Sound barrier

U.S. Navy F/A-18 breaking the sound barrier. The white halo is formed by condensed water droplets which are thought to result from a drop in air pressure around the aircraft (see Prandtl-Glauert Singularity).

1. Subsonic
2. Mach 1
3. Supersonic
4. Shock wave

Shadowgraph of a 7,62 x 51 NATO bullet in supersonic flight at Mach 2.4. Notice the shockwave (Mach cone) from the bullet's tip, a secondary shockwave coining from the cannellure and the turbulence in the air behind the bullet

In aerodynamics, the sound barrier usually refers to the point at which an aircraft moves from transonic to supersonic speed. The term came into use during World War II when a number of aircraft started to encounter the effects of compressibility, a grab-bag of unrelated aerodynamic effects. By the 1950s, aircraft started to routinely "break" the sound barrier.

History

Some common whips such as the bullwhip or sparewhip are able to move faster than sound: the tip of the whip breaks the sound barrier and causes a sharp crack—literally a sonic boom. Firearms since the 19th century have generally had a supersonic muzzle velocity. However, the sound barrier may have been first breached some 150 million years prior to the inventions of these implements. Some paleobiologists report that, based on computer models of their biomechanical capabilities, certain long-tailed dinosaurs such as apatosaurus and diplodocus may have possessed the ability to flick their tails at supersonic velocities, possibly used to generate an intimidating booming sound. This finding is theoretical and disputed by others in the field.

Early problems

The tip of the propeller on many early aircraft may reach supersonic speeds, producing a noticeable buzz that differentiates such aircraft. This is particularly noticeable on the Stearman, and noticeable on the T-6 Texan when it enters a sharp-breaking turn. This is undesirable, as the transonic air movement creates disruptive shock waves and turbulence. It is due to these effects that propellers are known to suffer from dramatically decreased performance as they approach the speed of sound. It is easy to demonstrate that the power needed to improve performance is so great that the weight of the required engine grows faster than the power output of the propeller. This problem was one of the issues that led to early research into jet engines, notably by Frank Whittle in England and Hans von Ohain in Germany, who were led to their research specifically in order to avoid these problems in high-speed flight.

Propeller aircraft were, nevertheless, able to approach the speed of sound in a dive. This led to numerous crashes for a variety of reasons. These included the rapidly increasing forces on the various control surfaces, which led to the aircraft becoming difficult to control to the point where many suffered from powered flight into terrain when the pilot was unable to overcome the force on the control stick. The Mitsubishi Zero was infamous for this problem, and several attempts to fix it only made the problem worse. In the case of the Supermarine Spitfire, the wings suffered from low torsional stiffness, and when ailerons were moved the wing tended to flex such that they counteracted the control input, leading to a condition known as control reversal. This was solved in later models with changes to the wing. The P-38 Lightning suffered from a particularly dangerous interaction of the airflow between the wings and tail surfaces in the dive that made it difficult to "pull out", a problem that was later solved with the addition of a "dive flap" that upset the airflow under these circumstances. Flutter due to the formation of shock waves on curved surfaces was another major problem, which led most famously to the breakup of de Havilland Swallow and death of its pilot, Geoffrey de Havilland, Jr.

All of these effects, although unrelated in most ways, led to the concept of a "barrier" that makes it difficult for an aircraft to break the speed of sound.

Early claims

There are, however, several claims that the sound barrier was broken during World War II. Hans Guido Mutke claimed to have broken the sound barrier on April 9, 1945 in a Messerschmitt Me 262. Mütke reported not just transonic buffeting but the resumption of normal control once a certain speed was exceeded, then a resumption of severe buffeting once the Me 262 slowed again. He also reported engine flame out. However, this claim is widely disputed by various experts believing the Me 262's structure could not support high transonic, let alone supersonic flight. The lack of area ruled fuselage and 10 percent thick wings did not prevent other aircraft from exceeding Mach 1 in dives. Chuck Yeager's Bell X1, the F-86 Sabre and the Convair Sea Dart seaplane exceeded Mach 1 without area rule fuselages. Computational tests carried out by Professor Otto Wagner of the München Technical University in 1999 suggest the Me 262 was capable of supersonic flight during steep dives. Recovering from the dive and the resumption of severe buffeting once subsonic flight was resumed would have been very likely to damage the craft terminally.

The comments about restoration of flight control and cessation of buffeting above Mach 1 are very significant in a 1946 document.

In his book Me-163, former Me-163 pilot Mano Ziegler claims that his friend, test pilot Heini Dittmar, broke the sound barrier when steep-diving the rocket plane and that several on the ground heard the sonic bangs. Heini Dittmar had been accurately and officially recorded at 1,004.5 km/h (623.8 mph) in level flight on October 2, 1941 in the prototype Me-163a V4. He reached this speed at less than full throttle as he was concerned by the transonic buffeting. The craft's Walter RII-203 rocket engine produced 7.34 kN (750 kgp / 1,650 lbf) thrust. The flight was made after a drop launch from a carrier plane to conserve fuel, a record that was kept secret until the war's end. The craft's potential performance in a powered dive is unknown but the production version of the rocket plane had an even more powerful engine. Ziegler claims that on July 6, 1944, Heini Dittmar flying a production comet Me-163BV18 VA + SP was measured traveling at a speed of 1,130 km/h. Similar claims for the Spitfire and other propeller aircraft are more suspect. It is now known that traditional airspeed gauges using a pitot tube give inaccurately high readings in the transonic, apparently due to shock waves interacting with the tube or the static source. This led to problems then known as "Mach jump".

