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Asupersonic transport (SST) or asupersonic airliner is aciviliansupersonic aircraft designed to transport passengers at speeds greater than thespeed of sound in terms ofair speed. To date, the only SSTs to see regular service have beenConcorde and theTupolev Tu-144, although theBoom TechnologyOverture SST is expected to start service in 2029, making it the third operational SST. The last passenger flight of the Tu-144 was in June 1978 and it was last flown in 1999 byNASA. Concorde's last commercial flight was in October 2003, with a November 26, 2003,ferry flight being its last flight.
Following the termination of flying by Concorde, there have been no SSTs in commercial service. However, several companies have proposedsupersonic business jet designs. Small SSTs have less environmental impact and design capability improves with continuing research which is aimed at producing an acceptable aircraft.
Supersonic airliners have been the objects of numerous ongoing design studies such as those ofBoom Technology. Drawbacks and design challenges are excessive noise generation (at takeoff and due tosonic booms during flight), high development costs, expensive construction materials, high fuel consumption, extremely high emissions, and an increased cost per seat over subsonic airliners. However, despite these challenges, Concorde was claimed to have operated profitably.[1]
Throughout the 1950s an SST looked possible from a technical standpoint, but it was not clear if it could be made economically viable. Because of differences inlift generation, aircraft operating at supersonic speeds have approximately one-half thelift-to-drag ratio of subsonic aircraft. 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. Therelative effect is reduced as the aircraft accelerates to higher speeds. Offsetting this increase in fuel use was the potential to greatly increasesortie 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.
Serious work on SST designs started in the mid-1950s, when the first generation of supersonicfighter aircraft were entering service. In Britain and France, government-subsidized SST programs quickly settled on thedelta wing in most studies, including theSud Aviation Super-Caravelle andBristol Type 223, althoughArmstrong-Whitworth proposed a more radical design, the Mach 1.2M-Wing.Avro Canada proposed several designs toTWA 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 ofHawker Siddeley's designs.[2] 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 theBristol Aeroplane Company andSud 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 withLockheed, stated to various magazines that an SST constructed of steel weighing 250,000 pounds (110,000 kg) 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 afterPan Am took out purchase options on the Concorde. Congress was soon funding an SST design effort, selecting the existingLockheed L-2000 andBoeing 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 toMach 3. The Soviet Union set out to produce its own design, theTu-144, which the western press nicknamed the "Concordski".[4]
The SST was seen as particularly offensive due to itssonic boom and the potential for its engine exhaust to damage theozone layer. Both problems impacted the thinking of lawmakers, and eventuallyCongress dropped funding for the US SST program inMarch 1971,[5][6][7][8][9] and all overland commercial supersonic flight was banned over the US.[10] Presidential advisorRussell Train warned that a fleet of 500 SSTs flying at 65,000 ft (20 km) 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 ofozone.[11]
Later, an additional threat to the ozone was hypothesized as a result of the exhaust'snitrogen oxides, a threat that was, in 1974, seemingly validated by anMIT team commissioned by theUnited States Department of Transportation.[12] However, while many purely theoretical models were indicating the potential for large ozone losses from SST nitrogen oxides (NOx), other scientists in the paper "Nitrogen Oxides, Nuclear Weapon Testing, Concorde and Stratospheric Ozone" turned to historical ozone monitoring andatmospheric nuclear testing to serve as a guide and means of comparison, observing that no detectable ozone loss was evident from approximately 213megatons of explosive energy being released in 1962, so therefore the equivalent amount of NOx from "1047" Concordes flying "10 hours a day", would likewise, not be unprecedented.[13] In 1981 models and observations were still irreconcilable.[14] More recent computer models in 1995 by David W. Fahey, an atmospheric scientist at theNational Oceanic and Atmospheric Administration, and others, suggest that the drop in ozone would be at most, "no more" than 1 to 2% if a fleet of 500 supersonic aircraft [were] operated.[15][16] 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" because "removing thesulfur in the fuel of the [Concorde]" would essentially eliminate the hypothesized 1%–2% ozone-destruction-reaction-pathway.[17]
Despite the model-observation discrepancy surrounding the ozone concern, in the mid-1970s, six years after its first supersonic test flight,[18] Concorde was now ready for service. The US political outcry was so high thatNew York banned the plane. This threatened 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. (atDulles inVirginia), 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 intoJFK.
