2 DOOMED SHUTTLES STAR-CROSSED COLUMBIA MAKES ITS LAST JOURNEY.

Byline: Charles F. Bostwick and James Nash Staff Writers

PALMDALE - Twenty-four-year-old Columbia was America's oldest space shuttle, destroyed on its 28th flight into space - overall, the 113th shuttle mission.

This was Columbia's first flight since it spent 17 months at U.S. Air Force Plant 42 for more than $90 million worth of inspections and modifications, which went 10 months longer than expected.

The modifications included lighter-weight and supposedly more durable thermal-protection covering, which shields the craft from the tremendous heat from the friction of re-entering the atmosphere at 16,000 mph.

Other shuttles earlier got similar upgrades, which replaced some areas of the heat-resistant silica ``tiles'' that cover much of the shuttles' exterior.

Columbia's last mission, more than 3 1/2 years ago, also suffered a potentially devastating problem: Five seconds after launch, two of the six main engine controllers went off-line. The electronic controllers automatically shut down if there is a power fluctuation.

Because of the provision for backup controllers, the July 1999 launch continued uneventfully.

But the incident was deemed serious enough by National Aeronautics and Space Administration to warrant forming an investigating panel to look into the problem. The investigating panel determined that a damaged wire had arced to a screw-head burr, tripping a circuit breaker.

The area where the wire located is normally kept covered and records indicated the cover had not been removed since the last time Columbia was in Palmdale for modifications four years earlier.

Noting that 20 percent of the shuttles' wiring - which totals more than 200 miles long - is in hard-to-access areas, the investigating panel recommended the wiring be inspected during the routine overhauls.

When Columbia arrived in Palmdale for its third overhaul and modification project in September 1999, about 350 Boeing engineers and technicians took apart the spacecraft and examined structural components for fatigue, corrosion or broken rivets or welds, as is done periodically to all the shuttles.

Besides the areas of new thermal protection, the modifications included better protection for its radiators against space debris, and touch-screen electronic instruments to replace the 1970s-era video screens and mechanical gauges.

The work was supposed to cost about $70 million and take 10 months, but it was extended twice to allow for what Boeing officials said were more detailed examinations, particularly of the wiring. The craft finally left for Kennedy Space Center in March 2001.

NASA decided the next year to do future overhauls and modifications in Florida, saying it could save millions of dollars, rather than in Palmdale, where all five shuttles - including the ill-fated Challenger - were assembled. The decision eliminated about 200 Antelope Valley jobs.

Attention focused Saturday on the complex patchwork of felt, ceramic and carbon tiles that make up the shuttle's thermal protection system, which is designed to keep the shuttle from burning up as it decelerates from 17,500 miles per hour to 200 mph while entering the Earth's atmosphere.

The Columbia was about 200,000 feet above the Earth, with temperatures as high as 3,000 degrees on the shuttle's nose and wings, when trouble developed.

Anthony Ingraffea, an aerospace engineer at Cornell University who has worked with NASA on thermal protection systems, said more heat was flowing into the shuttle at that point than any other during its planned return to Earth.

``The possibilities are that if enough tiles were loosened or damaged, you can get enough heat flow through the tiles and the load-bearing structures of the wings to cause failure,'' he said.

The field of thermal protection has advanced greatly since the first Columbia launch in 1981, he said. However, the shuttle's thermal shield is not completely replaced between missions because doing so would be prohibitively expensive, he said.

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Photo:

(color) The space shuttle Columbia, shown from the left side during liftoff in this Jan. 16 photo from the Kennedy Space Center in Cape Canaveral, Fla. Investigators are expected to focus on possible damage to protective thermal tiles on the left wing from a piece of flying debris during liftoff. However, the cause of the crash has not been confirmed.

Chris O'Meara/Associated Press