Reaching supersonic speeds requires a considerable amount of power to overcome wave drag, a powerful form of drag that starts at about Mach 0.8 and ends around Mach 1.2, the so-called transonic speed range. Between these speeds drag rises about three times over that of the normal drag just below that speed. Above the transonic the drag drops dramatically again, down to perhaps 30 to 50% higher than the drag at speeds below it. Once past this "hump", the plane is travelling perhaps twice as fast for 50% more drag, meaning that it is actually burning considerably less fuel to cover the same distance.
Offsetting this to a large degree is the general inefficiency of wings at high-supersonic speeds. At about Mach 2 a typical wing design will lose about 1/2 its lift. Considering that the primary measure of a design's efficiency is the so-called lift-to-drag ratio, this means that the aircraft is seeing little overall gain in fuel efficiency. For this reason a considerable amount of research was put into designing a planform for sustained supersonic cruise.
Throughout the 1950s the concept of an SST looked possible, but it was not clear whether or not it could be made economically viable. It seemed that there was a good argument for supersonic speeds on medium and long-ranged 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 and get there in the first place. The main advantage appeared to be practical; these designs would be flying at least three times as fast as existing subsonic transports, and would be able to replace three planes in service, and thereby lower costs in terms of manpower and maintenance.
Serious work on SST designs started in the mid-1950's, when the first generation of supersonic fighter aircraft were entering widespread 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-Witworth 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 the Concorde.
This development set off a wave of panic in the US industry, where it was thought that the Concorde would soon replace all other long range designs. Congress was soon funding an SST design effort of their own, 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. This panic also occurred in the Soviet Union which set to produce its own design of the Tu-144.
At about this time the western world was in the throes of the 1960s, an era when environmental concerns came to the fore for the first time. The SST was seen as particularly offensive due to sonic booms impacting people living on the ground in the plane's vicinity, and that the high-flying planes' exhaust would potentially damage the ozone layer. The former was originally thought to be not a serious issue due to the high altitudes that the planes flew at, but experiments with the USAFs North American B-70 Valkyrie proved otherwise in the mid-1960s. Both problems found a sympathetic ear in the public, who felt that SSTs would degrade the quality of life of the average citizen. Eventually Congress dropped funding for the US SST program in 1971, and all overland supersonic flight was banned.
It was at this point that the Concorde was ready for service. Public outcry was so high that New York banned the plane outright, the supersonic limitation not being enough on its own. This basically 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 that they didn't have it. It was not long before the Concorde was flying into JFK after all.
Public opinion was changing. The disaster stories being told during the 1960s were clearly blown out of proportion, and the high speed crossing seemed like a great idea. This started a second round of design studies in the US, under the name AST, for Advanced Supersonic Transport. Lockheed's SCV was an entirely new design for this category, while Boeing continued studies with the 2707 as a baseline.
However by this time the economics of the SST concept simply no longer made sense. When first designed, the SSTs were envisioned to compete with long-range aircraft seating 80 to 100 passengers, but with 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 is that the wide range of speeds that an SST operates at 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 simply faded away by the early 1980s.
Two more recent developments appear to alter the equations somewhat. During the original SST efforts in the 1960s it was suggested that careful shaping of the fuselage of the aircraft could 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, and proved the soundness of the design, which has the capability of reducing the boom by about one-half. This makes the boom from even very large designs "acceptable" (see sonic boom for details).
Engine design has also improved, with two new concepts showing promise. In the Mid Tandem Fan concept a large low-bypass turbofan engine centered in a large engine nacelle, driving a second, much larger, fan in the nacelle itself. This second fan can be controlled in speed, unlike a normal turbofan engine where the fan is connected to the turbine directly, and thereby "tuned" to different speeds and even turned off when supersonic. In the Mixed-Flow Turbofan with ejector concept, a low-bypass engine is centered in front of a long tube called the ejector, which is primarily a silencer device. The mixed-flow design does not have the advantages of the mid-tandem design in terms of low-speed efficiency, but is considerably simpler.
In April 1994, Aerospatiale, British Aerospace and Deutsche Aerospace (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 and Fiat started working together in 1991 on the development of a new engine. Investing no more than $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 the art, and how they applied to SST designs. This program went so far as to re-engine a Tu-144 aircraft in order to carry out a number of supersonic experiments in Russia in the mid-1990s.
Another development in the field of engines is the pulse detonation engine, which appears to be quickly gaining support as the "next design" for aircraft engines. These engines, often referred to as PDE's, offer even greater efficiencies than current turbofan engines, while allowing for high speed use. NASA maintains an active PDE research effort for just this purpose, with the baseline being a Mach 5 airliner.
Practicalities of demand, cost, construction difficulties and environmental impact have limited, and continue to limit, SST designs from becoming reality.
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