Supersonic transport

The Concorde supersonic transport has a delta wing, a slender fuselage and four underslung Olympus engines.

A supersonic transport (SST) is a civil aircraft designed to transport passengers at speeds greater than the speed of sound. Following the permanent cessation of flying by all Concordes, there are no SSTs in commercial service. The only SST to see regular international service was Concorde, and the only other design built in quantity was the Tupolev Tu-144. The first passenger flight of the Tu-144 was in June 1978, and Concorde's last flight was on November 26, 2003.

Challenges of supersonic flight

Fuel economy and weight

High fuel costs and low passenger capacity due to the aerodynamic requirement for a narrow fuselage have combined to make SSTs an expensive form of transportation compared with subsonic flight.

Reaching supersonic speeds requires considerable engine power to overcome wave drag, a powerful form of drag that starts at about Mach 0.8 and ends around Mach 1.2, the transonic speed range. Between these speeds the coefficient of drag (C_d) is approximately tripled. Above the transonic range C_d drops dramatically again, although it remains 30 to 50% higher than at subsonic speeds. In addition, drag increases in proportion to the square of the speed. However, this drag can be reduced back to near normal amounts by simply flying at a higher altitude where the air is far less dense. However, this also reduces lift.

In fact, the critical number is the lift to drag ratio of the wings/vehicles, since the aircraft weight must be held up by the lift, and the ratio gives the drag that must be opposed by the engines to maintain speed. Supersonic lift to drag ratios are invariably worse than subsonic.

For this reason a considerable amount of research was put into designing a planform for sustained supersonic cruise. At about Mach 2 a typical wing design will cut its lift-to-drag ratio in half (e.g., the Concorde vehicle managed a ratio of 7.4 whereas the subsonic Boeing 747's is 17.) Since the aircraft has to hold its own weight up, this means that the aircraft has to provide twice the thrust to maintain airspeed and altitude, and everything else being equal this requires twice the fuel.

Jet engine design differs significantly between supersonic and subsonic aircraft. Jet engines as a class can supply increased fuel efficiency at supersonic speeds because, even though the specific impulse efficiency drops off somewhat at higher speeds, the distance traveled is greater, and the dropoff is less than proportional to speed until well above Mach 2.

Originally when Concorde was being designed, high bypass jet engines had yet to be deployed on subsonic aircraft, and Concorde's turbojets would have been more competitive. When they were deployed this meant that subsonic jet engines suddenly became much more efficient. However, high bypass is a way of saving fuel by reducing the jet exhaust speed to better match a (subsonic) aircraft speeds; that would not improve efficiency at supersonic speeds, which need high exhaust speed for optimum efficiency. (The later TU-144s used high bypass jet engines, and this may have improved the TU-144's subsonic performance, but it did not achieve the same range as Concorde.)

In addition the higher speeds demand narrower wing and fuselage which lead to aeroelasticity problems which require heavier structures to minimise unwanted flexing. SSTs also require a much stronger (and therefore heavier) structure due to the fact that their fuselages are pressurized to a greater pressure differential (due to the difference between cabin pressure to the lower outside pressure at the high altitudes at which SSTs fly). These factors together meant that the empty weight per seat of Concorde is more than three times that of a Boeing 747.

The net upshot between the 747 and Concorde is that both aircraft use approximately the same amount of fuel to cover the same distance, but the 747 can carry more than four times as many passengers.

Sonic booms

The annoyance can be reduced by waiting to reach supersonic speeds until the aircraft is at high altitude over water; this is the technique used by Concorde. However, it precludes supersonic flights on transcontinental flights over populated areas. Supersonic aircraft seemingly inevitably have poor lift/drag ratios at subsonic speeds compared to subsonic aircraft and hence burn more fuel, and are economically disadvantageous for use over 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 shock waves that reach the ground. One way is to cause the shock waves to interfere with each other, greatly reducing sonic boom. This was difficult to test at that time due to the careful design it required, but the increasing power of computer-aided design has since made this considerably easier. In 2003 such a testbed aircraft was flown, the Shaped Sonic Boom Demonstration 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).

Damage to the ozone layer

The high altitude flight makes such damage theoretically more likely than with traditional aircraft. However, research showed that the comparatively tiny quantity of nitric oxides generated in the exhaust actually boosts the ozone layer.

Need to operate aircraft over a wide range of speeds

The design for 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 spent most of the flight at high speed. The Concorde designers were forced to spend a massive 5000 hours optimising 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.

North American Aviation solved 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 gave the Valkyrie the best lift:drag ratio of any supersonic powered manned aircraft ever built and allowed a much better aspect ratio on take-off and landing. Some recent SST designs are considering this as an option.

