Anyone can fly supersonically today. Just pay about $8,500 for a round-trip on the British-French Concorde and, before you know it, you're winging across the North Atlantic at Mach 2, or twice the speed of sound. It's hard to imagine that many people once believed it couldn't be done. But half a century ago, supersonic flight was a dream to some and a nightmare to others. During World War II, numerous Allied and Axis pilots came to grief when they dove high-performance fighters at full power during air combat and encountered mysterious forces that froze their controls or ripped their aircraft apart. Some said it was if they had hit a "brick wall in the sky." Thus was born the legend of a "sound barrier." It would die 50 years ago next Tuesday, but not until it had gained more infamy. In a highly publicized incident in September 1946, Britain's premier test pilot, Geoffrey de Havilland Jr., was killed while testing an experimental jet-powered aircraft called the "Swallow" in preparation for a run at the barrier. The plane disintegrated during a high-speed dive, and de Havilland's body washed up in the Thames River estuary 10 days later. The accident doomed supersonic flight research in Britain but not in the United States where the effort was deemed too far advanced to abandon. Ten weeks after de Havilland's death, on Dec. 9, 1946, the Bell XS-1 rocket plane made its first powered flight at what is now Edwards Air Force Base, about 75 miles northeast of Los Angeles. Later, the "S" (for "sonic") was dropped from the model name, and it became simply the X-1, "X" for experimental. The X-1 would dash the "sound barrier" legend but not without a struggle. Today, the bright orange, bullet-shaped, rocket-powered plane, one of three versions of the X-1 that would break the sound barrier, hangs in the main gallery of the Smithsonian Institution's National Air & Space Museum. It is just 31 feet long, shorter than an average city bus, and has a 28-foot wing span. The placard below the aircraft tells the story in cryptic fashion: "On Oct. 14, 1947, the airplane above, piloted by 24-year-old Charles E. Chuck' Yeager, USAF, became the first airplane to fly faster than the speed of sound. It reached 1127 kilometers per hour (700 miles per hour, Mach 1.06) at an altitude of 13,000 meters (43,000 feet) over the Mojave Desert near Muroc (now Rogers) Dry Lake, Calif. ... The X-1 was air launched from the bomb bay of a B-29 bomber at an altitude of 7,000 meters (23,000 feet). Under the power of its rocket engine, it climbed to test altitude and flew faster than the speed of sound." That makes it seem easy, but, of course, it wasn't. Yeager himself noted that "there are at least a dozen different ways the X-1 can kill you," and he probably understated the dangers. The plane carried 600 gallons of propellant -- ethyl alcohol fuel and liquid oxygen oxidizer -- that Yeager said could "blow up at the flick of an ignition switch." Yeager felt as if he were "strapped inside a live bomb that's about to be dropped out of a bomb bay." There was no way out in an emergency. The escape hatch was directly in front of the plane's ultra-thin wings, which could have sliced Yeager in half had he tried to bail out. Moreover, on his history-making flight, the high-spirited Yeager made things tougher on himself by showing up with two cracked ribs suffered less than 36 hours earlier. He had been thrown from a horse on a late-night gallop across the Mojave Desert and should have been home in bed instead of squeezed into the X-1 where his every move was agony. His injuries and his tightly-taped chest so restricted his movements that he had to use a sawed-off broom handle to reach far enough to latch the plane's door. But people expected nothing less from Yeager, who was becoming something of a living legend. Born and bred in West Virginia, he was a double ace in World War II, shooting down in one day the five enemy planes needed for designation as an "ace." Yeager was among the first Allied pilots to bring down a new German Messerschmidt-262. In another incident, he, too, was shot down but managed to escape from occupied France to Spain by climbing the Pyrenees Mountains, dragging a seriously wounded companion part of the way. Ordered back to America after his ordeal, he insisted on remaining in combat, taking his case all the way to the Supreme Allied Commander, Gen. Dwight D. Eisenhower, and winning. When the war ended, Yeager was assigned to Wright Field at Dayton, Ohio, and so impressed his superiors with his flying ability that they sent him to test pilot school although he lacked the college degrees and engineering