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 were no longer reasonable. When first designed, the SSTs were envisioned to compete with long-range aircraft seating 80 to 100 passengers such as theBoeing 707, but with newer aircraft such as theBoeing 747 carrying four times that, the speed and fuel advantages of the SST concept were taken 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 theturbofan engine with ever-increasingbypass ratios, the fan concept is difficult to use at supersonic speeds where the "proper" bypass is about 0.45,[19] 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 privatized, 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.[20]
On 25 July 2000,Air France Flight 4590 crashed shortly after take-off with all 109 occupants and four on ground killed; the only fatal incident involvingConcorde. Commercial service was suspended until November 2001, and Concorde aircraft were retired in 2003 after 27 years of commercial operations.
The last regular passenger flights landed atLondon Heathrow on October 24, 2003, fromNew York, a second flight fromEdinburgh, and a third which had taken off from Heathrow on a loop flight over theBay of Biscay.[21]
By the end of the 20th century, projects like theTupolev Tu-244,Tupolev Tu-344,SAI Quiet Supersonic Transport,Sukhoi-Gulfstream S-21,High Speed Civil Transport, etc. had not been realized.
For all vehicles traveling through air, the force ofdrag is proportional to thecoefficient 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 SSTs. 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 ofwave drag appears. This is a powerful form of drag that begins attransonic speeds (aroundMach 0.88). Around Mach 1, the peak coefficient of drag is four times that of subsonic drag. Above the transonic range, the coefficient drops drastically 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 abovetransonic speed is more efficient, it is still less efficient than flying subsonically.
Another issue in supersonic flight is thelift 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 designingwing 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 subsonicBoeing 747 has an L/D ratio of 17).[22] 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.
Jet engine design shifts significantly between supersonic and subsonic aircraft. Jet engines, as a class, can supply increasedfuel efficiency at supersonic speeds, even though theirspecific 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.
When Concorde was being designed byAérospatiale–BAC,high bypass jet engines ("turbofan" engines) had not yet been deployed on subsonic aircraft. Had Concorde entered service against earlier designs like theBoeing 707 orde Havilland Comet, it would have been much more competitive, though the 707 and DC-8 still carried more passengers. 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 theturboprop, where almost all of the jet thrust is used to power a very large fan – thepropeller. 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.[23]
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.
Supersonic vehicle speeds demand narrower wing and fuselage designs, and are subject to greater stresses and temperatures. This leads toaeroelasticity problems, which require heavier structures to minimize unwanted flexing. SSTs also require a much stronger (and therefore heavier) structure because theirfuselage must bepressurized 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.
Concorde and the Tu-144 were both constructed of conventional aluminum: Concorde ofHiduminium and Tu-144 ofduralumin. Modern, advanced materials were not to come out of development for a few decades. These materials, such ascarbon fibre andKevlar are much stronger for their weight (important to deal with stresses) as well as being more rigid. As per-seat weight of the structure is much higher in an SST design, structural improvements would have led to a greater proportional improvement than the same changes in a subsonic aircraft.
| Aircraft | Concorde[24] | Boeing 747-400[25] |
|---|---|---|
| Passenger miles/imperial gallon | 17 | 109 |
| Passenger miles/US gallon | 14 | 91 |
| Litres/passenger 100 km | 16.6 | 2.6 |
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.[26] 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.[20]
Extreme jet velocities used during take-off caused Concorde and Tu-144s to produce significant take-off noise. Communities near the airport were affected by high engine noise levels, which prompted some regulators to disfavor the practice. 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, particularly at low speeds/altitudes and at take-off.[27]
Therefore, a future SST might well benefit from avariable 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.
