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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 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.

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.

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 travelling 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 lose about half its lift. Considering that the primary measure of a design's efficiency is the lift-to-drag ratio, this means that the aircraft has little overall gain in fuel efficiency. For this reason a considerable amount of research was put into designing a platform for sustained supersonic cruise.
  • Sonic booms. These can be reduced in effect by waiting to reach supersonic speeds until the aircraft is at high altitude and over water.
  • Damage to the ozone layer. The high altitude flight makes such damage more likely than with traditional aircraft.
  • 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.
  • 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.

Serious work on SST designs started in the mid-1950's, 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 SSTs would degrade the quality of life. Eventually Congress dropped funding for the US SST program in 1971, and all overland 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.

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 test bed 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.

Engine design has also improved, with two concepts showing promise. In the Mid Tandem Fan concept a large low-bypass turbofan engine centred in a large engine nacelle, driving a second, much larger, fan in the nacelle itself. This second fan can be controlled in speed, unlike a normal turbofan engine where the fan is connected to the turbine directly, and thereby "tuned" to different speeds and even turned off when supersonic. In the Mixed-Flow Turbofan with ejector concept, a low-bypass engine is centred in front of a long tube called the ejector, which is primarily a silencer device. The mixed-flow design does not have the advantages of the mid-tandem 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.

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. 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. 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 in the early 2000's. The most recent reports of a supersonic business jet were of a Lockheed-Martin Skunk Works project.

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.

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