Flying at supersonic means that the airspeed is greater than the speed of sound in the vicinity of the aircraft . In flight physics, speeds are made dimensionless with the speed of sound and called Mach number (abbreviation M or Ma ), named after the physicist Ernst Mach . Supersonic flight means flying with Ma > 1.
Speed of sound in air
The speed of sound is defined by , where κ (kappa) is the ratio of the specific heats , the specific gas constant of the air and T is the thermodynamic temperature (measured in Kelvin ) (unit K ). It is therefore dependent on the temperature, but independent of the air pressure. The air humidity increases the product slightly , but the influence on the speed of sound is less than 1% even under tropical conditions. In dry air is and . For then there is a speed of sound of . With increasing flight altitude, the speed of sound decreases because of the lower temperatures. In the range of the usual flight altitudes above 11 km, the standard atmosphere has a temperature of . This results in a speed of sound of .
Approaches the plane of the sound speed ( Ma = 1), it is due to the compressibility of air to shock waves at various parts of the aircraft (See also compression shock ). As a result, the aerodynamic resistance (wind pressure) increases considerably until this limit, figuratively called the sound barrier , is overcome. The resistance then drops again (but remains higher than in the subsonic range). In normal operation, modern military engines provide sufficient thrust to be able to fly supersonically in level flight over the long term, which is known as supercruise . Older aircraft models require an afterburner for this or have to dive into a dive in order to be able to accelerate to supersonic speed. The airspeed at which the first supersonic areas and thus also compression shocks occur in the air flow around the aircraft is - depending on the design of the aircraft more or less significantly - below the speed of sound. If the humidity is sufficient, condensation clouds form in these areas, the rear end of which is marked by a collision ( cloud disk effect ) (see picture above). The speed range, in which areas with both higher and lower speeds of sound occur when the aircraft flows around, is called transonic and covers an airspeed range of approximately Ma = 0.8 to 1.2.
The development of the swept wing was very important for overcoming the sound barrier . This enabled the increase in resistance to be greatly reduced when approaching the speed of sound. In order to overcome the sound barrier with an airplane, however, it is usually also necessary to observe the area rule . Thereafter, the cross-section of the aircraft may only change slowly as a function of the axial position. Only when this rule was taken into account and after considerable changes to the fuselage could the YF-102A be the first aircraft to be accelerated to supersonic speed with its own engines in level flight.
The first prototype of the YF102A even broke the sound barrier on its very first flight (December 20, 1954, Lindbergh Field near San Diego) while still climbing .
Since the air does not flow away quickly enough, the dynamic pressure , in particular at the end faces, leads to a compression-related warming of the air and thus of the missile. This heating becomes so great at higher supersonic speeds that the strength of the aluminum commonly used in aircraft construction is impaired until it fails.
This shock wave has the shape of two cones, one on the aircraft nose and one on the aircraft tail. The cones open against the direction of flight. In the case of small aircraft or projectiles , these converge close enough to be perceived as a single bang; in large aircraft, the shock waves are clearly distinguishable and cause a “double bang” every few hundredths of a second (the human ear can detect very small time differences ( transit time difference )). At a great distance from the observer, the time interval between the two shock waves increases and can be several tenths of a second in large aircraft or space shuttles . The reason for this increase are slight differences in the speed of propagation of the shock waves; Unlike normal sound waves, the speed of propagation of shock waves depends on their amplitude .
Even if the bang is only perceived once at one point, there is by no means a single bang when the sound barrier is broken. The lower surface line of the cone determines the point in time when the bang reaches the recipient and he hears it, even before the perception z. B. the engine noise. In the meantime, however, the cone continues to move, which is why it reaches another receiver some distance away and hears another bang. The bang when flying at supersonic speed is only perceived by the observer after the observer has flown over it (delayed by the flight altitude, i.e. one second at 340 meters). The sound and thus the sonic boom of an object moving at supersonic speed is therefore “dragged along”.
