Supersonic transport
A supersonic transport or a supersonic airliner is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound in terms of air speed. To date, the only SSTs to see regular service have been Concorde and the Tupolev Tu-144, although the Boom Technology Overture SST is expected to start service in 2029, making it the third operational SST. The last passenger flight of the Tu-144 was in June 1978 and it was last flown in 1999 by NASA. Concorde's last commercial flight was in October 2003, with a November 26, 2003, ferry flight being its last flight.
Following the termination of flying by Concorde, there have been no SSTs in commercial service. However, several companies have proposed supersonic business jet designs. Small SSTs have less environmental impact and design capability improves with continuing research which is aimed at producing an acceptable aircraft.
Supersonic airliners have been the objects of numerous ongoing design studies such as those of Boom Technology. Drawbacks and design challenges are excessive noise generation, high development costs, expensive construction materials, high fuel consumption, extremely high emissions, and an increased cost per seat over subsonic airliners. However, despite these challenges, Concorde was claimed to have operated profitably.
History
Planning
Throughout the 1950s an SST looked possible from a technical standpoint, but it was not clear if it could be made economically viable. Because of differences in lift generation, aircraft operating at supersonic speeds have approximately one-half the lift-to-drag ratio of subsonic aircraft. This implies that for any given required amount of lift, the aircraft will have to supply about twice the thrust, leading to considerably greater fuel use. This effect is pronounced at speeds close to the speed of sound, as the aircraft is using twice the thrust to travel at about the same speed. The relative effect is reduced as the aircraft accelerates to higher speeds. Offsetting this increase in fuel use was the potential to greatly increase sortie rates of the aircraft, at least on medium and long-range flights where the aircraft spends a considerable amount of time in cruise. SST designs flying at least three times as fast as existing subsonic transports were possible, and would thus be able to replace as many as three planes in service, and thereby lower costs in terms of manpower and maintenance.file:supersonic.arp.750pix.jpg|thumb|Concorde landing
Serious work on SST designs started in the mid-1950s, when the first generation of supersonic fighter aircraft were entering service. In Britain and France, government-subsidised SST programs quickly settled on the delta wing in most studies, including the Sud Aviation Super-Caravelle and Bristol Type 223, although Armstrong-Whitworth proposed a more radical design, the Mach 1.2 M-Wing. Avro Canada proposed several designs to TWA that included Mach 1.6 double-ogee wing and Mach 1.2 delta-wing with separate tail and four under-wing engine configurations. Avro's team moved to the UK where its design formed the basis of Hawker Siddeley's designs. By the early 1960s, the designs had progressed to the point where the go-ahead for production was given, but costs were so high that the Bristol Aeroplane Company and Sud Aviation eventually merged their efforts in 1962 to produce Concorde.
In the early 1960s, various executives of US aerospace companies were telling the US public and Congress that there were no technical reasons an SST could not be produced. In April 1960, Burt C Monesmith, a vice president with Lockheed, stated to various magazines that an SST constructed of steel weighing could be developed for $160 million and in production lots of 200 or more sold for around $9 million. But it was the Anglo-French development of the Concorde that set off panic in the US industry, where it was thought that Concorde would soon replace all other long range designs, especially after Pan Am took out purchase options on the Concorde. Congress was soon funding an SST design effort, selecting the existing Lockheed L-2000 and Boeing 2707 designs, to produce an even more advanced, larger, faster and longer ranged design. The Boeing 2707 design was eventually selected for continued work, with design goals of ferrying around 300 passengers and having a cruising speed near to Mach 3. The Soviet Union set out to produce its own design, the Tu-144, which the western press nicknamed the "Concordski".
Environmental concerns
The SST was seen as particularly offensive due to its sonic boom and the potential for its engine exhaust to damage the ozone layer. Both problems impacted the thinking of lawmakers, and eventually Congress dropped funding for the US SST program in and all overland commercial supersonic flight was banned over the US. Presidential advisor Russell Train warned that a fleet of 500 SSTs flying at for a period of years could raise stratospheric water content by as much as 50% to 100%. According to Train, this could lead to greater ground-level heat and hamper the formation of ozone.Later, an additional threat to the ozone was hypothesized as a result of the exhaust's nitrogen oxides, a threat that was, in 1974, seemingly validated by an MIT team commissioned by the United States Department of Transportation. However, while many purely theoretical models were indicating the potential for large ozone losses from SST nitrogen oxides, other scientists in the paper "Nitrogen Oxides, Nuclear Weapon Testing, Concorde and Stratospheric Ozone" turned to historical ozone monitoring and atmospheric nuclear testing to serve as a guide and means of comparison, observing that no detectable ozone loss was evident from approximately 213 megatons of explosive energy being released in 1962, so therefore the equivalent amount of NOx from "1047" Concordes flying "10 hours a day", would likewise, not be unprecedented. In 1981 models and observations were still irreconcilable. More recent computer models in 1995 by David W. Fahey, an atmospheric scientist at the National Oceanic and Atmospheric Administration, and others, suggest that the drop in ozone would be at most, "no more" than 1 to 2% if a fleet of 500 supersonic aircraft operated. Fahey expressed that this would not be a fatal obstacle for an advanced SST development – while "a big caution flag... should not be a showstopper for advanced SST development" because "removing the sulfur in the fuel of the " would essentially eliminate the hypothesized 1%–2% ozone-destruction-reaction-pathway.
