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The sound barrier or sonic barrier is a popular term for the sudden increase in drag and other effects experienced by an aircraft or other object when it approaches supersonic speed. When aircraft first began to be able to reach close to supersonic speed, these effects were seen as constituting a barrier making supersonic speed very difficult or impossible.
In dry air at 20 °C (68 °F), the sound barrier is reached when an object moves at a speed of 343 metres per second (1,125 ft/s). The term came into use in this sense during World War II, when a number of aircraft started to encounter the effects of compressibility, a number of several unrelated aerodynamic effects that "struck" their aircraft like an impediment to further acceleration. By the 1950s, new aircraft designs routinely "broke" the sound barrier.[N 1]
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.
The sound barrier may have been first breached by living beings some 150 million years ago. 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. Meteorites entering the Earth's atmosphere usually, if not always, descend faster than sound.
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 North American 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 can compensate. This problem was one 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.
Nevertheless, propeller aircraft were able to approach the speed of sound in a dive. Unfortunately, doing so led to numerous crashes for a variety of reasons. Most infamously, in the Mitsubishi Zero, pilots flew full power into the terrain because the rapidly increasing forces acting on the control surfaces of their aircraft overpowered them. In this case, several attempts to fix it only made the problem worse. Likewise, the flexing caused by the low torsional stiffness of the Supermarine Spitfire's wings caused them, in turn, to counteract aileron control inputs, leading to a condition known as control reversal. This was solved in later models with changes to the wing. Worse still, a particularly dangerous interaction of the airflow between the wings and tail surfaces of diving Lockheed P-38 Lightnings made "pulling out" of dives difficult; however, the problem was later solved by 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. in 1946. A similar problem is thought to be the cause of the 1943 crash of the BI-1 rocket aircraft in the Soviet Union.
All of these effects, although unrelated in most ways, led to the concept of a "barrier" that makes it difficult for an aircraft to exceed the speed of sound.[N 2]
A powered flight faster than sound (600–680 meters/second (Mach 2) may have been achieved by the Soviet phosphorus-powered ramjet experiments of Yuri Pobedonostsev in 1933.
Another early 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 (1,200 m/s, or 3044 mph) during terminal descent.
There are, however, several claims that the sound barrier was broken during World War II by manned aircraft. In 1942, Republic Aviation issued a press release stating that Lts. Harold E. Comstock and Roger Dyar had exceeded the speed of sound during test dives in the P-47 Thunderbolt. Hans Guido Mutke claimed to have broken the sound barrier on 9 April 1945 in an early German jet aircraft, the Messerschmitt Me 262. Mutke 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 X-1, the North American F-86 Sabre (with Me 262 profile ) and the Convair Sea Dart seaplane exceeded Mach 1 without area-rule fuselages. Computational tests carried out by Professor Otto Wagner of the Munich 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.
Speeds of 950 km/h (590 mph) are reported to have been attained in a shallow dive 20° to 30° from the horizontal. No vertical dives were made. At speeds of 950 to 1,000 km/h (590 to 620 mph) the air flow around the aircraft reaches the speed of sound, and it is reported that the control surfaces no longer affect the direction of flight. The results vary with different airplanes: some wing over and dive while others dive gradually. It is also reported that once the speed of sound is exceeded, this condition disappears and normal control is restored.
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 Messerschmitt Me 163 "Komet" pilot Mano Ziegler claims that his friend, test pilot Heini Dittmar, broke the sound barrier when steep diving the rocket plane and that several people on the ground heard the sonic booms. 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 163 V4. He reached this speed at less than full throttle, as he was concerned by the transonic buffeting. The craft's Walter RII-203 cold 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 Me 163B test version of the series rocket plane had an even more powerful engine (HWK 109-509 A-2) and a greater wing sweep than the Me 163A. Ziegler claims that on July 6, 1944, Heini Dittmar, flying a test Me 163 B V18 VA + SP, was measured traveling at a speed of 1,130 km/h (702 mph).
The Luftwaffe test pilot Lothar Sieber (April 7, 1922 - March 1, 1945) may have inadvertently became the first man to break the sound barrier on 1 March 1945. This occurred while he was piloting a Bachem Ba 349 "Natter" for the first manned vertical takeoff of a rocket in history. In 55 seconds, he traveled a total of 14 km (8.7 miles). Unfortunately, there was a crash and he perished violently in this endeavor. Very little of his remains were found in the 15 ft deep crater, but he did receive a funeral with full military honors.
One of the highest recorded instrumented Mach Numbers attained for a propeller aircraft is the Mach 0.891 for a Spitfire PR XI, flown during dive tests at the Royal Aircraft Establishment, Farnborough in April 1944. The Spitfire, a photo-reconnaissance variant, the Mark XI, fitted with an extended 'rake type' multiple pitot system, was flown by Squadron Leader J. R. Tobin to this speed, corresponding to a corrected true airspeed (TAS) of 606 mph. A subsequent flight, flown by Sqn Ldr, Anthony Martindale, achieved Mach 0.92 but after engine overspeeding, and damage to the engine, resulted in a forced landing.
