do significant damage to the orbiter.''

During the Discovery's ascent on Tuesday, the foam broke off when the spacecraft was largely clear of the dense atmosphere and so floated away from the shuttle instead of being flung into it by torrents of air, as happened in 2003 when a 1.67-pound piece pierced the insulating skin of the Columbia, leading to its fiery destruction days later on re-entry.

Yesterday, experts inside NASA and out began reviewing the handling of the Discovery's external fuel tank, particularly the 37-foot, 19-pound PAL ramp running along a flank of the blimp-shaped repository for liquid hydrogen and oxygen.

Paul Hill, the lead flight director for the current mission, said yesterday at a news conference that the area where the foam ripped away had not been considered a problem spot.

''We have now found out differently,'' Mr. Hill said, ''and we're going to fix that piece as well,'' he said.

After the loss of the Columbia, a small army of engineers was deployed to focus on the external tank's crust of insulating foam, and particularly on those parts sprayed on by hand.

The engineers broke the problem down into its component parts: what were possible sources of debris, how would it fly once it broke free, and, if that trajectory caused it to collide with the spacecraft, what would the damage be.

Around 100 million computer simulations were run to assess how various pieces of foam or other debris, like ice, might fly and then strike the shuttle.

More than five million of those simulations looked specifically at debris from the liquid hydrogen protuberance airload ramp, better known around NASA by the acronym LH2 PAL ramp.

The goal, according to NASA documents, was a list of ''allowables,'' scenarios that could be tolerated because an ejected piece of foam either would miss the spacecraft entirely, be too small to pose a risk, or tend to strike some part of the fuselage that was strong enough to withstand the impact.

Now NASA research hubs around the country are girding for yet another grueling round of tests.

Angel M. Otero, the chief of the space operations division at NASA's Glenn Research Center in Cleveland, acknowledged that computer simulations or wind tunnel tests could go only so far. The only way to know how a complicated vehicle will work is to launch it into the world's biggest wind tunnel: the atmosphere itself.

''This was a test flight,'' Mr. Otero said. ''We needed to go back to flying to see if all the fixes we implemented did what they needed to do.''

The Glenn center first conducted wind-tunnel tests on the foam strip in 1983, after the only other shuttle flight in which NASA engineers found that some had fallen off.

In the past two years, the center conducted a battery of fresh wind tunnel tests, including tests to measure airflow around the PAL ramp, which is designed to deflect air away from a long channel containing cables and pressurized pipes so that turbulence does not endanger that vital hardware.

The early wind tests were meant to assess whether the ramp was needed at all. The answer was yes in 1983, and yes again in the last two years, NASA officials said.

There are few clues to why the section broke free. The external tank used on this flight of the Discovery was one that had been assembled and sprayed before the loss of the Columbia and the flurry of reviews and changed practices, said June Malone, a NASA spokeswoman.

But when a 10-foot section at the forward end of the PAL ramp was removed to fix a component underneath, tests of the foam showed that it had no bubbles or other potentially troublesome gaps. When that gap was filled in with fresh foam, the spraying was videotaped and each sprayer was observed by someone to watch for glitches, Ms. Malone said.

The part that flew off the tank on Tuesday broke free about 15 feet farther down the strip from the resprayed area, indicating there was little likelihood that the repair loosened the piece, she said.

New spraying methods have been developed for building the ramps for future fuel tanks, but NASA officials decided they were not vital to insure safety for this flight.

Chart/Photo: ''A Source Of Foam Debris''
NASA officials said cameras captured additional smaller pieces of foam breaking off the external tank during ascent, from a point near where a larger piece of debris originated. One of the smaller pieces may have struck the wing but appears not to have significantly damaged it.

The 37-foot-long protuberance air load (PAL) ramp protects the cable trays and pressure lines from air flow that originates near the solid rocket boosters and flows around the tank.

Unlike the foam on most of the external tank, the ramp is hand-sprayed, then trimmed into shape.

Smaller site of missing foam: 7 inches wide
Larger site of missing foam: 24 to 33 inches wide

(Photograph from NASA)

(Source by NASA)