Supersonic transport

The Concorde supersonic transport had an ogival delta wing, a slender fuselage and four underslung Rolls-Royce/Snecma Olympus 593 engines. G-BOAF was the last of the British Concordes to be completed and the last to fly in 2003.

Tupolev Tu-144 was the first SST to enter service and the first to leave it. Only 55 passenger flights were carried out before service ended due to safety concerns. A small number of cargo and test flights were also carried out after its retirement.

A supersonic transport (SST) is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. To date, the only SSTs to see regular service have been Concorde and the Tupolev Tu-144. The last passenger flight for 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 airborne operation. Following the permanent cessation of flying by Concorde, there are no remaining SSTs in commercial service.

Supersonic airliners have been the objects of numerous recent and ongoing design studies. Drawbacks and design challenges are excessive noise generation (at takeoff and due to sonic booms during flight), high development costs, expensive construction materials, great weight, and an increased cost per seat over subsonic airliners. Despite these challenges, Concorde was operated profitably in a niche market for over 27 years.

History

Throughout the 1950s an SST looked possible from a technical standpoint, but it was not clear if it could be made economically viable. Lift is generated using different means at supersonic speeds, and these methods are considerably less efficient than subsonic methods, with approximately one-half the lift-to-drag ratio. 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.

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-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. 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 250,000 pounds 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."

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 1971, and all overland commercial supersonic flight was banned. Presidential adviser Russell Train warned that a fleet of 500 SSTs flying at 65,000 ft. 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 found in the exhaust's nitrogen oxides, a threat that was, in 1974, seemingly validated by MIT. More recent analysis in 1995 by David W. Fahey, an atmospheric scientist at the National Oceanic and Atmospheric Administration, and others, found that the drop in ozone would be no more than 1 to 2% if a fleet of 500 supersonic aircraft was operated. Fahey expressed that this would not be a fatal obstacle for an advanced SST development - while "a big caution flag...[it] should not be a showstopper for advanced SST development.".

Nevertheless, in the mid-1970s, Concorde was now ready for service. The US political outcry was so high that New York banned the plane. This destroyed 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" (Advanced Supersonic Transport). 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 no longer made sense. 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 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 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.

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 privatised, cost reduction measures (notably the closing of the metallurgical wing testing site which had done enough temperature cycles to validate the aircraft through to 2010) 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.

The last regular passenger flights landed at London Heathrow Airport on Friday, October 24, 2003, just past 4 p.m.: Flight 002 from New York, a second flight from Edinburgh, Scotland, and the 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 realised.

Realised supersonic airliners

The Sinsheim Auto & Technik Museum in Germany is the only location where both Concorde and the Tu-144 are displayed together.

Concorde

Main article: Concorde aircraft histories

In total, 20 Concordes were built, including two prototypes, two pre-production aircraft and 16 production aircraft. Of the sixteen aircraft, two did not enter commercial service and eight were in service as of April 2003. All but two of these aircraft, a remarkably high percentage for any commercial fleet, are preserved; the two that are not preserved are F-BVFD (cn 211), parked as a spare-parts source in 1982 and scrapped in 1994, and F-BTSC (cn 203), which crashed in Paris on July 25, 2000.

Tupolev Tu-144

A total of sixteen airworthy Tupolev Tu-144s were built; a seventeenth Tu-144 (reg. 77116) was never completed. There was also at least one ground test airframe for static testing in parallel with the prototype 68001 development.

Challenges of supersonic passenger flight

Aerodynamics

For all vehicles traveling through air, the force of drag is proportional to the coefficient of drag (C_d), 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 SST. To some extent, supersonic aircraft also manage drag by flying at higher altitudes than subsonic aircraft, where the air density is lower.

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 0.88). Around Mach 1, the peak coefficient of drag is four times that of subsonic drag. Above the transonic range, the coefficient drops dramatically 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 (L/D 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 (e.g., Concorde managed a ratio of 7.14, whereas the subsonic Boeing 747 has an L/D ratio of 17). 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.

Engines

Jet engine design shifts significantly between supersonic and subsonic aircraft. Jet engines, as a class, can supply increased fuel efficiency at supersonic speeds, even though their specific fuel consumption is greater at higher speeds. Because their speed over the ground is greater, this decrease in efficiency is less than proportional to speed until well above Mach 2, and the consumption per unit distance is lower.