Thesonic 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 controversialOklahoma City sonic boom tests and studies of theUSAF's North AmericanXB-70 Valkyrie proved otherwise (seeSonic boom § Abatement). By 1964, whether civilian supersonic aircraft would be licensed was unclear, because of the problem.[28]
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 asvariable-sweep wings are employed), and hence burn more fuel, which results in their use being economically disadvantageous on such flight paths.
Concorde had anoverpressure of 1.94 lb/sq ft (93 Pa) (133 dBA SPL). Overpressures over 1.5 lb/sq ft (72 Pa) (131 dBA SPL) often cause complaints.[29]
If the intensity of the boom can be reduced, then this may make even very large designs of supersonic aircraft acceptable for overland flight. 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 theshock waves to interfere with each other, greatly reducing the sonic boom. This was difficult to test at the time, but the increasing power ofcomputer-aided design has since made this considerably easier. In 2003, aShaped 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 (seeSonic boom § Abatement).
When it comes to public policy, for example, the FAA prohibits commercial airplanes from flying at supersonic speeds above sovereign land governed by the United States because of the negative impact the sonic boom brings to humans and animal populations below.[30]
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 (alift to drag ratio of about 4) at slow speed, but it travelled at high speed for most of the flight. Designers of Concorde spent 5000 hours optimizing the vehicle shape in wind tunnel tests to maximize the overall performance over the entire flightplan.[citation needed]
TheBoeing 2707 featuredswing 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 theXB-70 Valkyrie. By lowering the outer panels of the wings at high Mach numbers, they were able to take advantage ofcompression lift on the underside of the aircraft. This improved the L/D ratio by about 30%.
Aircraft are surrounded by an air layer the temperature of which increases with aircraft speed. As a result, the skin of the aircraft gets hotter with increasing supersonic speeds (kinetic heating from the high speed boundary layer[31]). Heat from the sun also raises the skin temperature. Heat transfers into the aircraft structure which also gets hotter. By the early 1960s many investigations in the United States, Britain and France had shown equilibrium skin temperatures varying from 130 degC at Mach 2.2 to 330 degC at Mach 3.[32]
Subsonic aircraft are often made ofaluminium alloys. However such alloys, while being light and strong, is not able to withstand temperatures much over 127 °C; above 127 °C the aluminium gradually loses its properties that were brought about by age hardening.[33] For aircraft that have flown at Mach 3, materials such asstainless steel (XB-70 Valkyrie,MiG-25) ortitanium (SR-71,Sukhoi T-4) have been used.
The range of an aircraft depends on three efficiencies which appear in theBreguet range equation. They are the aerodynamic efficiency, which says how much wanted lift can be produced without too much unwanted drag, powerplant efficiency, which says how much fuel is converted into moving the aircraft against its drag resistance, and structural efficiency, which says how heavy the structure is compared to the fuel and passengers it can carry.
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Airlines potentially value very fast aircraft, because it enables the aircraft to make more flights per day, providing a higher return on investment. Also, passengers generally prefer faster, shorter-duration trips to slower, longer-duration trips, so operating faster aircraft can give an airline a competitive advantage, even to the extent that many customers will willingly pay higher fares for the benefit of saving time and/or arriving sooner.[34] 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.[35] The proposed American SSTs were intended to fly at Mach 3, partly for this reason. However, allowing for acceleration and deceleration time, a trans-Atlantic trip on a Mach 3 SST would be less than three times as fast as a Mach 1 trip.
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 necessarily higher, all other factors being equal, as well as making that price more sensitive to the price of oil. (It also makes supersonic flights less friendly to the environment and sustainability, two growing concerns of the general public, including air travelers.)
Investing in research and development work to design a new SST can be considered as an effort to push the speed limit of air transport. Generally, other than an urge for new technological achievement, the major driving force for such an effort is competitive pressure 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.[citation needed] An example of this phenomenon ishigh-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 among themselves. This phenomenon also reduces the airline desirability of SSTs, because, for very long-distance transportation (a couple of thousand 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 moderately to reduce cost and increase service quality than pay much more for a speed increase.[citation needed] Also, for-profit companies generally prefer low risk business plans with high probabilities of appreciable profit, but an expensive leading-edge technological research and development program is a high-risk enterprise, as it is possible that the program will fail for unforeseeable technical reasons or will meet cost overruns so great as to force the company, due to financial resource limits, to abandon the effort before it yields any marketable SST technology, causing potentially all investment to be lost.
