Throughout the 1950s an SST looked possible, but it was not clear whether or
not 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.
Serious work on SST designs started in the mid-1950's, 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 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. 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 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. Eventually Congress dropped funding
for the US SST program in 1971, and all overland supersonic flight was banned.
Concorde was now ready for service. The US public 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 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 about the damage SST
flights could do was blown out of proportion, and the high speed ocean 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 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 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.
Two recent developments appear to alter the economics. 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 test bed 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 a half.
This may make the boom from even very large designs acceptable.
Engine design has also improved, with two concepts showing promise. In the
Mid Tandem Fan concept a large low-bypass turbofan engine centred 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 centred 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 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.
Although the Concorde and Tu-144 were certainly the first aircraft to carry
commercial passengers at supersonic speeds, they were not the first commercial
airliners to break the sound barrier. On August 21, 1961 a Douglas DC-8 broke
the sound barrier at Mach 1.012 or 660 mph while in a controlled dive through
41,088 feet. The purpose of the flight was to collect data on a new leading-edge
design for the wing. Boeing reports that the 747 broke the sound barrier during
certification tests.