British Airways Concorde at Filton Aerodrome, Bristol, England shows the slender fuselage necessary for supersonic flight

When Concorde was being designed by Aérospatiale–BAC, high bypass jet engines ("turbofan" engines) had not yet been deployed on subsonic aircraft. Had Concorde entered service against earlier designs like the Boeing 707 or de Havilland Comet, it would have been much more competitive. When these high bypass jet engines reached commercial service in the 1960s, subsonic jet engines immediately became much more efficient, closer to the efficiency of turbojets at supersonic speeds. One major advantage of the SST disappeared.

Turbofan engines improve efficiency by increasing the amount of cold low-pressure air they accelerate, using some of the energy normally used to accelerate hot air in the classic non-bypass turbojet. The ultimate expression of this design is the turboprop, where almost all of the jet thrust is used to power a very large fan – the propeller. The efficiency curve of the fan design means that the amount of bypass that maximizes overall engine efficiency is a function of forward speed, which decreases from propellers, to fans, to no bypass at all as speed increases. Additionally, the large frontal area taken up by the low-pressure fan at the front of the engine increases drag, especially at supersonic speeds, and means the bypass ratios are much more limited than on subsonic aircraft.

For example, the early Tu-144S was fitted with a low bypass turbofan engine which was much less efficient than Concorde's turbojets in supersonic flight. The later TU-144D featured turbojet engines with comparable efficiency. These limitations meant that SST designs were not able to take advantage of the dramatic improvements in fuel economy that high bypass engines brought to the subsonic market, but they were already more efficient than their subsonic turbofan counterparts.

Structural issues

Supersonic vehicle speeds demand narrower wing and fuselage designs, and are subject to greater stresses and temperatures. This leads to aeroelasticity problems, which require heavier structures to minimize unwanted flexing. SSTs also require a much stronger (and therefore heavier) structure because their fuselage must be pressurized to a greater differential than subsonic aircraft, which do not operate at the high altitudes necessary for supersonic flight. These factors together meant that the empty weight per seat of Concorde is more than three times that of a Boeing 747.

However, Concorde and the TU-144 were both constructed of conventional aluminum (Hiduminium in Concorde's case), and (duralumin), whereas more modern materials such as carbon fibre and Kevlar are much stronger in tension for their weight (important to deal with pressurization stresses) as well as being more rigid. As the per-seat weight of the structure is much higher in an SST design, any improvements will lead to a greater percentage improvement than the same changes in a subsonic aircraft.

High costs

Concorde fuel efficiency comparison

Aircraft	Concorde	Boeing 747-400
passenger miles/imperial gallon	17	109
passenger miles/US gallon	14	91
litres/passenger 100 km	16.6	3.1

Higher fuel costs and lower passenger capacities due to the aerodynamic requirement for a narrow fuselage make SSTs an expensive form of commercial civil transportation compared with subsonic aircraft. For example, the Boeing 747 can carry more than three times as many passengers as Concorde while using approximately the same amount of fuel.

Nevertheless, fuel costs are not the bulk of the price for most subsonic aircraft passenger tickets. For the transatlantic business market that SST aircraft were utilized for, Concorde was actually very successful, and was able to sustain a higher ticket price. Now that commercial SST aircraft have stopped flying, it has become clearer that Concorde made substantial profit for British Airways.

Takeoff noise

One of the problems with Concorde and the Tu-144's operation was the high engine noise levels, associated with very high jet velocities used during take-off, and even more importantly flying over communities near the airport. 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 which causes problems particularly at low speeds/altitudes and at take-off.

Therefore, a future SST might well benefit from a 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.

Sonic boom

The sonic boom was not thought to be a serious issue due to the high altitudes at which the planes flew, but experiments in the mid-1960s such as the controversial Oklahoma City sonic boom tests and studies of the USAF's North American XB-70 Valkyrie proved otherwise.

