Breaking the Sound Barrier

Overview

In 1947, Chuck Yeager (1923- ) became the first person to fly faster than the speed of sound. Not only did he prove that the "sound barrier" could be broken, but he set in motion development that has led to the high-speed fighters that are critical to military defense. Commercial use of high-speed aircraft has been limited primarily to the Concorde, but plans exist for hypersonic flight (Mach 5 or higher).

Background

Aircraft went through a period of rapid development and commercialization after World War I, with the development of airmail and regular passenger flights. In addition, competitions, most notably the race to cross the Atlantic solo, captured public imagination and attracted investment. Airplanes became an essential weapon during World War II. The need for speed and maneuverability, particularly among fighters, spurred advances in design, including the development of jet engines.

Air superiority, particularly with a nuclear payload, became a life and death concern after the war. With the Eastern Bloc and Western Alliance locked in the Cold War, massive investments went into the design, construction, and testing of new aircraft. During what is now called the golden age of flying, test pilots regularly pushed new aircraft to their limits. One limit that many considered absolute was the speed of sound, Mach 1 (760 mph [1,223 kph] at sea level).

During World War II research into supersonic flight, pilots noticed that when aircraft velocity approached the speed of sound, the planes started to become uncontrollable. This happened when sound waves actually began to get ahead of their source, creating localized shockwaves that disrupted airflow around the aircraft. In particular, this phenomenon, called "compressibility," caused tremendous air turbulence around the wings. Compressibility was so difficult to handle that the British, after losing one of their best pilots, terminated their supersonic research flights.

The U.S. team, however, saw an engineering challenge and began further testing. The vehicle they chose was Bell's X-1, a solidly built, bullet-shaped craft that was launched from a bomber. The pilot was an Air Force captain named Chuck Yeager, who took over the mission after the commercial test pilot demanded more money. After series of powerless glides and incrementally faster tests of the X-1's rockets, Yeager hit a problem when he brought the vehicle up to 0.94 Mach. He lost pitch control, the ability to keep the nose from rising or falling. The control for the elevator on the wing refused to move.

The X-1 had hit the "barrier" that had led other aircraft to crash or disintegrate. The shock waves caused the airplane to shake violently and changed the aerodynamics so that it lost lift and began to drag. Sensor data showed that turbulence near the wing actually froze the hinge on the wing's elevator, which was why Yeager couldn't move it. The team was faced with a decision—quit or risk having the X-1 disintegrate with Yeager inside. The team's engineer, Jack Ridley, thought there was an alternative way to maintain control as Mach 1 was approached. He suggested Yeager use the horizontal stabilizer (a wing-like structure high on the tail, some distance from the air turbulence) to control the pitch. On October 14, 1947, Yeager ignited all four rockets and brought the X-1 up to speed. Again, he lost normal pitch control at Mach 0.94, but he was able to use the stabilizer as planned and regained command of the vehicle. Passing through Mach 0.96, the normal controls became operative again. Yeager took the X-1 up to Mach 1.07 (700 mph [1,126 kph]), and he was able to land the vehicle safely after having created the first man-made sonic boom.

Impact

Breaking the sound barrier meant breaking a psychological barrier as well as an engineering one. Once the possibility was proven, research was able to proceed, moving up to higher speeds and higher altitudes.

Public reaction to the breaking of the sound barrier was muted because security concerns kept it secret for eight months. The American military wanted as much of a lead in aircraft development over the Soviets as they could get. High-speed aircraft represented a significant military advantage for four reasons: First, in a dogfight, a speedier aircraft could provide the ability to acquire targets and, with the right design, outmaneuver them. Second, faster aircraft that could outrun the enemy could retreat from dangerous situations where they did not have the advantage. Third, at high speeds, the probability of being shot down was reduced, making these jets good platforms for surveillance. Fourth, supersonic aircraft could be stationed at a safe distance from a target and still be accessible in a short period of time.

Each of these benefits has played a role in defense operations, but today some limits of supersonic flight are recognized. According to Yeager, most dogfights are done at Mach 0.9 to 1.2. Heatseeking missiles eliminate the need to get on the enemy's tail, so anything over Mach 2.2 just uses more fuel. Since missiles travel faster than jets, running away is no longer an option, even for supersonic aircraft. Missiles have also helped neutralize the surveillance potential of supersonic aircraft, particularly when satellites offer an alternative. Distance to target remains a vital capability, but not for supersonic bombers, which are not a factor in defense due to their inability to meet cost and performance specifications.

