SPACE SHUTTLES STREAMLINE FOR NEW MILLENNIUM; NEW DESIGNS KEEP FLEET APACE.

Byline: Warren E. Leary The New York Times

America's aging space shuttles, often criticized as monuments to 1970s technology, are slowly being replaced by much more modern spacecraft: themselves.

With the possible exception of a few science fiction counterparts, the nation's fleet of space shuttles, which began flying humans into space 18 years ago, have become the most recognized spaceships in the world. The stubby-winged space planes, with twin rocket boosters and giant fuel tanks strapped to their bellies, appear little changed since the first one flew in 1981.

But appearances can be deceiving. The shuttles have been steadily modified during their lifetimes, NASA engineers and managers say, and are much more modern beneath their familiar skin than most people realize.

The shuttles underwent numerous, highly publicized safety modifications after the Challenger explosion in 1986 took the lives of seven crew members. These changes, among thousands of major and minor design modifications that were made since the shuttles started flying, included redesigned solid-fuel booster rockets, more spacecraft sensors and a parachute escape system for the crew.

But later improvements that have not drawn as much attention include new main engines that engineers say are safer and more reliable because they have far fewer welded parts and operate under lower pressures; external fuel tanks made of a new, lighter aluminum-lithium alloy, providing a 7,500-pound weight reduction that translates into increased cargo capacity; improved brakes and the addition of tail parachutes for surer, shorter landings, and added protection against the impact of orbital debris to the leading edges of the wings and heat radiators inside the cargo bay doors.

To illustrate the evolving shuttle, officials at the Kennedy Space Center showed off the shuttle Atlantis in April, giving journalists unprecedented access to the orbiter, which recently completed a major upgrade in Palmdale.

During a 10-month, $70 million overhaul at U.S. Air Force Plant 42, the Atlantis was taken out of flight rotation and given more than 130 major modifications while being disassembled, cleaned, tested and put back together again. Such reconditioning typically occurs every four or five years.

The 86-ton space plane returned to Kennedy in September weighing 1,000 pounds less and full of enhancements, including a modern cockpit with digital display screens and a new navigation system and other electronics that rival the latest airliners.

``This new cockpit is a major improvement and should make the shuttle much easier and safer to fly,'' said Air Force Col. James D. Halsell Jr., who will command the Atlantis on its first mission after the overhaul, a supply flight to the international space station tentatively set for December.

Dressed in a white ``clean room'' suit that includes a hood over his head to prevent stray hairs from contaminating the Atlantis' near-sterile environment, Halsell showed similarly clad visitors the shuttle's new ``glass cockpit.'' Gone are four black-and-white cathode ray television screens and numerous mechanical instruments and gauges, replaced by an array of color active-matrix displays like those found on portable computers.

The nine flat-screen displays in front of the pilots, plus two more in the rear cockpit, can be programmed to show all kinds of flight information and sensor data.

Shuttle pilots said the new displays, each with a square 6.7-inch viewing area showing better organized and color-coded data, should help them to obtain and understand information more quickly during the critical 8-1/2-minute trip from takeoff to orbit and subsequent landing.

``The cockpit displays are the most visible sign of changes in the shuttles, but a lot of the other modifications will be of huge benefit to the crews,'' said Lt. Col. Scott J. Horowitz, who will pilot the Atlantis on its next flight. ``The shuttle will be flying for a long time because nothing else can do its job. This thing will outlive my career and a lot of other people's, too.''

With continued updating, officials say, these shuttles could conceivably fly for an additional 20 or 30 years.

Elric McHenry, the shuttle program development manager, said Congress and the Clinton administration would soon decide what to do about a replacement for the shuttle. But whatever the decision, he said, the shuttle fleet would continue to fly for at least a decade, and probably longer, because there are no alternatives on the immediate horizon.

``It's prudent for the shuttle program to assume that for the next 10 years, we will be it,'' McHenry said at a news briefing, ``and I personally believe we will fly much longer than that.''

Other space experts agree. Dr. John M. Logsdon, director of the Space Policy Institute at George Washington University in Washington, said the only shuttle alternatives that existed were on paper and that he believed the current spacecraft would continue flying for 15 to 20 years. ``The shuttle is the only re-usable, manned spacecraft that we've got,'' he said.

John E. Pike, space policy expert for the Federation of American Scientists, said the shuttle could be the spacecraft equivalent of the much-modified B-52 bomber, which the Air Force recently said could remain in service more than 70 years after its introduction. ``The shuttle could continue flying like the B-52, with the only limit being its airframe life,'' Pike said.

Daniel S. Goldin, the administrator of the National Aeronautics and Space Administration, said the versatility of the shuttle made it difficult to find alternatives, particularly for servicing the international space station now being constructed in orbit. People have suggested alternative vehicles that would only carry crews to and from the station, or that would serve as unpiloted cargo haulers, but no proposal is up to all the shuttle's jobs, he told reporters in April.

``The shuttle is a rocket for 8-1/2 minutes, capable of carrying a crew and cargo, but the shuttle is also a spacecraft that can operate independently in space for eight to 14 days,'' Goldin said. If a major problem occurred aboard the space station that required outside repairs, he said, there would be a need for a craft like the shuttle, in which crews could live and work for long periods.

Officials noted that the shuttle fleet was still less than one-fourth of the way through its design lifetime and that continued modernization was making it safer, more capable and less expensive to fly with each year. Each of the $2 billion shuttles was designed for 100 flights and the most used orbiters, the Columbia and Discovery, had so far flown 25 missions each.

While the shuttles technically could fly for 30 more years with updating, McHenry said the program probably would not last that long for economic reasons. As new technology, like different types of engines or electronics, emerges for better, cheaper spacecraft, he said, it might make more economic sense to apply it to a new vehicle rather than adapt it to the shuttles.

NASA spends about $100 million a year to develop and install modifications and upgrades for the orbiter fleet, McHenry said. For example, the agency spent about $200 million to develop the equipment and software for the new cockpit, he said, including $9 million to install it in Atlantis. It should cost slightly less to put the new flight deck into each of the other shuttles as they come up for refitting.

McHenry said NASA was considering numerous other modifications, including installing a new system of thrusters, using less toxic fuels, to control the shuttles in space, improving the life of the fuel cells that generate electricity on missions and developing a new front landing gear for better safety under hazardous landing conditions.

Even more radical changes have been suggested, like replacing the solid-fuel booster rockets with safer, liquid-fuel boosters designed to fly back to their launching site. But with a projected development cost of more than $5 billion, experts question whether the investment would be worth it.