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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.[1]
Contents
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 average 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.
In the early 1960s various executives of U.S. aerospace companies were telling the U.S. public and U.S. 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.[3] 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 Concorde purchase options. 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 design was eventually selected for continued work. The Soviet Union set out to produce its own design, the Tu-144, which was nicknamed[by whom?] 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 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.
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,[6] 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.[7]
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
Main article: Concorde aircraft histories
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 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.Challenges of supersonic passenger flight
Aerodynamics
For all vehicles traveling through air, the force of drag is proportional to the coefficient of drag (Cd), 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.
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 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).[8] 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 mile is lower.
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.[9]
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 (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
| Aircraft | Concorde[10] | Boeing 747-400[11] |
|---|---|---|
| passenger miles/imperial gallon | 17 | 109 |
| passenger miles/US gallon | 14 | 91 |
| litres/passenger 100 km | 16.6 | 3.1 |
Nevertheless, fuel costs are not the bulk of the price for most subsonic aircraft passenger tickets.[citation needed] 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.[7]
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 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.[12]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.
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.[13]
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.
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 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.[13]
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).
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.[citation needed] For aircraft that fly at Mach 3, materials such as stainless steel (XB-70 Valkyrie) 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 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.[citation needed]
Airline desirability of SSTs
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.[14] 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,[15][16] 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.[17][18] 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 2020–2025.[19] On 18 June 2011, the Zero Emission High Speed Transport or ZEHST concept aircraft was unveiled by EADS at the Paris Air Show.[20] The ZEHST, a hypersonic aircraft to be capable of 3,000 mph (4,800 km/h), is a result of the collaboration efforts between EADS and Japan.[21]
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.[22] The follow-on research effort, LAPCAT II began in 2008 and is to last four years.[23]
In May 2008, it was reported that Aerion Corporation had $3 billion of pre-order sales on its Aerion SBJ supersonic business jet.[24] In late 2010, the project continued with a testbed flight of a section of the wing.[25]
Hypersonic transport
See also: Hypersonic and Reaction Engines A2
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.[citation needed]
Scramjets currently are not practical for passenger-carrying vehicles.[why?][citation needed]
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.
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