Supersonic transport - AbsoluteAstronomy.com

A supersonic transport (SST) is a civilian supersonic aircraft

Supersonic aircraft

A supersonic aircraft is designed to exceed the speed of sound in at least some of its normal flight configurations.-Overview:The great majority of supersonic aircraft today are military or experimental aircraft...

designed to transport passengers at speeds greater than the speed of sound

Speed of sound

The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

. The only SSTs to see regular service to date have been Concorde

Concorde

Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...

and the Tupolev Tu-144

Tupolev Tu-144

The Tupolev Tu-144 was a Soviet supersonic transport aircraft and remains one of only two SSTs to enter commercial service, the other being the Concorde...

. The last passenger flight of the Tu-144 was in June 1978 with its last ever flight in 1999 (as utilized by NASA). Concorde's last commercial flight was in October 2003, with a November 26, 2003 ferry flight being the last ever airborne operation for the model. Following the permanent cessation of flying by Concorde, there are no SSTs in commercial service.

Supersonic airliners' greater speed and efficiency over their conventional counterparts have made them objects of numerous recent and ongoing design studies. Drawbacks and design challenges are excessive noise generation (due to sonic boom

Sonic boom

A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion...

s), 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. 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-1950s, when the first generation of supersonic fighter aircraft

Fighter aircraft

A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

were entering service. In Europe, government-subsidized SST programs quickly settled on the delta wing

Delta wing

The delta wing is a wing planform in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta .-Delta-shaped stabilizers:...

in most studies, including the Sud Aviation Super-Caravelle

Sud Aviation Super-Caravelle

The Sud Aviation Super-Caravelle was an early design for a supersonic transport. Unlike most competing designs which envisioned larger trans-Atlantic aircraft and led to the likes of the Boeing 2707, the Super-Caravelle was a much smaller, shorter range design intended to replace their earlier and...

and Bristol 223

Bristol 223

The Bristol Aeroplane Company Type 223 was an early design for a supersonic transport. In the late 1950s and early 1960s the company studied a number of models as part of a large British inter-company effort funded by the government. These models eventually culminated in the Type 223, a...

, although Armstrong-Whitworth proposed a more radical design, the Mach 1.2 M-Wing. Avro Canada

Avro Canada

Commonly known as Avro Canada, this company started in 1945 as an aircraft plant and became within thirteen years the third-largest company in Canada, one of the largest 100 companies in the world, and directly employing over 50,000...

proposed several designs to TWA

Twa

The Twa are any of several hunting peoples of Africa who live interdependently with agricultural Bantu populations, and generally hold a socially subordinate position: They provide the farming population with game in exchange for agricultural products....

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 Bristol and Sud eventually merged their efforts in 1962 to produce Concorde.

This development set off panic in the US industry, where it was thought that Concorde would soon replace all other long range designs. Congress was soon funding an SST design effort, selecting the existing Lockheed L-2000

Lockheed L-2000

The Lockheed L-2000 was Lockheed Corporation's entry in a government-funded competition to build the United States' first supersonic transport in the 1960s. The L-2000 lost the contract to the Boeing 2707, but that competing design was ultimately canceled for political, environmental and economic...

and Boeing 2707

Boeing 2707

The Boeing 2707 was developed as the first American supersonic transport . After winning a competition for a government-funded contract to build an American SST, Boeing began development at its facilities in Seattle, Washington...

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, which was nicknamed the "Concordski."

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

Sonic boom

and the potential for its engine exhaust to damage the ozone layer

Ozone layer

The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...

. 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.

Concorde was now ready for service. The US political outcry was so high that New York

New York

New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

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

John F. Kennedy International Airport

John F. Kennedy International Airport is an international airport located in the borough of Queens in New York City, about southeast of Lower Manhattan. It is the busiest international air passenger gateway to the United States, handling more international traffic than any other airport in North...

.

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

Boeing 707

The Boeing 707 is a four-engine narrow-body commercial passenger jet airliner developed by Boeing in the early 1950s. Its name is most commonly pronounced as "Seven Oh Seven". The first airline to operate the 707 was Pan American World Airways, inaugurating the type's first commercial flight on...

