Supersonic transport
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A supersonic transport (SST) is a civil aircraft designed to transport passengers at speeds greater than the speed of sound. As of 2005, there are no more SSTs used in regular commercial service. The only SST to see regular service was the Concorde, and the only other design built in quantity was the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978, and the Concorde's last flight was on 26 November 2003.
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Supersonic aircraft design
Planes designed for supersonic flight usually have a narrow fuselage and swept-back delta wings to limit the effects of turbulence at supersonic speeds.
Some aircraft have a "Coke Bottle" fuselage, based on the 'constant area rule', which means that they taper in the middle slightly. This can reduce transonic drag.
Challenges of supersonic flight
Operation costs
High fuel costs and low passenger capacity (due to the aerodynamic requirement for a narrow fuselage) have combined to make SSTs an expensive form of transportation compared with the cost of subsonic flight.
Reaching supersonic speeds requires considerable engine power to overcome wave drag, a powerful form of drag that starts at about Mach 0.8 and ends around Mach 1.2, the transonic speed range. Between these speeds drag rises about three times over that of the normal drag just below that speed. Above the transonic range the drag drops dramatically again, down to perhaps 30 to 50% higher than the drag at speeds below it. Once past this "hump", the plane is traveling perhaps twice as fast for 50% more drag, meaning that it is actually burning considerably less fuel to cover the same distance.
Offsetting this is the inefficiency of wings at high-supersonic speeds. At about Mach 2 a typical wing design will cut its lift-to-drag ratio in half. Since the aircraft has to hold its weight up, this means that the aircraft has to provide twice the thrust to maintain airspeed and altitude; so there's little or no overall gain in fuel efficiency. For this reason a considerable amount of research was put into designing a planform for sustained supersonic cruise.
Meanwhile jet engines can supply increased miles per gallon efficiency at supersonic speeds- because even though the specific impulse efficiency drops off somewhat at higher speeds, the distance traveled is greater, and the drop off is less than proportional to miles per second until well past mach 2.
The combination of all these factors is that, for example, Concorde's fuel consumption was higher than subsonic aircraft, but not stupidly so (Concorde managed very roughly 14 miles per gallon per passenger).
Sonic booms
These can be reduced in effect by waiting to reach supersonic speeds until the aircraft is at high altitude over water.
Damage to the ozone layer
The high altitude flight makes such damage theoretically more likely than with traditional aircraft. However, research showed that the comparatively tiny quantity of nitric oxides generated in the exhaust actually boosts the ozone layer.
Need to operate aircraft over a wide range of speeds
The design for aircraft needs to change with its speed for optimal performance. Thus, an SST would ideally change shape during flight to maintain optimal performance at both subsonic and supersonic speeds. Such a design would introduce complexity which increases maintenance needs, operations costs, and safety concerns.
In practice all supersonic transports have used essentially the same shape for subsonic and supersonic flight, and a compromise in performance is chosen, often to the detriment of low speed flight. For example Concorde had very high drag (lift to drag ratio of about 4) at slow speed, but it spent most of the flight at high speed.
Some designs of supersonic transports posessed swing wings, to give higher efficiency at low speeds.
Higher landing/takeoff speeds
This requires longer runways and raises safety concerns.
History
Throughout the 1950s an SST looked possible, but it was not clear whether or not it could be made economically viable. There was a good argument for supersonic speeds on medium- and long-range flights at least, where the increased speed and potential good economy once supersonic would offset the tremendous amount of fuel needed to overcome the wave drag. The main advantage appeared to be practical; these designs would be flying at least three times as fast as existing subsonic transports, and would be able to replace three planes in service, and thereby lower costs in terms of manpower and maintenance.
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Serious work on SST designs started in the mid-1950s, when the first generation of supersonic fighter aircraft were entering service. In Europe, government-subsidized SST programs quickly settled on the delta wing in most studies, including the Sud Aviation Super-Caravelle and Bristol 223, although Armstrong-Whitworth proposed a more radical design, the Mach 1.2 M-Wing. 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 Bristol and Sud eventually merged their efforts in 1962 to produce the Concorde.
This development set off a wave of panic in the US industry, where it was thought that the Concorde would soon replace all other long range designs. Congress was soon funding an SST design effort of their own, selecting the existing Lockheed L-2000 and Boeing 2707 designs, to produce an even more advanced, larger, faster and longer ranged design. The Boeing design was eventually selected for continued work. The Soviet Union set out to produce its own design, the Tu-144.
In the 1960s environmental concerns came to the fore for the first time. The SST was seen as particularly offensive due to its sonic boom and the potential for its engine exhaust to damage the ozone layer. The sonic boom was not thought to be a serious issue due to the high altitudes at which the planes flew, but experiments with the USAFs North American B-70 Valkyrie proved otherwise in the mid-1960s. Both problems found a sympathetic ear in the public, who felt that sonic booms and potential ozone layer damage from SSTs would degrade their quality of life. Eventually Congress dropped funding for the US SST program in 1971, and all overland commercial supersonic flight was banned.
Concorde was now ready for service. The US public outcry was so high that New York banned the plane outright. This destroyed the aircraft's economic prospects -- it had been built with the London-New York route in mind. However the plane was allowed into Washington, DC, and the service was so popular that New Yorkers were soon complaining that they didn't have it. It was not long before the Concorde was flying into JFK after all.
Public opinion was changing. The disaster stories about the damage SST flights could do was blown out of proportion, and the high speed ocean crossing seemed like a great idea. This started a second round of design studies in the US, under the name AST, for Advanced Supersonic Transport. Lockheed's SCV was an entirely new design for this category, while Boeing continued studies with the 2707 as a baseline.