TheInternational Council on Clean Transportation (ICCT) estimates a SST would burn 5 to 7 times as much fuel per passenger.[36] The ICCT shows that a New York to London supersonic flight would consume more than twice as muchfuel per passenger than in subsonicbusiness-class, six times as much as foreconomy class, and three times as much as subsonic business for Los Angeles to Sydney.[37] Designers can either meet existing environmental standards with advanced technology orlobby policymakers to establish new standards for SSTs.[38]
If there were 2,000 SSTs in 2035, there would be 5,000 flights per day at 160 airports and the SST fleet would emit ~96 million metric tons of CO2 per year (likeAmerican,Delta andSouthwest combined in 2017), 1.6 to 2.4 gigatonnes of CO2 over their 25-year lifetime: one-fifth of the international aviationcarbon budget if aviation maintains itsemissions share to stay under a 1.5 °Cclimate trajectory.Noise exposed area around airports could double compared to existing subsonic aircraft of the same size, with more than 300 operations per day atDubai andLondon Heathrow, and over 100 inLos Angeles,Singapore,San Francisco,New York-JFK,Frankfurt, andBangkok. Frequentsonic booms would be heard in Canada, Germany, Iraq, Ireland, Israel, Romania, Turkey, and parts of the United States, up to 150–200 per day or one every five minutes.[39]
On August 21, 1961, aDouglas DC-8-43 (registration N9604Z) exceeded Mach 1 in a controlled dive during a test flight at Edwards Air Force Base. The crew were William Magruder (pilot), Paul Patten (copilot), Joseph Tomich (flight engineer), and Richard H. Edwards (flight test engineer).[40] This is the first supersonic flight by a civilian airliner.[40]
In total, 20 Concordes were built: two prototypes, two development aircraft and 16 production aircraft. Of the sixteen production aircraft, two did not enter commercial service and eight remained in service as of April 2003. All but two of these aircraft are preserved; the two that are not are F-BVFD (cn 211), parked as a spare-parts source in 1982 and scrapped in 1994, and F-BTSC (cn 203), whichcrashed outside Paris on July 25, 2000, killing 100 passengers, 9 crew members, and 4 people on the ground.
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.
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The desire for a second-generation supersonic aircraft has remained within some elements of the aviation industry,[41][42] and several concepts have emerged since the retirement of Concorde.
According toAviation Week, the market for supersonic airliners costing $200 million could be 1,300 over a 10-year period, worth $260 billion.[43] Development and certification is probably a $4 billion operation.[44]
In November 2003,EADS—the parent company ofAirbus—announced that it was considering working with Japanese companies to develop a larger, faster replacement for Concorde.[46][47] 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 (Next Generation Supersonic Transport,NEXST, thenZero Emission Hyper Sonic Transport). If pursued to commercial deployment, it would be expected to be in service around 2020–25.[48]
In May 2008, it was reported thatAerion Corporation had $3 billion of pre-order sales on itsAerion SBJ supersonic business jet.[49] In late 2010, the project continued with a testbed flight of a section of the wing.[50] TheAerion AS2 was proposed as a 12-seat trijet, with a range of 4,750 nmi (8,800 km; 5,470 mi) at Mach 1.4 over water or 5,300 nmi (9,800 km; 6,100 mi) at Mach 0.95 over land, although "boomless" Mach 1.1 flight was claimed to be possible. Backed by Airbus and with 20 launch orders from Flexjet, first deliveries were pushed back from 2023 by two years whenGE Aviation was selected in May 2017 for a joint engine study. In May 2021 the company announced that they would be ceasing operations due to inability to raise capital.[51]
TheSAI Quiet Supersonic Transport is a 12-passenger design fromLockheed 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.[52]
The supersonicTupolev Tu-444 orGulfstream X-54 have also been proposed.