, but with newer aircraft such as the Boeing 747

Boeing 747

The Boeing 747 is a wide-body commercial airliner and cargo transport, often referred to by its original nickname, Jumbo Jet, or Queen of the Skies. It is among the world's most recognizable aircraft, and was the first wide-body ever produced...

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

Turbofan

The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...

engine with ever-increasing bypass ratio

Bypass ratio

The term bypass ratio relates to the design of turbofan engines, commonly used in aviation. It is defined as the ratio between the mass flow rate of air drawn in by the fan bypassing the engine core to the mass flow rate passing through the engine core....

s, 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 Heathrow Airport on Friday, October 24, 2003 just past 4 p.m. – Flight 002 from New York, one from Edinburgh, Scotland, and the third which had taken off from Heathrow on a loop flight over the Bay of Biscay.

Aircraft histories

Concorde

In total, 20 Concordes were built, including two prototypes, two pre-production aircraft and 16 production aircraft. Of the 16 aircraft, two did not enter commercial service and 8 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

Air France Flight 4590

Air France Flight 4590 was a Concorde flight operated by Air France which was scheduled to run from Charles de Gaulle International Airport near Paris, to John F. Kennedy International Airport in New York City. On 2000, it crashed in Gonesse, France. All one hundred passengers and nine crew...

on July 25, 2000.

Tupolev Tu-144

A total of 16 airworthy 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.

Aerodynamics

For all vehicles traveling through air, the force of drag

Drag (physics)

In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

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.
As speeds approach the speed of sound, the additional phenomenon of wave drag

Wave drag

In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.- Overview :...

appears. This is a powerful form of drag that begins at transonic

Transonic

Transonic speed is an aeronautics term referring to the condition of flight in which a range of velocities of airflow exist surrounding and flowing past an air vehicle or an airfoil that are concurrently below, at, and above the speed of sound in the range of Mach 0.8 to 1.2, i.e. 600–900 mph...

speeds (around Mach 0.8

Mach number

Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

). Around Mach 1, the peak coefficient of drag can be up to four times that of subsonic drag. Above the transonic range, the coefficient drops dramatically again, although it remains 30 to 50% higher 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

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 planform

Planform

In aviation, a planform is the shape and layout of a fixed-wing aircraft's fuselage and wing. Of all the myriad planforms used, they can typically be grouped into those used for low-speed flight, found on general aviation aircraft, and those used for high-speed flight, found on many military...

s for sustained supersonic cruise. At about Mach 2, a typical wing design will cut its L/D ratio in half (e.g., the Concorde

Concorde

vehicle 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

A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

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 mile is lower.

When Concorde was being designed by Aérospatiale

Aérospatiale

Aérospatiale was a French aerospace manufacturer that built both civilian and military aircraft, rockets and satellites. It was originally known as Société Nationale Industrielle Aérospatiale...

–BAC

British Aircraft Corporation

The British Aircraft Corporation was a British aircraft manufacturer formed from the government-pressured merger of English Electric Aviation Ltd., Vickers-Armstrongs , the Bristol Aeroplane Company and Hunting Aircraft in 1960. Bristol, English Electric and Vickers became "parents" of BAC with...

, high bypass jet engines ("turbofan

Turbofan

" engines) had not yet been deployed on subsonic aircraft. Had the Concorde competed against earlier designs like the Boeing 707

Boeing 707

or de Havilland Comet

De Havilland Comet

The de Havilland DH 106 Comet was the world's first commercial jet airliner to reach production. Developed and manufactured by de Havilland at the Hatfield, Hertfordshire, United Kingdom headquarters, it first flew in 1949 and was a landmark in aeronautical design...

, 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

Turboprop

A turboprop engine is a type of turbine engine which drives an aircraft propeller using a reduction gear.The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller...

, where almost all of the jet thrust is used to power a very large fan - the propeller

Propeller

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...