Concorde
Despite the model-observation discrepancy surrounding the ozone concern, in the mid-1970s, six years after its first supersonic test flight, Concorde was now ready for service. The US political outcry was so high that the state of New York banned the plane. This threatened the aircraft's economic prospects — it had been built with the London–New York route in mind. The plane was allowed into Washington, D.C., and the service was so popular that New Yorkers were soon complaining because they did not have it. It was not long before Concorde was flying into JFK.Along with shifting political considerations, the flying public continued to show interest in high-speed ocean crossings. This started additional design studies in the US, under the name "AST". Lockheed's SCV was a new design for this category, while Boeing continued studies with the 2707 as a baseline.
By this time, the economics of past SST concepts were no longer reasonable. When first designed, the SSTs were envisioned to compete with long-range aircraft seating 80 to 100 passengers such as the Boeing 707, but with newer aircraft such as the Boeing 747 carrying four times that, the speed and fuel advantages of the SST concept were taken away by sheer size.
Another problem was that the wide range of speeds over which an SST operates makes it difficult to improve engines. While subsonic engines had made great strides in increased efficiency through the 1960s with the introduction of the turbofan engine with ever-increasing bypass ratios, the fan concept is difficult to use at supersonic speeds where the "proper" bypass is about 0.45, as opposed to 2.0 or higher for subsonic designs. For both of these reasons the SST designs were doomed by higher operational costs, and the AST programs vanished by the early 1980s.
Profitability
Concorde only sold to British Airways and Air France, with subsidized purchases that were to return 80% of the profits to the government. In practice for almost all of the length of the arrangement, there was no profit to be shared. After Concorde was privatized, cost reduction measures and ticket price raises led to substantial profits.Since Concorde stopped flying, it has been revealed that over the life of Concorde, the plane did prove profitable, at least to British Airways. Concorde operating costs over nearly 28 years of operation were approximately £1 billion, with revenues of £1.75 billion.
Final flights
On 25 July 2000, Air France Flight 4590 crashed shortly after take-off with all 109 occupants and four on ground killed; the only fatal incident involving Concorde. Commercial service was suspended until November 2001, and Concorde aircraft were retired in 2003 after 27 years of commercial operations.The last regular passenger flights landed at London Heathrow on October 24, 2003, from New York, a second flight from Edinburgh, and a third which had taken off from Heathrow on a loop flight over the Bay of Biscay.
By the end of the 20th century, projects like the Tupolev Tu-244, Tupolev Tu-344, SAI Quiet Supersonic Transport, Sukhoi-Gulfstream S-21, High Speed Civil Transport, etc. had not been realized.
Design
Aerodynamics
For all vehicles traveling through air, the force of drag is proportional to the coefficient of drag, to the square of the airspeed and to the air density. Since drag rises rapidly with speed, a key priority of supersonic aircraft design is to minimize this force by lowering the coefficient of drag. This gives rise to the highly streamlined shapes of SSTs. To some extent, supersonic aircraft also manage drag by flying at higher altitudes than subsonic aircraft, where the air density is lower.file:Qualitive variation of cd with mach number.png|thumb|right|Qualitative variation in Cd factor with Mach number for aircraft
As speeds approach the speed of sound, the additional phenomenon of wave drag appears. This is a powerful form of drag that begins at transonic speeds. Around Mach 1, the peak coefficient of drag is four times that of subsonic drag. Above the transonic range, the coefficient drops drastically again, although remains 20% higher by Mach 2.5 than at subsonic speeds. Supersonic aircraft must have considerably more power than subsonic aircraft require to overcome this wave drag, and although cruising performance above transonic speed is more efficient, it is still less efficient than flying subsonically.
Another issue in supersonic flight is the lift to drag ratio of the wings. At supersonic speeds, airfoils generate lift in an entirely different manner than at subsonic speeds, and are invariably less efficient. For this reason, considerable research has been put into designing wing planforms for sustained supersonic cruise. At about Mach 2, a typical wing design will cut its L/D ratio in half. Because an aircraft's design must provide enough lift to overcome its own weight, a reduction of its L/D ratio at supersonic speeds requires additional thrust to maintain its airspeed and altitude.