Attempts to break the sound barrier

The first self-propelled vehicle to break the sound barrier was probably the first successful test launch of the German V-2 ballistic missile on October 3, 1942, at Peenemünde in Germany. By September 1944, the V-2s routinely achieved Mach 4 (4,900 km/h) during terminal descent.

In 1942 the United Kingdom's Ministry of Aviation began a top secret project with Miles Aircraft to develop the world's first aircraft capable of breaking the sound barrier. The project resulted in the development of the prototype Miles M.52 jet aircraft, which was designed to reach 1,000 mph (417 m/s; 1,600 km/h) at 36,000 feet (11 km) in 1 minute 30 sec.

The aircraft's design introduced many innovations which are still used on today's supersonic aircraft. The single most important development was the all-moving tailplane, giving extra control to counteract the Mach tuck which allowed control to be maintained to and beyond supersonic speeds. This was wind-tunnel tested at Mach 0.86 in 1944 in the UK. In the immediate postwar era new data from captured German records suggested that major savings in drag could be had through a variety of means such as swept wings, and Director of Scientific Research, Sir Ben Lockspeiser, decided to cancel the project in light of this new information. Later experimentation with the Miles M.52 design proved that the aircraft would indeed have broken the sound barrier, with an unpiloted 3/10 scale replica of the M.52 achieving Mach 1.5 in October 1948. By that time, the sound barrier had been broken by the Americans, and also by the British De Havilland DH 108.

Sound barrier officially broken

U.S. efforts progressed apace soon after Britain had disclosed all its research and designs to the U.S. government, on the promise that U.S. information would be shared the other way. The U.S. failed to disclose any information in return, stating the Pentagon had deemed the project Top Secret. They took the technological information provided by the British and began work on the Bell XS-1. The final version of the Bell XS-1 has many design similarities to the original Miles version. Also featuring the all-moving tail, the XS-1 was later known as the X-1. It was in the X-1 that Chuck Yeager was credited with being the first man to break the sound barrier in level flight on 14 October 1947, flying at an altitude of 45,000 ft (13.7 km). George Welch made a plausible but officially unverified claim to have broken the sound barrier on 1 October 1947, while flying an XP-86 Sabre. He also claimed to have repeated his supersonic flight on October 14, 1947, 30 minutes before Yeager broke the sound barrier in the Bell X-1. Although evidence from witnesses and instruments strongly imply that Welch achieved supersonic speed, the flights were not properly monitored and are not officially recognized. The XP-86 officially achieved supersonic speed on 26 April 1948.

On October 14, 1947, just under a month after the United States Air Force had been created as a separate service, the tests culminated in the first manned supersonic flight, piloted by Air Force Captain Charles "Chuck" Yeager in aircraft #46-062, which he had christened "Glamorous Glennis", after his wife. The rocket-powered aircraft was launched from the bomb bay of a specially modified B-29 and glided to a landing on a runway. XS-1 flight number 50 is the first one where the X-1 recorded supersonic flight, at Mach 1.06 (361 m/s, 1,299 km/h, 807.2 mph) peak speed; however, Yeager and many other personnel believe Flight #49 (also with Yeager piloting), which reached a top recorded speed of Mach 0.997 (339 m/s, 1,221 km/h), may have, in fact, exceeded Mach 1. (The measurements were not accurate to three significant figures and no sonic boom was recorded for that flight.)

As a result of the X-1's initial supersonic flight, the National Aeronautics Association voted its 1948 Collier Trophy to be shared by the three main participants in the program. Honored at the White House by President Harry S. Truman were Larry Bell for Bell Aircraft, Captain Yeager for piloting the flights, and John Stack for the NACA contributions.

Jackie Cochran was the first woman to break the sound barrier on May 18, 1953, in a Canadair Sabre, with Yeager as her wingman.

The sound barrier fades

As the science of high-speed flight became more widely understood, a number of changes led to the eventual disappearance of the "sound barrier". Among these were the introduction of swept wings, the area rule, and engines of ever increasing performance. By the 1950s many combat aircraft could routinely break the sound barrier in level flight, although they often suffered from control problems when doing so, such as Mach tuck. Modern aircraft can transition through the "barrier" without it even being noticeable.

By the late 1950s the issue was so well understood that many companies started investing in the development of supersonic airliners, or SSTs, believing that to be the next "natural" step in airliner evolution. History has proven this not to be the case, at least yet, but Concorde and the Tupolev Tu-144 both entered service in the 1970s regardless.

Although Concorde and the Tu-144 were the first aircraft to carry commercial passengers at supersonic speeds, they were not the first or only commercial airliners to break the sound barrier. On August 21, 1961, a Douglas DC-8 broke the sound barrier at Mach 1.012 or 1,240 km/h (776.2 mph) while in a controlled dive through 41,088 feet (12,510 m). The purpose of the flight was to collect data on a new leading-edge design for the wing. A China Airlines 747 almost certainly broke the sound barrier in an unplanned descent from 41,000 ft (12,500 m) to 9,500 ft (2,900 m) after an in-flight upset on February 19, 1985. It also reached over 5g.

Breaking the sound barrier on land

The sound barrier was first broken in a vehicle in a sustained way on land in 1948 by a rocket-powered test vehicle at Muroc Air Force Base (now Edwards AFB) in California, United States. It was powered by 6,000 lbs (27 kN) of thrust, reaching 1,019 mph (1,640 km/h).

On 15 October 1997, in a vehicle designed and built by a team led by Richard Noble, British driver (and Royal Air Force pilot) Andy Green became the first person to break the sound barrier in a land vehicle. The vehicle, called the ThrustSSC ("Super Sonic Car"), captured the record exactly 50 years and one day after Yeager's flight.