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In March 2016,Boom Technology revealed that it is in the development phases of building a 40-passenger supersonic jet capable of flying Mach 1.7, claiming that the design simulation shows that it will be quieter and 30% more efficient than the Concorde and will be able to fly Los Angeles to Sydney in 6 hours. It is planned to go into service in 2029.[53]
For its economic viability, NASA research since 2006 has focused on reducing thesonic boom to allow supersonic flight over land.[54] In 2016, NASA announced it had signed a contract for the design of a modernlow-noise SSTprototype.[55] The designing team is led byLockheed Martin Aeronautics.[55]NASA should fly a low-boom demonstrator in 2019, reduced from double bangs to soft thumps by airframe shaping, to inquire community response, in support of a prospectiveFAA andICAO ban lift in the early 2020s. TheLockheed Martin X-59 QueSST X-plane will mimic the shockwave signature of a Mach 1.6 to 1.8, 80- to 100-seat airliner for 75 PNLdB compared with 105 PNLdB for Concorde.[54]
TheTsAGI exhibited at the 2017MAKS Air Show in Moscow a scale model of its Supersonic Business Jet / Commercial Jet which should produce a low sonic boom permitting supersonic flight over land, optimised for 2,100 km/h (1,300 mph) cruise and 7,400–8,600 km (4,600–5,300 mi) range. The scientific research aims to optimise for both Mach 0.8–0.9transonic and Mach 1.5–2.0 supersonic speeds, a similar design is tested in awind tunnel while the engines are conceptualised at theCentral Institute for Aviation Motors and designs are studied byAviadvigatel andNPO Saturn.[56]
At the October 2017NBAA convention in Las Vegas, with NASA supporting only research, various companies faced engineering challenges to propose aircraft with no engine available, variable top speeds and operating models:[57]
| Model | Passengers | Cruise | Range(nmi) | MTOW | Total Thrust | Thrust/weight |
|---|---|---|---|---|---|---|
| Tupolev Tu-144 | 150 | Mach 2.0 | 3,500 nmi (6,500 km) | 207 t (456,000 lb) | 960 kN (216,000 lbf) | 0.44 |
| Concorde | 120 | Mach 2.02 | 3,900 nmi (7,200 km) | 185 t (408,000 lb) | 676 kN (152,000 lbf) | 0.37 |
| Boom Overture | 64-80 | Mach 1.7[61] | 4,250 nmi (7,870 km) | 77.1 t (170,000 lb) | 200–270 kN (45,000–60,000 lbf) | 0.26–0.35 |
Of the four billion airpassengers in 2017, over 650 million flewlong-haul between 2,000 and 7,000 miles (3,200 and 11,300 km), including 72 million inbusiness andfirst class, reaching 128 million by 2025;Spike projects 13 million would be interested in supersonic transport then.[62]
In October 2018, thereauthorization of theFAA planned noise standards for supersonic transports, giving developers a regulatory certainty for their designs, mostly their engine choice.Rules for supersonicflight-testing authorization in the U.S. andnoise certification will be proposed by theFAA by early 2019.[63] The FAA should make a proposition for landing-and-takeoff noise before March 31, 2020, for a rule after 2022; and for overland sonic boom from the end of 2020, whileNASA plans to fly theLockheed Martin X-59 QueSST low-boom flight demonstrator from 2021 forICAO standards in 2025.[64]
In June 2019, inspired by the NASA quiet supersonic initiative andX-59 QueSST, Lockheed Martin unveiled theQuiet Supersonic Technology Airliner,[65] a Mach 1.8, transpacific airliner concept for 40 passengers. Lowerairport noise andsonic boom are allowed byshaped-boom design; integrated low-noise propulsion; swept-wing supersonic naturallaminar flow; and the cockpitexternal vision system (XVS). The 225 ft (69 m) long design is significantly longer than theConcorde, featuring an almost 70 ft (21 m) long nose and a 78 ft (24 m) cabin. The sharply sweptdelta wing has a 73 ft (22 m) span, slightly narrower than the Concorde.[66]