With increasing speed, the cones become “tighter” around the aircraft, and at the same time - due to the higher energy that is transferred to the air per unit of travel - their amplitude and thus the volume of the sonic boom increase. The volume of the bang also depends on the amount of air displaced and thus on the size of the aircraft . The energy E released per distance s is included
where the drag coefficient is in the supersonic range and is mostly about twice the value in the subsonic range. Furthermore, the frontal area of the aircraft, the air density and the airspeed are relative to the surrounding air. The power released into the air is correspondingly at constant flight speed
The energy per unit of distance is decisive for the amplitude and thus for the volume of the bang, while the performance has a direct influence on fuel consumption.
At very high altitudes, the cones no longer touch the ground, but are converted into very low-frequency sound waves , and the bang is no longer perceived there ( see also: infrasound ). With very large missiles or extremely high supersonic speeds, the pressure wave can nevertheless be strong and / or concentrated enough in time that audible sound waves or even shock waves reach the ground. This is e.g. B. the case with the re-entry of space shuttles or the entry of larger meteoroids .
For these noise reasons, the Concorde normally only increased the flight speed to supersonic over uninhabited areas (usually over the open sea). The 1986 flight of a Concorde from Paris to Leipzig is a specialty . From the rotating radio beacon (VOR) Trent on Rügen to the VOR Fürstenwalde , the machine flew at supersonic speed over the area of the GDR . The GDR carried out noise measurements and transmitted the results to the French side.
On July 1, 1941, Heini Dittmar reached a speed of 1004 km / h with the Me 163 BV18 Komet VA + SP high above the Luftwaffe test site Peenemünde-West and was the first pilot to achieve a speed in the critical range during his flight Mach number reached. In Der Spiegel magazine , Dittmar was quoted as follows: "These so-called Mach phenomena, which I experienced as the first pilot, were the first knocking on the sound barrier."
In Great Britain in 1943 the forces felt by fighter pilots in extreme situations were researched with propeller planes; a Mach number of 0.9 was achieved in dives at altitudes of 12 kilometers (around 40,000 feet ). But it was also clear that higher speeds would not be achievable because the propeller generated more drag than propulsive force at these speeds. Regardless of this, engine performance would be required that can not be achieved with reciprocating piston engines .
The German fighter pilot Hans Guido Mutke claims to have broken the sound barrier on April 9, 1945 with a Messerschmitt Me 262 . However, there is no evidence to support the claim. Wolfgang Czaia, the German test pilot of the Me-262 replica project, also considers this claim to be unrealistic. As a test pilot, he has flown in both of the Me 262s that have previously been replicated in the USA and therefore knows their data and parameters very well. Pilots of the first jet aircraft realized that with the technology of the time, breaking the sound barrier was unlikely. At speeds above Mach 0.95, heavy mechanical loads occurred and the control effect was lost. In individual cases, this caused the machines to crash or break apart.
On October 1, 1947, George Welch broke the sound barrier with a prototype of the North American F-86 Saber in a 40-degree dive. However, since the speedometer was not calibrated to the appropriate altitude and there was no speed measurement from the ground, the flight was not officially rated.
On October 14, 1947, the American test pilot Chuck Yeager demonstrably broke the sound barrier in a Bell X-1 at an altitude of about 15,000 m. During the previous flight attempts he had to struggle with the shock waves and a resulting reduction in the effectiveness of the elevator . Only the idea of moving the entire horizontal stabilizer with electric motors instead of muscle power made this pioneering act possible. The fuselage of the X-1 rocket plane still had the shape of a scaled-up rifle projectile, which is aerodynamically unfavorable in aircraft. Regular supersonic flight only became possible after aircraft with swept wings were constructed and the area rule was observed .
The first jet-powered production aircraft that reached supersonic speed in a slight orbit inclination flight was a prototype of the North American F-86 Saber (XP-86 Saber, April 26, 1948), a prototype of the Convair F-102 was the first to succeed on December 20, 1954 own engines even in a slight climb. In the summer of 1953, the French Jacqueline Auriol became the first woman to fly a Dassault-Breguet Mystère supersonic. The first official FAI speed record at supersonic speed was achieved by a North American F-100 on August 20, 1955.