. 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

Aeroelasticity

Aeroelasticity is the science which studies the interactions among inertial, elastic, and aerodynamic forces. It was defined by Arthur Collar in 1947 as "the study of the mutual interaction that takes place within the triangle of the inertial, elastic, and aerodynamic forces acting on structural...

problems, which require heavier structures to minimize unwanted flexing. SSTs also require a much stronger (and therefore heavier) structure because their fuselage

Fuselage

The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...

must be pressurized

Pressurization

Pressurization is the application of pressure in a given situation or environment; and more specifically refers to the process by which atmospheric pressure is maintained in an isolated or semi-isolated atmospheric environment .-See also:* Cabin pressurization* Compressed air* Decompression* Gas...

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 (duralumin

Duralumin

Duralumin is the trade name of one of the earliest types of age-hardenable aluminium alloys. The main alloying constituents are copper, manganese, and magnesium. A commonly used modern equivalent of this alloy type is AA2024, which contains 4.4% copper, 1.5% magnesium, 0.6% manganese and 93.5%...

), whereas more modern materials such as carbon fibre and Kevlar

Kevlar

Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed at DuPont in 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires...

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

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 the 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 and sonic booms

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

Nacelle

The nacelle is a cover housing that holds engines, fuel, or equipment on an aircraft. In some cases—for instance in the typical "Farman" type "pusher" aircraft, or the World War II-era P-38 Lightning—an aircraft's cockpit may also be housed in a nacelle, which essentially fills the...

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

Variable Cycle Engine

A variable cycle engine is an engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic.The next generation of Supersonic transport may require some form of VCE...

, 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.

The sonic boom

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 Oklahoma City sonic boom tests

Oklahoma City sonic boom tests

The Oklahoma City sonic boom tests, also known as Operation Bongo II, refer to a controversial experiment in which 1,253 sonic booms were carried out over Oklahoma City, Oklahoma over a period of six months in 1964...

and studies of the USAF's North American XB-70 Valkyrie

XB-70 Valkyrie

The North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep-penetration strategic bomber for the United States Air Force's Strategic Air Command...

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 swing wing are employed), and hence burn more fuel, which results in their use being economically disadvantageous on such flight paths.

Additionally, 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

Shock Waves

Shock Waves, , is a 1977 horror movie written and directed by Ken Wiederhorn...

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

Computer-aided design

Computer-aided design , also known as computer-aided design and drafting , is the use of computer technology for the process of design and design-documentation. Computer Aided Drafting describes the process of drafting with a computer...

has since made this considerably easier. In 2003, a Shaped Sonic Boom Demonstration

Shaped Sonic Boom Demonstration

The NASA Shaped Sonic Boom Demonstration, also known as the Shaped Sonic Boom Experiment, was a two-year program that used a Northrop F-5E with a modified fuselage to demonstrate that the aircraft's shock wave, and accompanying sonic boom, can be shaped, and thereby reduced...

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.

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

Sonic boom

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

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

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

North American Aviation

North American Aviation was a major US aerospace manufacturer, responsible for a number of historic aircraft, including the T-6 Texan trainer, the P-51 Mustang fighter, the B-25 Mitchell bomber, the F-86 Sabre jet fighter, the X-15 rocket plane, and the XB-70, as well as Apollo Command and Service...

had an unusual approach to this problem with the XB-70 Valkyrie

XB-70 Valkyrie

The North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep-penetration strategic bomber for the United States Air Force's Strategic Air Command...

. By lowering the outer panels of the wings at high Mach numbers, they were able to take advantage of compression lift

Compression lift

In aerodynamics, compression lift refers to an aircraft that uses shock waves generated by its own supersonic flight to generate lift. This can lead to dramatic improvements in lift for supersonic/hypersonic aircraft, which often fly at high altitudes and thus suffer from decreased lift due to the...

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

Temper

Temperare is the Latin origin of words like "temperature" and "tempering"; it and "tempo" come, in turn, from tempus...

and is weakened. For aircraft that fly at Mach 3, materials such as stainless steel (XB-70 Valkyrie

XB-70 Valkyrie

The North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep-penetration strategic bomber for the United States Air Force's Strategic Air Command...

) or titanium (SR-71) 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

Louis Charles Breguet

Louis Charles Breguet was a French aircraft designer and builder, one of the early aviation pioneers.- Biography :...

range equation.