The annoyance of a sonic boom can be avoided by waiting until the aircraft is at high altitude over water before reaching supersonic speeds; this was the technique used by Concorde. However, it precludes supersonic flight over populated areas. Supersonic aircraft have poor lift/drag ratios at subsonic speeds as compared to subsonic aircraft (unless technologies such as Variable-sweep wings are employed), and hence burn more fuel, which results in their use being economically disadvantageous on such flight paths.

Concorde had an overpressure of 1.94 lb/ft^2. Overpressures over 1.5 lb/ft^2 often cause complaints.

If the intensity of the boom can be reduced, then this may make even very large designs of supersonic aircraft acceptable for overland flight (see sonic boom). Research suggests that changes to the nose cone and tail can reduce the intensity of the sonic boom below that needed to cause complaints. During the original SST efforts in the 1960s, it was suggested that careful shaping of the fuselage of the aircraft could reduce the intensity of the sonic boom's shock waves that reach the ground. One design caused the shock waves to interfere with each other, greatly reducing sonic boom. This was difficult to test at the time, but the increasing power of computer-aided design has since made this considerably easier. In 2003, a Shaped Sonic Boom Demonstration aircraft was flown which proved the soundness of the design and demonstrated the capability of reducing the boom by about half. Even lengthening the vehicle (without significantly increasing the weight) would seem to reduce the boom intensity.

Need to operate aircraft over a wide range of speeds

The aerodynamic design of a supersonic 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 (a lift to drag ratio of about 4) at slow speed, but it travelled at high speed for most of the flight. Designers of Concorde were forced to spend a massive 5000 hours optimizing the vehicle shape in wind tunnel tests to maximise the overall performance over the entire flightplan.

The Boeing 2707 featured swing wings to give higher efficiency at low speeds, but the increased space required for such a feature produced capacity problems that proved ultimately insurmountable.

North American Aviation had an unusual approach to this problem with the XB-70 Valkyrie. By lowering the outer panels of the wings at high Mach numbers, they were able to take advantage of compression lift on the underside of the aircraft. This improved the L/D ratio by about 30%.

Skin temperature

As a supersonic aircraft flies, it adiabatically compresses the air in front of the vehicle. This causes an increase in the temperature of the air resulting in heating of the aircraft.

Normal subsonic aircraft are traditionally made of aluminium. However aluminium, while being light and strong, is not able to withstand temperatures much over 127 °C; above 127 °C the aluminium gradually loses its temper and is weakened. For aircraft that fly at Mach 3, materials such as stainless steel (XB-70 Valkyrie) or titanium (SR-71, Sukhoi T-4) have been used, at considerable increase in expense, as the properties of these materials make the aircraft much more difficult to manufacture.

Poor range

The range of supersonic aircraft can be estimated with the Breguet range equation.

The high per-passenger takeoff weight makes it difficult to obtain a good fuel fraction. This issue, along with the challenge presented by supersonic lift/drag ratios, greatly limits the range of supersonic transports. Because long distance routes were not a viable option, airlines had little interest in buying the jets.

Airline desirability of SSTs

Aeroflot Tupolev Tu-144 at the Paris Air Show in 1975.

Airlines buy aircraft as a means of making money, and wish to make as much return on investment as possible from their assets.

Airlines potentially value very fast aircraft, because it enables the aircraft to make more flights per day, providing a higher return on investment. However, Concorde's high noise levels around airports, time zone issues, and insufficient speed meant that only a single return trip could be made per day, so the extra speed was not an advantage to the airline other than as a selling feature to its customers. The American SSTs were intended to fly at Mach 3, partly for this reason. However, allowing for acceleration and deceleration time, a trans-Atlantic trip would not be 3 times faster.

Since SSTs produce sonic booms at supersonic speeds they are rarely permitted to fly supersonic over land, and must fly supersonic over sea instead. Since they are inefficient at subsonic speeds compared to subsonic aircraft, range is deteriorated and the number of routes that the aircraft can fly non-stop is reduced. This also reduces the desirability of such aircraft for most airlines.

Supersonic aircraft have higher per-passenger fuel consumption than subsonic aircraft; this makes the ticket price more sensitive to the price of oil.

