WASHINGTON — WASHINGTON -- The troubled B-2 stealth bomber may be vulnerable to advanced heat-seeking detection systems being developed by the Soviet Union, raising new questions about its ability to sneak through sophisticated air defenses in the next few years, according to scientists and Pentagon advisers on infrared technology.
The potential problem stems from the presence of hot spots -- chiefly sections of the aircraft's non-metallic skin and the intakes, tailpipes and exhaust plumes of four turbo-fan jet engines -- despite design features that were intended to shield or cool them down.
"A tremendous amount of attention has been paid to it, but that doesn't necessarily mean success," said a Massachusetts aeronautics expert with access to classified B-2 design and performance data.
He and other specialists interviewed last week disagreed over how easily an approaching stealth bomber might be discovered by advanced heat-seeking sensors, known to the military as infrared search and track systems (IRST). But they all acknowledged that, once spotted by radar or other methods of detection, the B-2 risked being targeted successfully by IRST systems aboard enemy aircraft.
"If a fighter can get in range of its tail, the [B-2] plane is dead," said the aeronautics expert, who insisted on anonymity.
Questions about the bomber's stealthiness have been mounting since the Air Force disclosed last month that one of three existing planes had failed to meet radar-evading performance standards during a July 26 flight test. The aircraft's radar image, or "signature," was slightly larger than it should have been, Air Force officials explained.
Although the Air Force ruled out a design flaw or production mishap, it has been unable to resolve the problem or to allay congressional concerns about the plane's ability to escape detection. Nonetheless, Pentagon spokesman Pete Williams has declared the B-2 the "most survivable aircraft in the world."
More recently, questions have arisen about future missions for the bomber, which was conceived to penetrate deep into Soviet airspace to destroy mobile missiles and command bunkers with nuclear weapons.
President Bush's recent nuclear arms cuts included cancellation a short-range nuclear attack missile that would have been a key armament of the B-2 bomber. Combined with the apparent collapse of the Soviet military threat, this undercut the rationale for the $65 billion stealth program, with little more than a conventional warfare mission left for the world's most expensive plane, B-2 critics argue.
Much of the scientific concern over the plane's vulnerability to infrared sensors rests with knowledge that the Soviets are testing and fielding advanced IRST systems that would make it easier to find and target heat sources from distances farther than ever before. These systems, which would have greater reliability and be less prone to false alarms than current sensors, appear to be intended as a backup in case radar systems are jammed.
Some scientists said they were focusing on this development because Soviet technology may be exported to potential U.S. adversaries seeking to build a diversified air defense network.
The U.S. Navy and Air Force -- which still regard the Soviet Union as the primary strategic threat to U.S. security -- are engaged in similar development projects, but they have not equipped aircraft with IRST systems as the Soviets already have done, scientists said.
"These are detectors the B-2 was not designed to elude," said a researcher whose firm has advised the military on measuring "infrared signatures" of aircraft and other combat hardware. Reducing the heat displayed by the plane was "not a design priority," he said.
Other scientists said they thought the Air Force had vigorously addressed the infrared problem but agreed that the job of reducing heat sources on the B-2 has been tougher than shrinking the plane's radar image.
"For one thing, the skin can be heated at the leading edge" due to friction as the aircraft cuts through the air, one scientist said. He added that the black carbon-composite skin, which does not reflect most of the radar waves striking it, is not prone to reflect sunlight and heat as well, keeping the B-2 airframe's exterior temperature higher than desired.
Jack Kerrebrock, professor of aeronautics at the Massachusetts Institute of Technology and a former NASA official, observed that "some attempt has been made to shield the tailpipes from view." But, he added, "It's kind of hard to blow hot gas out back without getting hot surfaces."
"If things are hot, you can see them," Dr. Kerrebrock said.
He and other scientists said it was vital to consider the vulnerability of the bomber once it is detected. "It's in the endgame where the infrared problem becomes important -- your ability to attack the bomber once you know it's coming," Dr. Kerrebrock said.
The other aeronautics expert, who would not be identified, reasoned that as long as the plane was stealthy to radar, the infrared problem would not be an "Achilles heel." But "if the radar part has failed, the importance of the infrared signature is high," this scientist said.
The B-2 got its stealthy name because its revolutionary batwing shape, contours and skin are supposed to absorb, rather than deflect, radar waves to reduce the plane's susceptibility to radar detection. Because radars are the main sensor for most air defenses, the B-2 radar signature was designed to be very small, perhaps one-ten-thousandth the size of an aircraft made of a conventional design and material, scientists said.
In March 1990, the Air Force issued an unclassified report on "B-2 Survivability," saying that "calculations and computer models, all backed up by actual field experiments," had been used to measure the bomber's capabilities against radars, infrared systems and other sensors. It expressed confidence that the B-2 could defeat any air defense threat but said actual flight tests would still be needed to confirm this claim.
For new technology such as advanced infrared sensors, the Air Force report said only that "intense" investigations had found that the B-2 could be detected but that no system provided "a robust air defense capability" to track and kill the B-2 at ranges of 25 to 200 miles.
But, the Air Force added, "This is not to say that more effective approaches may not be developed in the future."
The Air Force has never claimed the plane would be invisible, only harder to find. Officials have also argued that spotting an incoming B-2 would not be enough to stop it because a successful air defense system must track and target the plane to guide missiles in for the kill.