When a B-2 stealth bomber flew into the Paris Air Show in June, the crowds surged forward to catch a glimpse of the semimystical flying wing.
It was hardly surprising, because until three years ago the B-2 radar-evading aircraft was the U.S. Air Force's best-kept secret. Its appearance in Paris was the first time the B-2 bomber has been seen outside the United States.
"Stealth" -- the innovative technology that enables aircraft to become invisible to radar -- became famous in the Persian Gulf war, when the first generation of stealthy aircraft, the F-117A, flew nightly missions over Baghdad.
The F-117A had been in service with the Air Force since 1981, but the program was so secret that the government only admitted it existed in 1988.
The B-2 is the next generation of stealth aircraft and is designed to carry nuclear weapons deep into enemy territory. Even though the public at the air show could see the aircraft, which each cost a mind-boggling $900 million, the B-2 or "Spirit" retained most of its mystery.
Armed guards kept everyone at a distance, and the aircraft stayed on the ground for only an hour to change crews before heading back to the United States. The B-2's engines were not even turned off during the stop.
U.S. determination to keep all but the outline of the aircraft secret apparently extended to putting large radar transmitters on the bomber for the trip.
Since everyone knew it was coming, the Air Force wanted to make sure that even friendly radars could not try to pick up some trace of the jet. So to hide its true nature, the B-2 blasted out a vast radar signal. It was like an alley cat crashing dustbin lids together to disguise its stalking technique when it is not really trying to catch mice.
In reality, however, stealth gives an aircraft great advantages. It avoids detection from the ground, allowing bombers to fly unobserved over enemy territory, and allows fighters to sneak up on their opposition without being seen.
The pilot who can see his opponent minutes before he can be detected has a significant tactical advantage.
Stealth -- which translates into French as furtive -- has proved such a successful technology for the United States that it is now finding its way onto missiles, ships and helicopters. And while most of the details remain highly secret, some of the basic principles on which stealth operates are beginning to come to light.
There are two tell-tale signals given off by aircraft that have traditionally been used to track them. Since its invention shortly before World War II, radar, a form of high-frequency radio wave, has been the principal method of tracking aircraft.
A beam is sent out from a radarset, which reflects off the metallic surfaces of the aircraft and back to the detector. Considerable effort has also gone into tracking aircraft using the infrared heat trails left behind by jet engines.
Stealth aircraft try to tackle both problems. To avoid radar, the designers have used secret radar-absorbing materials on many surfaces which damp any reflected waves and reduce the signal's strength.
However, the real breakthrough in stealth technology was to shape the aircraft so that any radar reflections that do occur are deflected away in one direction so that they never return to the source of the signal.
To do that, engineers had to smooth the aircraft so that there are as few radar reflecting surfaces as possible. All of the spikes, roundedexternal missiles, fuel tanks and irregular wing surfaces had to go.
The designers were aiming for a simple shape which uses the same angles everywhere, so that the signals bounce in one direction away from the source of the radar.
For example, on the B-2, which is made by Northrop-Grumman, the saw-tooth angles at the rear of the aircraft all match, while on the even more recent F-22 advanced fighter, currently being developed by Boeing and Lockheed Martin for the U.S. Air Force, the rake angles on the engine intakes match those of the tailplanes.
The shaping and smoothing has to be so precise that not even a rivet head can stand out on a wing surface in case it catches a glinting reflection of a radar beam.
"Just one bolt out of place andyour stealth goes right out of the window," says Mickey Blackwell, president of Lockheed Martin's aircraft division, which owns the "skunk works" research laboratory where much stealth technology has been developed.
The snag with all of this is that the shape that they developed also had to be able to fly.
There is an inherent conflict between what the stealth shapers would like to do to avoid detection and what aerodynamics engineers want to do to make an aircraft agile.
Early stealth designs had the flight characteristics of bricks.
More recent designs have refined the art, and Lockheed Martin claims that the new F-22 fighter has regained all the aerodynamic agility of a fully acrobatic fighter while retaining the furtiveness of the first-generation F-117A stealth aircraft used in the gulf war.
Another problem is that the stealth designers had to get rid of all the missiles and bombs, which are the main point of combat aircraft. To overcome this, all weapons have been hidden inside the aircraft.
For example, the new F-22 fighter has four medium-range air-to-air missiles concealed inside its main body section. The B-2 can carry two large nuclear weapons inside its improbably thin fuselage.
"Miniaturization has helped us a lot here," Mr. Blackwell says. "We have been able to shrink equipment enough to allow us to fit weapons inside the plane."
Only at the point of firing does a side bay door on the F-22 slide away and the missile pops out, generating a momentary radar reflection. By then it is much too late for the prey.
Other tricks of the stealth trade are to hide the turbine blades of the jet engines, which sparkle like ballroom mirrored globes in a radar beam.
In the F-22, for instance, the engines are hidden from the outside by an S-bend air intake, which is lined with radar-absorbing materials. Any radar signal which gets as far as the turbines bounces off the internal walls so much before it gets back out that the signal is lost.
To cope with heat from jet exhausts, the gases can be vented across the top of the aircraft, as in the B-2, so that the heat is invisible from the ground, which is useful in a high-altitude bomber.
The F-22 is a fighter, which is more likely to be seen from any angle, so its heat is dissipated through a complex series of nozzles and heat-conducting materials to lessen the heat signature.
Even though the heat and radar problems are well on their way to being solved, at least by the United States, others remain. The F-22 is designed to cruise at supersonic speeds, generating a tell-tale shock wave. The noise of jet engines is another giveaway. And stealth aircraft can still be seen in daylight.
"We keep working on these issues and we'll solve them one by one," Mr. Blackwell says.
Which suggests that in its top-secret "black" laboratories, the United States is already working on ways to damp sonic booms and cut engine noise.
It also hints that the days of the invisible aircraft might not be too far away.
Bernard Gray is defense correspondent for the Financial Times of London.