Similar claims for 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 regime, apparently due to shock waves interacting with the tube or the static source. This led to problems then known as "Mach jump".
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 turbojet powered aircraft, which was designed to reach 1,000 mph (417 m/s; 1,600 km/h) (over twice the existing speed record) in level flight, and to climb to an altitude of 36,000 ft (11 km) in 1 minute 30 sec.
Although the project was eventually cancelled, the research was used to construct an unmanned missile that went on to achieve a speed of Mach 1.38 in a successful, controlled transonic and supersonic level test flight; a unique achievement at that time which validated the aerodynamics of the M.52.
A huge number of advanced features were incorporated into the resulting M.52 design, many of which hint at a detailed knowledge of supersonic aerodynamics. In particular, the design featured a conical nose and sharp wing leading edges, as it was known that round-nosed projectiles could not be stabilised at supersonic speeds. The design used very thin wings of biconvex section proposed by Jakob Ackeret for low drag. The wing tips were "clipped" to keep them clear of the conical shock wave generated by the nose of the aircraft. The fuselage had the minimum cross-section allowable around the centrifugal engine with fuel tanks in a saddle over the top.
Another critical addition was the use of a power operated stabilator, also known as the all-moving tail or flying tail, a key to supersonic flight control which contrasted with traditional hinged tailplanes (horizontal stabilizers) connected mechanically to the pilots control column. Conventional control surfaces became ineffective at the high subsonic speeds then being achieved by fighters in dives, due to the aerodynamic forces caused by the formation of shockwaves at the hinge and the rearward movement of the centre of pressure, which together could override the control forces that could be applied mechanically by the pilot, hindering recovery from the dive. A major impediment to early transonic flight was control reversal, the phenomenon which caused flight inputs (stick, rudder) to switch direction at high speed; it was the cause of many accidents and near-accidents. An all-flying tail is considered to be a minimum condition of enabling aircraft to break the transonic barrier safely, without losing pilot control. The Miles M.52 was the first instance of this solution, and has since been universally applied.
Initially, the aircraft was to use Frank Whittle's latest engine, the Power Jets W.2/700, which would only reach supersonic speed in a shallow dive. To develop a fully supersonic version of the aircraft a new innovation was incorporated; a reheat jetpipe - also known as an afterburner. Extra fuel was to be burned in the tailpipe to avoid overheating the turbine blades, making use of unused oxygen in the exhaust. Finally the design included another critical element, the use of a shock cone in the nose to slow the incoming air to the subsonic speeds needed by the engine.
The British Air Ministry signed an agreement with the United States to exchange all its high-speed research, data and designs and Bell Aircraft company was given access to the drawings and research on the M.52, but the U.S. reneged on the agreement and no data was forthcoming in return. Bell's supersonic design was still using a conventional tail and they were battling the problem of control.
They utilized the information to initiate work on the Bell X-1. The final version of the Bell X-1 was very similar in design to the original Miles M.52 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 October 14, 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 April 26, 1948.
On 14 October 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. 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.
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 transit 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 yet to be the case. Although the Concorde and the Tupolev Tu-144 entered service in the 1970s, both have since been retired. The last flight of a Concorde in service was in 2003.
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 21 August 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 may have broken 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 19 February 1985. It also reached over 5g.
On January 12, 1948, a Northrop unmanned rocket sled became the first land vehicle to break the sound barrier. At a military test facility at Muroc Air Force Base (now Edwards AFB), California, it reached a peak speed of 1,019 mph (1,640 km/h) before jumping the rails. 
On October 15, 1997, in a vehicle designed and built by a team led by Richard Noble, Royal Air Force pilot Andy Green became the first person to break the sound barrier in a land vehicle in compliance with Fédération Internationale de l'Automobile rules. The vehicle, called the ThrustSSC ("Super Sonic Car"), captured the record 50 years and one day after Yeager's first supersonic flight.
In January 2010, it was reported that Felix Baumgartner was working with a team of scientists and sponsor Red Bull to attempt the highest sky-dive on record. The project would see Baumgartner attempt to jump 120,000 ft (36,580 m) from a helium balloon and become the first parachutist to break the sound barrier. The launch was scheduled for October 9, 2012, but was aborted due to adverse weather; subsequently the capsule was launched instead on October 14. Baumgartner's feat also marked the 65th anniversary of U.S. test pilot Chuck Yeager's successful attempt to become the first man to officially break the sound barrier in an aircraft.
Baumgartner landed in eastern New Mexico after jumping from a world record 128,100 feet (39,045 m), or 24.26 miles, and broke the sound barrier as he traveled at speeds up to 833.9 mph (1342 km/h or Mach 1.26). In the press conference after his jump, it was announced he was in freefall for 4 minutes, 18 seconds, the second longest freefall after the 1960 jump of Joseph Kittinger for 4 minutes, 36 seconds.
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