Military aircraft that can fly at supersonic speeds have been around since the early 1950s . Fighter planes reach Mach 2, the MiG-25 interceptor was able to reach Mach 3 for a short time and the SR-71 reconnaissance aircraft permanently. Rocket planes like the X-15 reached seven times the speed of sound, scramjets like the test missile X-43A reached just under Mach 10 (9.6). Military aircraft or scientific test missiles operating at supersonic speeds are still in use today. One of the most notable supersonic aircraft is the XB-70 . This is a supersonic bomber designed for a sustained speed of Mach 3.
On its return to earth, the space shuttle flew in the supersonic range without any propulsion (initially about 27 times the speed of sound, i.e. about 33,300 km / h).
Civilian supersonic flight
The first civilian supersonic aircraft was the Soviet TU-144 . It was the first commercial aircraft to reach double the speed of sound (2150 km / h) on May 26, 1970, but it was more of a political and technical success than an economic one. On June 3, 1973, the fourth Tu-144S ever built (the first production machine) crashed on the suburb of Goussainville at the air show in Le Bourget near Paris . Because of the high costs in flight operations, the TU-144 was decommissioned in 1978.
In contrast , the British-French Concorde , which was developed at almost the same time at high costs, successfully ran its scheduled service from 1976 to 2003 at over Mach 2. In July 2000, however, a Concorde on Air France flight 4590 crashed as a result of a (through a foreign object triggered on the runway) devastating chain reaction shortly after take-off in Gonesse near Paris. 113 people were killed in the accident. Air France and British Airways then temporarily stopped flight operations for Concorde and improved the kerosene tanks in the wings. In 2001 France and England decided after a brief resumption of flights to put the Concorde out of service. The important flight routes to the USA had long had a deficit because of resistance there. The last flight of a Concorde took place on November 26, 2003.
Other aircraft manufacturers such as Boeing with the Model 733/2707 also developed supersonic passenger aircraft during this time, but stopped their development after the success of the Concorde and in the wake of the later oil crisis. To this day there have been repeated efforts to build a further developed successor for the Concorde. However, these failed until the end due to the high development and operating costs. Another early passenger aircraft that reached supersonic speed was a Douglas DC-8 . However, this happened in the descent and the aircraft was actually not designed for it.
In 1993, Sukhoi proposed a 50-seater model at the Paris Air Show that was somewhat similar to the Sukhoi T-4 , and sought funding from Western and Arab countries.
In June 2005, France and Japan signed an agreement on the occasion of the air show in Le Bourget , according to which the two countries will provide 1.5 million euros annually in research funds for the development of a joint civilian supersonic aircraft .
There were various plans for new supersonic concepts , but orders for the 12-seater Aerion AS2 business aircraft have been in place since 2015, even if experts were very skeptical at the time. Another project for 18 passengers was called Spike S-512 .
In 2017, at the Paris Air Show in Le Bourget , the American company Boom Technology announced that it would manufacture a commercial supersonic passenger aircraft by 2023. The aircraft should be able to reach Mach 2.2 and offer space for 55 passengers. The maiden flight of the smaller test version Boom XB-1 , called Baby Boom , is scheduled for the end of 2018 . In contrast to other projects, the company works with non-refundable down payments from five major airlines.
On April 3, 2018, NASA communicated the order for around 200 million euros to the US defense company Lockheed Martin to develop a supersonic jet, the X-plane , with as little as possible, i.e. a quiet sonic boom , by the end of 2021 . A speed of 1500 km / h at an altitude of 16 km is targeted.
Without an aircraft
Felix Baumgartner is the first person who reached supersonic speed in free fall without additional propulsion. On October 14, 2012, exactly 65 years after Chuck Yeager's first supersonic flight , the 43-year-old Austrian jumped from 39 kilometers (128,100 feet; stratosphere ) as part of the Red Bull Stratos project and reached speeds of up to 1342.8 km / h (Mach 1.24).
With this jump, the extreme athlete also broke other world records (see main article ).
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