The high per-passenger takeoff weight makes it difficult to obtain a good fuel fraction. This, together with the relatively poor supersonic lift/drag ratios, supersonic aircraft have historically had relatively poor range. This meant that a lot of routes were non viable, and this in turn helped mean that they sold poorly with airlines.

Airline desirability of SSTs

Airlines buy aircraft as a means of making money, and wish to make as much return on investment

Return on investment

Return on investment is one way of considering profits in relation to capital invested. Return on assets , return on net assets , return on capital and return on invested capital are similar measures with variations on how “investment” is defined.Marketing not only influences net profits but also...

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, this only would have cut 20 minutes off a transatlantic trip which would probably not have been enough to perform an extra roundtrip, and the aircraft would have been much more expensive for the airlines to purchase.

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 nonstop 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 SST's, 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.

Hypersonic transports

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

Rocket

A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

or scramjet

Scramjet

A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...

engines; pulse detonation engine

Pulse detonation engine

A pulse detonation engine, or "PDE", is a type of propulsion system that uses detonation waves to combust the fuel and oxidizer mixture. The engine is pulsed because the mixture must be renewed in the combustion chamber between each detonation wave initiated by an ignition source. Theoretically, a...

s have also been proposed.

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

Rocket engined vehicles while technically practical (either as ballistic

External ballistics

External ballistics is the part of the science of ballistics that deals with the behaviour of a non-powered projectile in flight. External ballistics is frequently associated with firearms, and deals with the behaviour of the bullet after it exits the barrel and before it hits the target.-Forces...

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.

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.

Current research and development

In April 1994, Aérospatiale

Aérospatiale

Aérospatiale was a French aerospace manufacturer that built both civilian and military aircraft, rockets and satellites. It was originally known as Société Nationale Industrielle Aérospatiale...

, British Aerospace

British Aerospace

British Aerospace plc was a UK aircraft, munitions and defence-systems manufacturer. Its head office was in the Warwick House in the Farnborough Aerospace Centre in Farnborough, Hampshire...

and Deutsche Aerospace AG (DASA) created the European Supersonic Research Program (ESRP) with plans for a second-generation Concorde to enter service in 2010. The plane was to be called the Avion de Transport Supersonique Futur

Avion de Transport Supersonique Futur

The Avion de Transport Supersonique Futur also known as Alliance was a concept design for supersonic aircraft considered by British Aerospace and Aérospatiale. The plane was to be based on the experience learned from the supersonic Concorde, and was to fly at Mach 2 or higher...

. In parallel, SNECMA

Snecma

Snecma is a major French manufacturer of engines for commercial and military aircraft, and for space vehicles. The name is an acronym for Société Nationale d'Étude et de Construction de Moteurs d'Aviation .In 2005, the Snecma group, which included Snecma ,...

, Rolls-Royce

Rolls-Royce plc

Rolls-Royce Group plc is a global power systems company headquartered in the City of Westminster, London, United Kingdom. It is the world’s second-largest maker of aircraft engines , and also has major businesses in the marine propulsion and energy sectors. Through its defence-related activities...

, MTU München

MTU Aero Engines

MTU Aero Engines GmbH is Germany's leading aircraft engine manufacturer. MTU develops, manufactures and provides service support for military and civil aircraft engines...

and Fiat

Fiat

FIAT, an acronym for Fabbrica Italiana Automobili Torino , is an Italian automobile manufacturer, engine manufacturer, financial, and industrial group based in Turin in the Italian region of Piedmont. Fiat was founded in 1899 by a group of investors including Giovanni Agnelli...

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 (10,186 km) aircraft, with the baseline design looking very much like an enlarged Concorde with canard

Canard (aeronautics)

In aeronautics, canard is an airframe configuration of fixed-wing aircraft in which the forward surface is smaller than the rearward, the former being known as the "canard", while the latter is the main wing...

s.

Meanwhile NASA

NASA

The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

started a series of projects to study advances in the state of SST design. As part of the High Speed Civil Transport

High Speed Civil Transport

The High Speed Civil Transport , also known as High-Speed Research ,was a NASA project to design a supersonic transport. The aircraft was to be a future supersonic passenger aircraft, able to fly Mach 2, or twice the speed of sound. The project started in 1990 and ended during 1999...

program a Tu-144 aircraft was re-engined in order to carry out supersonic experiments in Russia in the mid-1990s, but development was ended in 1999.