background of most of his colleagues. After completing the course, he was picked to fly the X-1 over more than 100 senior pilots in the Flight Test Division. The officer who selected him said none of the others "could quite match his skill in the cockpit." Equally important were Yeager's "ability to perform" and "coolness under pressure," qualities that author Tom Wolfe later would describe as "the right stuff." In retrospect, achieving supersonic flight obviously was not as daunting as thought. After all, bullets travel at supersonic speeds and maintain shape and stability. And near the close of World War II, Germany fired V-2 rockets that flew to England at more than five times the speed of sound. These facts suggested that aerodynamics -- the way a plane's surfaces respond to air flowing past -- was the problem and that modifying craft could be the solution. Some saw this clearly. One was Air Force Capt. Jack Ridley, Yeager's engineering guru on the X-1 program who later flew the plane himself. "The only barrier is bad aerodynamics and bad planning," he said. Aeronautical engineers have a name for the problem that airplanes encounter in high-speed flight. They call it "compressibility." When a plane flies well below the speed of sound, air behaves like an incompressible fluid. As the plane's surfaces push on the air, it is shoved aside. But as the plane's speed approaches the speed of sound, the air becomes more and more compressible. Instead of moving aside to let the plane pass, it acts more like squeezed rubber and pushes back. This change in air's behavior plays havoc with the plane's lift and drag. The moveable control surfaces on the aircraft's wings and tail are shaped to behave properly when air is incompressible but do not act the same when air turns rubbery. A related problem involves the shock wave produced when the plane exceeds the speed of sound. This is what produces the sonic boom. Before reaching the ground, the wave can deliver a powerful thump to the plane's surfaces. Portions of a supersonic aircraft continuously pummeled by the shock wave can heat up, sometimes enough to cause damage if they are made of inappropriate materials. {For a more detailed explanation of the shock wave and the sonic boom, see the diagram below.} Once, Yeager was flying the X-1 for the eighth time but had not yet attempted supersonic speeds, the various aerodynamic effects suddenly caused him to lose control of the elevator, the moveable surface on the tail's horizontal stabilizer. As the plane reached 94 percent of the speed of sound, a shock wave prevented the elevator from working, and Yeager couldn't control the craft's nose up/nose down attitude. This briefly put the entire X-1 program in jeopardy. In the mid-1940s, flight into the transonic region was aviation's "black hole," which Yeager and Ridley called "the ughknown," because wind-tunnel tests had provided no reliable information. When technicians cranked up air flow in a wind tunnel close to the speed of sound, aircraft models created shock waves that bounced off the walls and choked off the flow of air. At higher speeds, however, things smoothed out again as the models no longer were subjected to a mix of subsonic and supersonic forces. This seemed to promise a bright future if the mysteries of the "sound barrier" could be solved. But given the problems with wind tunnels, the answers would have to come through actual flight testing. Once the scientists and engineers knew precisely what happened to the air flow around the plane as it approached the speed of sound, they could make necessary adjustments in design and flight procedures. Thus, in 1944, the military and NASA's predecessor, the National Advisory Committee for Aeronautics (NACA), collaborated to convince people that, as one official put it, the sound barrier was simply a "steep hill." In March 1945, Bell Aircraft won a contract with the Army Air Force to build three XS-1 airframes. It produced the No. 1 ship, which Yeager would fly into the history books, within 10 months. Just weeks after it rolled out of the factory in December 1945, the engineless plane was airlifted by a modified B-29 to an Army air field near Orlando, Fla., for glide tests. In a series of tests, the B-29 carried the X-1 aloft and released it with a pilot to glide back to a runway. The following March, it was transported back to the Bell plant at Niagara Falls, N.Y., where technicians installed the power plant -- a four-chambered rocket engine, manufactured by Reaction Motors, Inc. This would give the X-1 the thrust needed to make a run at the sound barrier in level flight rather than a more dangerous