Still, supersonic aircraft offered so much of a military advantage that they became one of the three legs of the U.S. nuclear defense (along with missiles and submarines). In addition, dozens of nations now include supersonic fighters, armed with conventional weapons, in their arsenals. For instance, there are over 1,000 F-15s currently in use. The production and development of supersonic fighters represent a multibillion dollar industry with significant foreign exchange. (A single U.S. sale of 160 F-16s to Turkey in 1987 was valued at $4 billion.)

The economic impact of commercial supersonic flight has been minimal. With the exception of the British-French alliance that created the Concorde (which first flew in 1976), major development projects were scrapped in the 1970s due to environmental concerns and high costs. The Soviet Tu-144, which made its first flight in 1968, suffered a very public reversal when an aircraft crashed at the Paris Air Show in 1973. The model was grounded in 1978 after only 102 passenger flights. The American SST became a focus of concern because of sonic booms and ozone-destroying pollutants. (The latter led to worldwide action protecting the Earth's ozone layer, based on work for which Paul Crutzen [1933- ] shared the 1995 Nobel Prize in chemistry.) Environmental groups successfully stopped government funding, and the aircraft industry put its development efforts into other subsonic projects, most notably jumbo jets like the Boeing 747. Further, the environmental groups translated the momentum for their cause into a ban on commercial supersonic flight over the continental United States and tight restrictions on flights in and out of coastal cities. This severely hampered the economics of commercial applications. In fact, only 14 of the 70 Concordes planned were built, and the program isn't likely to ever recover its development costs.

Pilots have had a place in the popular imagination almost from the time of the Wright brothers, and test pilots had their heyday, becoming popular heroes in the 1950s. Yeager never got a ticker-tape parade, but he has never lacked for speaking engagements and even helped make a John Wayne movie. Many test pilots, impressed by Yeager's accomplishments and his desire to "push the envelope," went on to become astronauts, who in turn attracted a whole generation into science. Yeager himself became one of the key figures in Tom Wolfe's book The Right Stuff (and the movie by the same name), which inspired renewed interest in his accomplishments.

The dreams of supersonic flight persist. New research is underway, partly aimed at creating a successor to the Concorde fleet, which retires in 2005. Curiously, there is an American-Russian initiative that uses a Tu-144 as part of its test program. More ambitious is the work toward hypersonic flight (Mach 5 or better). The speed record for manned flight is Mach 6.7, set in 1967, but the current target is Mach 10. A radio-controlled prototype vehicle, the A43-HyperX, underwent ground testing by NASA in 2000, and many believe that the military already has a hypersonic vehicle, popularly known as Aurora.

Renewed interest in faster vehicles came in part as a result of the development of new materials, encouraging tests of a new engine concept called scramjet and new design options that may mitigate the intensity of sonic booms. The Lawrence Livermore National Laboratory's proposal for the HyperSoar vehicle would skip the aircraft off the Earth's atmosphere in a way that would allow it to provide air to the engines (inherently more efficient than using rockets) while shedding excess heat in space. Such an aircraft could take passengers from San Francisco to Tokyo in less than two hours. A HyperSoar bomber could reach any target on the planet without refueling. HyperSoar could even provide the first stage for launching payloads into space.

PETER J. ANDREWS

Other

The Boeing Company. "Faster and Faster." http://www.boeing.com/defense-space/military/f15/barrier/faster.htm

PBS Nova Online. "Supersonic Spies." http://www.pbs.org/wgbh/nova/supersonic/

THE TEST PILOT MYSTIQUE

The jet test pilot emerged, in the late 1940s, as a new kind of American hero. There had, of course, been pilot-heroes before: barnstormers, air racers, explorers, airmail pioneers, and the aces of two world wars. The older pilot-heroes were defined, in the public's mind, by their reckless daring, raw courage, and flamboyant behavior. The new heroes were something else altogether. Their job was to gather precise data on the handling of experimental aircraft and determine where the limits of their performance lay. Test flying demanded the ability to fly routine maneuvers time after time with absolute precision while staying alert and ready to deal, coolly and quickly, with any unexpected crisis. Test pilots judged themselves by their ability to push a plane to the very edge of its performance "envelope" and pull it back at the last instant before control was lost. The public's image of test pilots revolved around their supreme grace under pressure—their ability to deal matter-of-factly with life-threatening situations. This image of the test pilot was introduced by Chuck Yeager, who broke the sound barrier in 1947, but it was made famous by the test-pilot astronauts who went to the Moon in the 1960s and 70s.

A. BOWDOIN VAN RIPER