Japan has a supersonic transport research program. In 2005, it was announced that a Japanese-French joint venture would continue research into a design the plane would be called Next Generation Supersonic Transport

Next Generation Supersonic Transport

The Next Generation Supersonic Transport is an SST being developed by the Japanese Space Agency JAXA. By comparison to the Aérospatiale-BAC SST Concorde, the new design will carry three times the passengers , fly at roughly the same speed , and have twice the range. The goal is to achieve a...

, JAXA hopes the Next Generation Supersonic Transport would be flying by 2015. An 11.5-meter model was successfully flight-tested in October 2005.

Another area that has seen research interest is the supersonic business jet

Supersonic business jet

A supersonic business jet would be a small business jet, intended to travel at speeds above Mach 1.0. Typically intended to transport about ten passengers, SSBJs are about the same size as traditional subsonic business jets...

(SSBJ). Some business jet customers are prepared to pay heavily for decreased travel times, and the noise issues are less serious in a smaller craft. Sukhoi

Sukhoi

Sukhoi Company is a major Russian aircraft manufacturer, headquartered in Begovoy District, Northern Administrative Okrug, Moscow, famous for its fighters...

and Gulfstream

Gulfstream Aerospace

Gulfstream Aerospace Corporation is a producer of several models of jet aircraft. Gulfstream has been a unit of General Dynamics since 1999.The company has produced more than 1,500 aircraft for corporate, government, private, and military customers around the world...

co-investigated such a craft in the mid-1990s, as did Dassault Aviation

Dassault Aviation

Dassault Aviation is a French aircraft manufacturer of military, regional and business jets, a subsidiary of Dassault Group.It was founded in 1930 by Marcel Bloch as Société des Avions Marcel Bloch or "MB". After World War II, Marcel Bloch changed his name to Marcel Dassault, and the name of the...

in the early 2000s. Aerion Corporation's Aerion SBJ

Aerion SBJ

|-See also:-External links:* * * Aviation Week & Space Technology* *...

, the SAI Quiet Supersonic Transport and Tupolev

Tupolev

Tupolev is a Russian aerospace and defence company, headquartered in Basmanny District, Central Administrative Okrug, Moscow. Known officially as Public Stock Company Tupolev, it is the successor of the Tupolev OKB or Tupolev Design Bureau headed by the Soviet aerospace engineer A.N. Tupolev...

's Tu-444

Tupolev Tu-444

|-See also:-External links:* *...

are current SSBJ projects.

Another development in the field of engines is the pulse detonation engine

Pulse detonation engine

. These engines, often referred to as PDEs, offer even greater efficiencies than current turbofan

Turbofan

engines, while allowing for high speed use. NASA

NASA

maintains a PDE research effort, with the baseline being a Mach 5 airliner. A PDE was test flown successfully in 2008.

At the most exotic, high supersonic designs like Reaction Engines 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 from.

There is also a very long distance supersonic/hypersonic transport version of Skylon, the A2

Reaction Engines A2

|-See also:-External links:* *...

, being evaluated by the European Union

European Union

The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

as part of the LAPCAT

LAPCAT

LAPCAT was a 36 month European FP6 study to examine ways to produce engines for a Mach 4-8 hypersonic aircraft. The project ended in April 2008...

project, which would travel at Mach 5 and would be capable of travelling Brussels to Sydney in 4.6 hours.

Tupolev

Tupolev

planned to build the Tupolev Tu-244

Tupolev Tu-244

|-See also:-References:* * Gordon, Yefim and Vladimir Rigamant. OKB Tupolev: A History of the Design Bureau and its Aircraft. Hinckley, UK: Midland Publishing, 2005. ISBN 1-85780-214-4....

, but was cancelled in 1993.

External links

Overview: Civil Engines Extract from article on civil engine market, including discussion of SST – August 2006
Current Supersonic Flight Possibilities Today supersonic flights are possible in Military Jet Fighters only
The Rise & Fall Of The SST on Vectorsite.net

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