dive. The plane also was fitted with thinner wings and a horizontal tail designed to minimize the effects of shock waves produced at transonic speeds. When Bell resumed flight testing at Muroc in September 1946, it used the No. 2 aircraft and entrusted the controls to Chalmers "Slick" Goodlin, 23, a strikingly handsome man who looked more like Hollywood's idea of a test pilot than the real thing. Over the next nine months, Goodlin made 20 powered flights in the two aircraft, eventually reaching a speed of Mach 0.82, as indicated by the cockpit's Machmeter. This instrument gave a more useful measure of speed than would have a standard air speed indicator because the speed of sound is not constant. Sound travels faster in the more dense air at sea level than in the thinner air high above Earth. It ranges from about 760 mph at sea level to approximately 660 mph at 36,000 feet. Sound travels even faster in water. Because of this difference, aeronautical engineers use "Mach numbers" to designate the percentage of the speed of sound at whatever altitude a plane happens to be flying. The meter factors together air speed and air pressure. The system was named for Ernst Mach, a 19th-century Austrian scientist who discovered many of these phenomena after studying the flight of artillery shells. In July 1947, the Army Air Force took over the X-1 program, days before Congress voted to make the AAF an independent service, today's Air Force. The transfer proved a good move for taxpayers as Goodlin, a Bell employee, had been negotiating for a $150,000 bonus to continue flight testing while Yeager already was on the government payroll as an Air Force captain. The Air Force/NACA plan called for a gradual assault on Mach 1, but Yeager always pushed "the outside of the envelope." Told not to exceed Mach 0.82 on his first powered flight, he took the X-1 to Mach 0.85 in spectacular fashion. Instead of jettisoning fuel and flying straight home after completing the planned test, Yeager dove to a mere 3,000 feet and, using only one rocket chamber, streaked past the control tower, where the Wright Field "brass" were observing the test. Then Yeager kicked in the other three rocket chambers in rapid succession. The X-1 went almost straight up, hitting Mach 0.85 before exhausting its fuel. The X-1 could fly for 2112 minutes on all four chambers, five minutes on two and 10 minutes on one.). The brass were impressed but not amused. By-the-book NACA officials were livid, questioning Yeager's "sense of discipline." The young pilot received a gentle admonition from his boss at Wright Field -- "please approach higher speeds progressively and safely to the limit of your best judgment." Yeager promised to straighten up and fly right and, for the most part, kept his word. Sticking to flight plans, he took the X-1 to Mach 0.89 on his second flight, Mach 0.91 on the fourth and, creeping every closer to the magic number, Mach 0.925 on the fifth. The X-1 now had pulled to within 50 mph of the speed of sound at its normal altitude and was experiencing the severe buffeting and control problems that had helped to create the sound barrier myth in the first place. On the eighth flight, when it reached Mach 0.94, Yeager lost elevator control when a shock wave affected the point where the elevator was hinged to the rest of the tail's horizontal stabilizer. Yeager said it felt as if the control cables had snapped when he pulled back on the wheel and nothing happened. Fortunately, X-1 designers had anticipated this problem and built a stabilizer that could be moved as a whole to different angles to offset loss of elevator control. After much discussion and ground testing, it was decided to try this approach, and Yeager noted that it "gave me just enough pitch control to keep me safe." Also on the eighth flight, Yeager faced another problem. After he reached Mach 0.997, the inside of the windshield became encrusted with so much frost that he couldn't see through it or scrape it away. He had to be talked down to a safe landing by a pilot, ironically named Dick Frost, in a chase plane. In later flights, this problem was prevented by coating the inside of the windshield with shampoo. Four days later, a 24-year-old Yeager gritted his teeth against the pain of his cracked ribs and made the flight that Aviation Week magazine said "ranks in historic importance with the first flight of the Wright Flyer...." By then, Yeager knew the X-1 intimately as he pushed the plane, named "Glamorous Glennis" after his wife, steadily through the transonic range until the Machmeter needle hit 0.98, fluctuated and swung beyond the scale. "I thought I