However by this time the economics of the SST concept no longer made sense. When first designed, the SSTs were envisioned to compete with long-range aircraft seating 80 to 100 passengers, but with aircraft such as the Boeing 747 carrying four times that, the speed and fuel advantages of the SST concept were washed 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 increasing efficiencies 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 0.7, as opposed to 2.0 or higher for the subsonic designs. For both of these reasons the SST designs were doomed to higher operational costs, and the AST programs faded away by the early 1980s.
Recent developments
Two recent developments appear to alter the economics. During the original SST efforts in the 1960s it was suggested that careful shaping of the fuselage of the aircraft could cause the shock waves to interfere with each other, greatly reducing sonic boom. This was difficult to test at that time due to the careful design it required, but the increasing power of computer-aided design has since made this considerably easier. In 2003 such a testbed aircraft was flown, the Shaped Sonic Boom Demonstration which proved the soundness of the design and demonstrated the capability of reducing the boom by about a half. This may make the boom from even very large designs acceptable (see sonic boom for details).
One of the main problems with Concorde and the Tu-144 operations was the high engine noise levels, associated with very high jet velocities used during Take-off. SST engines need a fairly high specific thrust (net thrust/airflow) during supersonic cruise, to minimize engine cross-sectional area and, thereby, nacelle drag. Unfortunately this implies a high jet velocity, which makes the engines noisy at Take-off.
Therefore, a future SST requires some sort of Variable Cycle Engine, where the specific thrust (and therefore jet velocity and noise) is low at Take-off, but is forced high during Supersonic Cruise. Transition between the two modes would occur at some point during the Climb and back again during the Descent (to minimize jet noise upon Approach). The difficulty is devising a Variable Cycle Engine configuration that meets the requirement for a low cross-sectional area during Supersonic Cruise.
Two concepts show promise:-
In the Mid Tandem Fan concept a high specific flow single stage fan is located between the HP and LP compressors of a turbojet core. Only bypass air is allowed to pass through the fan, the LP compressor exit flow passing through special passages within the fan disc, directly underneath the fan rotor blades. Some of the bypass air enters the engine via an auxiliary intake. During Take-off and Approach the engine behaves much like a normal civil turbofan, with an acceptable jet noise level (i.e. low specific thrust). However, for Supersonic Cruise, the fan variable inlet guide vanes and auxiliary intake close-off to minimize bypass flow and increase specific thrust. In this mode the engine acts more like a 'leaky' turbojet (e.g. F404)
In the Mixed-Flow Turbofan with Ejector concept, a low-bypass ratio engine is mounted in front of a long tube, called an ejector. This silencer device is deployed during Take-off and Approach. Turbofan exhaust gases induce additional air into the ejector via an auxiliary air intake, thereby reducing the specific thrust/mean jet velocity of the final exhaust. The mixed-flow design does not have the advantages of the mid-tandem fan design in terms of low-speed efficiency, but is considerably simpler.
In April 1994, Aerospatiale, British Aerospace and Deutsche Aerospace AG (DASA) created the European Supersonic Research Program (ESRP) with plans for a second-generation Concorde to enter service in 2010. In parallel, SNECMA, Rolls-Royce, MTU München and Fiat started working together in 1991 on the development of a new engine. Investing no more than US$12 million per year, mainly company funded, the research program covers materials, aerodynamics, systems and engine integration for a reference configuration. The ESRP exploratory study is based on a Mach 2, 250-seat, 5,500 nautical mile-range aircraft, with the baseline design looking very much like an enlarged Concorde with canards.
Meanwhile NASA started a series of projects to study advances in the state of SST design. As part of the program a Tu-144 aircraft was re-engined in order to carry out supersonic experiments in Russia in the mid-1990s.
Although the Concorde and Tu-144 were certainly the first aircraft to carry commercial passengers at supersonic speeds, they were not the first commercial airliners to break the sound barrier. On August 21, 1961 a Douglas DC-8 broke the sound barrier at Mach 1.012 or 660 mph while in a controlled dive through 41,088 feet. The purpose of the flight was to collect data on a new leading-edge design for the wing. Boeing reports that the 747 broke the sound barrier during certification tests. A China Airlines 747 almost certainly broke the sound barrier in an unplanned descent from 41 000 feet to 9500 feet after an in-flight upset on 19 February 1985. It also pulled over 5g. [1]
Future
Japan has had a supersonic transport research program for some years. In 2005, it was announced that a Japanese-French joint venture would continue research into a design, in the hope of designing a craft that could be flying by 2015 [2]. A 11.5-meter model was successfully flight-tested in October 2005 [3]. Like all of these research projects, it remains to be seen whether such an aircraft is economically viable.
Another area that has seen research interest is the supersonic business jet (SSBJ). High-end business jet customers are prepared to pay heavily for decreased travel times and the noise issues are less serious in a smaller craft. Sukhoi investigated such a craft in the mid-1990's, as did Dassault Aviation in the early 2000's. Aerion Corporation's Aerion SBJ and Tupolev's Tu-444 are two current SSBJ projects.
Another development in the field of engines is the pulse detonation engine, which appears to be gaining support as the "next design" for aircraft engines. These engines, often referred to as PDE's, offer even greater efficiencies than current turbofan engines, while allowing for high speed use. NASA maintains a PDE research effort, with the baseline being a Mach 5 airliner.
At the most exotic, high supersonic designs like Skylon would seem to be capable of reaching Mach 5.5 within the atmosphere, before activating a rocket engine and entering orbit. The design can later reenter the atmosphere and land back on the runway it took off on.