Design goals are a 4,200–5,300 nmi (7,800–9,800 km) range and a 9,500–10,500 ft (2,900–3,200 m) takeoff field length, a 75-80 PLdB sonic boom and a cruise of Mach 1.6–1.7 over land and Mach 1.7-1.8 over water.Twin tail-mounted nonafterburning 40,000 lbf (180 kN) engines are located between V-tails. Integrated low-noise propulsion include advancedplug nozzle designs,noise shielding concepts and distortion-tolerantfan blades.[66]
In 2019,Exosonic, Inc was founded with the goal of developing a 70-passenger supersonic jet capable of flying Mach 1.8 and with a range of 5,000 nmi (9,300 km; 5,800 mi). The company was aiming to introduce the jet commercially in the 2030s.[67][68] In April 2021, Exosonic was awarded a contract to develop a supersonic jet which could have been used for Air Force One.[69]
In August 2020,Virgin Galactic with Rolls-Royce unveiled the concept of a Mach 3 capable twinjetdelta wing aircraft that can carry up to 19 passengers.[70][71]
NASA is working with 2 teams led by Boeing and Northrop Grumman on developing concepts for a Mach 4 airliner.[72]
In April 2024, Boom received FAA licensure for Mach 1 and beyond tests of its XB-1 to be conducted at the Black Mountain Supersonic Corridor, in Mojave, California.[73]
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 userocket orscramjet 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 asballistic transports or assemiballistic 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]
At the June 2011Paris Air Show, EADS unveiled itsZEHST concept, cruising at Mach 4 (4,400 km/h; 2,400 kn) at 105,000 ft (32,000 m) and attracting Japanese interest.[74] The GermanSpaceLiner is a suborbital hypersonic winged passenger spaceplane project under preliminary development.[when?]
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. The British companyReaction Engines Limited, with 50% EU money, has been engaged in a research programme calledLAPCAT, which examined a design for a hydrogen-fueled plane carrying 300 passengers called theA2, potentially capable of flying at Mach 5+ nonstop from Brussels to Sydney in 4.6 hours.[75] The follow-on research effort,LAPCAT II began in 2008 and was to last four years.[76]
STRATOFLY MR3 is an EU research program (German Aerospace Center,ONERA and universities) with the goal of developing acryogenic fuel 300-passenger airliner capable to fly at about 10,000 km/h (Mach 8) above 30 km of altitude.[77][78]
Destinus,Hermeus, and Venus Aerospace are developing hypersonic passenger aircraft.[79][80][81][82]
Boeing unveiled at theAIAA 2018 conference a Mach 6 (6,500 km/h; 3,500 kn) passenger airliner. Crossing the Atlantic in 2 hours or the Pacific in 3 at 100,000 ft (30 km) would enable same-day return flights, increasing airlines' assetutilization. Using atitanium airframe, its capacity would be smaller than aBoeing 737 but larger than a long-rangebusiness jet. A reusable demonstrator could be flown as early as 2023 or 2024 for a potential entry into service from the late 2030s. Aerodynamics would benefit from theBoeing X-51 Waverider experience, riding the leading edgeshockwave for lowerinduced drag.Flow control would enhancelift at slower speeds, and avoiding afterburners on takeoff would reducenoise.[83] The Boeing hypersonic airliner would be powered by aturboramjet, aturbofan that transitions to aramjet at Mach 6 would avoid the need for a scramjet, similar to theSR-71 Blackbird'sPratt & Whitney J58, but shutting off theturbine at higher speeds. It would be integrated in anaxisymmetric annular layout with a singleintake andnozzle, and a bypass duct around the turbine engine to a combinationafterburner/ramjet at the rear. It would need advancedcooling technology like theheat exchanger developed byReaction Engines, maybe usingliquid methane and/orjet fuel.[83] Cruising at 90,000–100,000 feet (27,000–30,000 m) makesdepressurization a higher risk. Mach 6 was chosen as the limit achievable with availabletechnology. It would have a highcapacity utilization, being able to cross the Atlantic four or five times a day, up from a possible twice a day with theConcorde.[84]
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