Making investment for research and development work to design a new SST can be thought as an effort to push the speed limit of air transport. Generally, other than an urge for a technological achievement, the major driving force for such an effort is competition from other modes of transport. Competition between different service providers within a mode of transport does not typically lead to such technological investments to increase the speed. Instead, the service providers prefer to compete in service quality and cost. An example of this phenomenon is high-speed rail. The speed limit of rail transport had been pushed so hard to enable it to effectively compete with road and air transport. But this achievement was not done for different rail operating companies to compete between themselves. This phenomenon also reduces the airline desirability of SSTs, because, in very long distances (a couple of thousands of kilometers), competition between different modes of transport is rather like a single-horse race: air transport does not have a significant competitor. The only competition is between the airline companies, and they would rather pay to reduce cost and increase service quality than an expensive speed increase.

Under development

The desire for a second-generation supersonic aircraft has remained within some elements of the aviation industry, and several concepts emerged quickly following the retirement of Concorde.

In November 2003, EADS—the parent company of Airbus—announced that it was considering working with Japanese companies to develop a larger, faster replacement for Concorde. In October 2005, JAXA, the Japan Aerospace eXploration Agency, undertook aerodynamic testing of a scale model of an airliner designed to carry 300 passengers at Mach 2 (working name NEXST). If pursued to commercial deployment, it would be expected to be in service around 2020–25.

In May 2008, it was reported that Aerion Corporation had $3 billion of pre-order sales on its Aerion SBJ supersonic business jet. In late 2010, the project continued with a testbed flight of a section of the wing.

A conceptual design by Lockheed Martin, as presented to NASA Aeronautics Research Mission Directorate in April 2010.

Supersonic Aerospace International's Quiet Supersonic Transport is a 12 passenger design from Lockheed Martin that is to cruise at Mach 1.6, and is to create a sonic boom only 1% as strong as that generated by Concorde.

In the 21st century some supersonic airliners and business jets (Aerion AS2, HyperMach SonicStar, Next Generation Supersonic Transport, Tupolev Tu-444, Gulfstream X-54, LAPCAT, Reaction Engines A2, Spike S-512, Zero Emission Hyper Sonic Transport) were under development.

Hypersonic transport

See also: Hypersonic speed, Hypersonic flight, Reaction Engines A2, and Scramjet

While conventional turbo and ramjet engines are able to remain reasonably efficient up to Mach 5.5, some ideas for very high-speed flight above Mach 6 are also sometimes discussed, with the aim of reducing travel times down to one or two hours anywhere in the world.

These vehicle proposals very typically either use rocket or scramjet engines; pulse detonation engines have also been proposed.

There are many difficulties with such flight, both technical and economic.

Rocket-engined vehicles, while technically practical (either as ballistic transports or as semiballistic transports using wings), would use a very large amount of propellant and operate best at speeds between about Mach 8 and orbital speeds. Rockets compete best with air-breathing jet engines on cost at very long range; however, even for antipodal travel, costs would be only somewhat lower than orbital launch costs.

Scramjets currently are not practical for passenger-carrying vehicles due to technological limitations.

Precooled jet engines are jet engines with a heat exchanger at the inlet that cools the air at very high speeds. These engines may be practical and efficient at up to about Mach 5.5, and this is an area of research in Europe and Japan.

On 18 June 2011, the Zero Emission High Speed Transport or ZEHST concept hypersonic aircraft was unveiled by EADS at the Paris Air Show. The ZEHST, a hypersonic aircraft to be capable of 4,800 km/h (2,600 kn; 3,000 mph), is a result of the collaboration efforts between EADS and Japan.

The British company Reaction Engines Limited, with 50% EU money, has been engaged in a research programme called LAPCAT, which examined a design for a hydrogen-fuelled plane carrying 300 passengers called the A2, potentially capable of flying at Mach 5+ non-stop from Brussels to Sydney in 4.6 hours. The follow-on research effort, LAPCAT II began in 2008 and was to last four years.

Also the German SpaceLiner project of suborbital hypersonic winged passenger spaceplane is under preliminary development.

See also

• Supercruise

External links

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• Advanced Concept Studies for Supersonic Commercial Transports Entering Service in the 2018 to 2020 Period: Phase I Final Report National Aeronautics and Space Administration
• . Today supersonic flights are possible in military jet fighters only.
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