A supersonic transport (SST) is a civil aircraft designed to transport passengers at speeds greater than the speed of sound. The only SST to see regular international service was Concorde, and the only other design built in quantity was the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978, and Concorde's last flight was on November 26, 2003. Following the permanent cessation of flying by all Concorde, there are no SSTs in commercial service.
However, as speeds approach the speed of sound, the phenomenon of wave drag appears. This is a powerful form of drag that begins at about Mach 0.8, and ends at about Mach 1.2, (transonic speeds). Between these speeds, the peak coefficient of drag (Cd) can be up to four times that of subsonic drag. Above the transonic range, Cd 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 speed is more efficient, it is still rather 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 planforms for sustained supersonic cruise. At about Mach 2, a typical wing design will cut its L/D ratio in half (e.g., the 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.
When Concorde was being designed by Aérospatiale-BAC, high bypass jet engines had not yet been deployed on subsonic aircraft, and Concorde would have been 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. A bypass design is more fuel efficient at subsonic speeds, as they can reduce the jet exhaust speed to better match that of the aircraft. This capability would not improve efficiency, indeed would reduce it, during supersonic cruise, where the smaller size of turbojet engines gives low drag and better net efficiency. For example the early TU-144S was fitted with a low bypass jet engine but which was much less efficient than Concorde's turbojets. The later TU-144D featured a turbojet engine and was comparable.
Modern jet engines employ a high overall pressure ratio wherever possible, which, for fundamental reasons, yields better fuel efficiency and it may be that a more modern design would give even better fuel efficiency than Concorde's engines.
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, when mixed with polymers being more rigid, so it's likely that considerable improvements could be made, far more so than with conventional aircraft.
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.
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 is 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 wave 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, 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.
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).
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 spent most of the flight at high speed. The Concorde designers 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.
Some designs of supersonic transports possessed 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%.
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.
Normal subsonic aircraft are traditionally made of aluminium. However aluminium while being light and strong is not able to take temperatures much over 127 C otherwise the aluminium gradually loses its temper and it is weakened. This corresponds to an airspeed of about Mach 2.2.
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.
Since SSTs emit sonic booms at supersonic speeds, and are not usually particularly efficient at subsonic speeds, this reduces the routes that the aircraft can be used on, and this massively reduces the desirability of such aircraft for most airlines.
Since supersonic aircraft use more fuel, this translates into higher ticket prices, but since this is borne by the customers, this is not of itself a disadvantage to the airline. However, because more of the ticket price is fuel, variations in the price of oil cause greater elasticity in the price of a ticket, and hence in the numbers of passengers, and having to fly aircraft partly empty is rarely cost-effective.
Airlines potentially value very fast aircraft, because it permits the aircraft to make more flights per day, which allows for 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 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.
Serious work on SST designs started in the mid-1950s, when the first generation of supersonic fighter aircraft were entering service. In Europe, government-subsidized SST programs quickly settled on the delta wing in most studies, including the Sud Aviation Super-Caravelle and Bristol 223, although Armstrong-Whitworth proposed a more radical design, the Mach 1.2 M-Wing. By the early 1960s, the designs had progressed to the point where the go-ahead for production was given, but costs were so high that Bristol and Sud eventually merged their efforts in 1962 to produce 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 and Boeing 2707 designs, to produce an even more advanced, larger, faster and longer ranged design. The Boeing design was eventually selected for continued work. The Soviet Union set out to produce its own design, the Tu-144.
In the 1960s environmental concerns came to the fore for the first time. The SST was seen as particularly offensive due to its sonic boom and the potential for its engine exhaust to damage the ozone layer. 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 banned the plane outright. This destroyed the aircraft's economic prospects — it had been built with the London-New York route in mind. However, the plane was allowed into Washington, DC, and the service was so popular that New Yorkers were soon complaining because they did not have it. It was not long before Concorde was flying into JFK after all.
Along with shifting political considerations, the flying public continued to show interest in high-speed ocean crossings. This started a second round of design studies in the US, under the name AST, for Advanced Supersonic Transport. Lockheed's SCV was a new design for this category, while Boeing continued studies with the 2707 as a baseline.
However 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 increasing efficiencies through the 1960s with the introduction of the turbofan engine with ever-increasing bypass ratios, the fan concept is difficult to use at supersonic speeds where the "proper" bypass is about 0.45, as opposed to 2.0 or higher for subsonic designs. For both of these reasons the SST designs were doomed to higher operational costs, and the AST programs faded away by the early 1980s.
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.
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.
There are many difficulties with such flight, both technical and economic.
Rocket engined vehicles while technically practical (either as ballistic transports or as semiballistic transports using wings) would use a very large amount of propellant and operate best at speeds between about Mach 8 and orbital speeds. Rockets compete best with air breathing jet engines on cost at very long range, however even for antipodal travel costs would be only somewhat lower than orbital launch costs.
Scramjets currently are not practical for passenger carrying vehicles.
Meanwhile NASA started a series of projects to study advances in the state of SST design. As part of the High Speed Civil Transport 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, 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 (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 and Gulfstream co-investigated such a craft in the mid-1990s, as did Dassault Aviation in the early 2000s. Aerion Corporation's Aerion SBJ and Tupolev's Tu-444 are two current SSBJ projects. Other companies advertise SSBJs as well.
Another development in the field of engines is the pulse detonation engine. These engines, often referred to as PDEs, offer even greater efficiencies than current turbofan engines, while allowing for high speed use. NASA maintains a PDE research effort, with the baseline being a Mach 5 airliner. A PDE was recently test flown successfully .
At the most exotic, high supersonic designs like 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, being evaluated by the European Union as part of the LAPCAT project, which would travel at Mach 5 and would be capable of travelling Brussels to Sydney in 4.6 hours.