was seeing things," Yeager wrote later. "We were flying supersonic! And it was as smooth as a baby's bottom." Eight miles below, Yeager's colleagues learned the news almost as quickly as he did when the double sonic boom, generated by the X-1 shock waves, rolled across the desert floor. On the way down, Yeager did victory rolls. The "sound barrier" no longer blocked the advance of aviation, but fame did not come quickly to Yeager. The Air Force sat on the story for eight months, fearing that knowledge of the success might give a Cold War advantage to the Soviet Union. It even refused to confirm a sketchy account that appeared in Aviation Week magazine Dec. 22, 1947. Eventually, Yeager emerged from the experience as perhaps the most celebrated aviator since Charles Lindbergh, and, he later noted, "the awards, medals and plaques began piling up." One was the prestigious Collier Trophy, which he shared with two other major players in the X-1 program, Larry Bell of Bell Aircraft and John Stack of NACA. But fame did little to advance Yeager's career and historians speculate that it probably hindered it by creating envy and resentment among senior Air Force officers. He didn't earn the silver star of a brigadier general until 1968, seven years before his retirement, when he had all but given up hope of receiving one. He called it his "miracle star." Yeager's face would become familiar to a subsequent generation of television watchers who saw him making product endorsements in a series of commercials. Now 74 and living in California, Yeager remains what one admiring magazine writer called "an American original." Although he bristles when people ask whether he thinks he has "the right stuff," he has no false modesty about his own achievements. "I don't deny that I was damned good," he wrote in his autobiography. "If there is such a thing as the best,' I was one of the title contenders." John G. Leyden was public affairs officer of the Federal Aviation Administration and information officer for its Supersonic Transport (SST) program. Retired, he freelances articles about history. CAPTION: THE BOOM AT THE BARRIER The sonic boom, produced when objects travel faster than the speed of sound, results from a complex natural phenomenon. Here it is in steps: 1. SPEED = 0 If a sound source is not moving, the sound spreads in expanding spheres, simplified here to circles. Each circle represents the crest of a wave moving like a ripple in water. Each wave crest is a region of high air pressure separated from its neighbors by circles of low air pressure. 2. SPEED = Mach 0.67 If the source moves, the sound waves bunch up on one side. This is because the source is catching up with its own sound waves, but only in one direction. If the wave crests overlap, they reinforce one another. The result is a superwave of even higher pressure. 3. SPEED = Mach 1.33 Once the source exceeds the speed of sound -- outrunning its own sound -- the wave pattern begins to resemble a triangle. The sides of the triangle flaring back from the front are a shock wave -- the region in which the peaks of many sound waves are superimposed and combine to produce a loud boom. 4. SPEED = Mach 1.33 Here is a three-dimensional view of the situation in Step 3. The plane produces a shock wave that expands as the plane flies. The boom is produced when air moving over the top of a wing rejoins air flowing under the wing. Because the top path is longer, the air must move faster. Thus the air pressure above the wing is lower than that below. (This is what creates lift.) When the two air masses rejoin, their impact produces a boom. SOURCE: How Things Work: The Physics of Everyday Life, by Louis A. Bloomfield COMPARING SPEEDS IN MILES PER HOUR Human walking

3 Human running

34.3 Race horse

44.9 Cheetah running

65 Car on interstate highway

65 Fastest train

250 Passenger jetliner

575 (McDonnell Douglas DC9) Speed of sound

761 (At sea level, 59 degrees) Concorde

1,450 Fastest jet fighter

4,500 Space shuttle in orbit

17,000 SOURCE: The Sizesaurus, by Stephen Strauss CAPTION: Chuck Yeager aboard the Bell X-1 in the 1940s. CAPTION: Chuck Yeager, 74, at a gathering of retired test pilots last June at Maxwell Air Force Base in Montgomery, Ala. CAPTION: The U. S. Postal Service plans to issue this commemorative stamp, showing the X-1, on Oct. 14, the 50th anniversary of Chuck Yeager's historic flight. Also that day, at 10:29 a.m. PDT, the same minute he did it the first time, Yeager is to break the sound barrier again. The pilot will be flying an F-15 at Edwards Air Force Base in the Mojave Desert, with Bob Hoover again piloting the chase plane.