Takeoff noise

One of the problems with Concorde and the Tu-144's operation was the high engine noise levels, associated with very high jet velocities used during take-off, and even more importantly flying over communities near to the airport. SST engines need a fairly high specific thrust (net thrust/airflow) during supersonic cruise, to minimize engine cross-sectional area and, thereby, nacelle drag. Unfortunately this implies a high jet velocity, which makes the engines noisy which causes problems particularly at low speeds/altitudes and at take-off.

Therefore, a future SST might well benefit from a Variable Cycle Engine, where the specific thrust (and therefore jet velocity and noise) is low at take-off, but is forced high during Supersonic Cruise. Transition between the two modes would occur at some point during the Climb and back again during the Descent (to minimize jet noise upon Approach). The difficulty is devising a Variable Cycle Engine configuration that meets the requirement for a low cross-sectional area during Supersonic Cruise.

Several concepts show promise:-

In the Tandem Fan, the engine has two fans, both mounted on the low pressure (LP) shaft, with a significant axial gap between the units. In normal flight, the engine is in the Series Mode, with the flow leaving the front fan passing directly into the second fan, the engine behaving much like a normal turbofan. However, for take-off, climb-out, final-descent and approach, the front fan is allowed to discharge directly through an auxiliary nozzle on the underside of the powerplant nacelle. Auxiliary intakes are opened on each side of the powerplant, allowing air to enter the rear fan and progress through the rest of the engine. Operating the fans in this Parallel Mode substantially increases the total airflow of the engine at a thrust, resulting in a lower jet velocity and a quieter engine. Back in the 1970s, Boeing modified a Pratt & Whitney JT8D to a Tandem Fan configuration and successfully demonstrated the switch from Series to Parallel operation (and vice-versa) with the engine running, albeit at part power.

In the Mid Tandem Fan concept, a high specific flow single stage fan is located between the high pressure (HP) and LP compressors of a turbojet core. Only bypass air is allowed to pass through the fan, the LP compressor exit flow passing through special passages within the fan disc, directly underneath the fan rotor blades. Some of the bypass air enters the engine via an auxiliary intake. During take-off and Approach the engine behaves much like a normal civil turbofan, with an acceptable jet noise level (i.e., low specific thrust). However, for Supersonic Cruise, the fan variable inlet guide vanes and auxiliary intake close-off to minimize bypass flow and increase specific thrust. In this mode the engine acts more like a 'leaky' turbojet (e.g. the F404).

In the Mixed-Flow Turbofan with Ejector concept, a low bypass ratio engine is mounted in front of a long tube, called an ejector. This silencer device is deployed during take-off and approach. Turbofan exhaust gases induce additional air into the ejector via an auxiliary air intake, thereby reducing the specific thrust/mean jet velocity of the final exhaust. The mixed-flow design does not have the advantages of the mid-tandem fan design in terms of low-speed efficiency, but is considerably simpler.

History

Throughout the 1950s an SST looked possible, 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.

An unusual angle on a Concorde landing

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. The sonic boom was not thought to be a serious issue due to the high altitudes at which the planes flew, but experiments with the USAF's North American XB-70 Valkyrie proved otherwise in the mid-1960s. 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.

Tupolev Tu-144LL

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 like the Boeing 707, but with newer aircrafts 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 0.7, as opposed to 2.0 or higher for the 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.

Current research and development

In April 1994, Aerospatiale, British Aerospace and Deutsche Aerospace AG (DASA) created the European Supersonic Research Program (ESRP) with plans for a second-generation Concorde to enter service in 2010. In parallel, SNECMA, Rolls-Royce, MTU München and Fiat 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 aircraft, with the baseline design looking very much like an enlarged Concorde with canards.

Meanwhile NASA started a series of projects to study advances in the state of SST design. As part of the program a Tu-144 aircraft was re-engined in order to carry out supersonic experiments in Russia in the mid-1990s.

Japan has a supersonic transport research program. In 2005, it was announced that a Japanese-French joint venture would continue research into a design, in the hope of designing a craft that could 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, which appears to be gaining support as the "next design" for aircraft engines. 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. However, PDEs have severe noise and longevity issues, and no aircraft has ever flown powered by a PDE engine in unclassified tests.

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 suborbital supersonic transport version of Skylon being evaluated by the EU, LAPCAT, which would travel at Mach 5 and would be capable of travelling Brussels to Sydney in 2-4 hours.

Subsonic transports that have broken the sound barrier

A Douglas DC-8 achieved supersonic speed during a test dive and Boeing reports that a 747 broke the sound barrier during certification trials.

See also

• Supersonic
• Hypersonic

External links

• Skunk Works plans worldwide network of Thunderbirds-style supersonic jets
• Overview: Civil Engines Extract from article on civil engine market, including discussion of SST - August 2006