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Commercial Aviation - Supersonic Hits Sound Barrier

Issue: 06-2009By Group Captain (Retd) Joseph Noronha, Goa

Around the 1960s, when environmental concerns also emerged, the supersonic transport was seen as particularly offensive due to its sonic boom and the potential for its engine exhaust to damage the ozone layer. Within a decade or so, SSTs were sidelined.

“You can be in London at 10 o’clock and in New York at 10 o’clock. I have never found another way of being in two places at once.”
— Sir David Frost, aboard the Concorde

Dawn of the commercial jet age in 1952 sparked off a worldwide craze for speed. Passengers flocked to the airlines by the thousands, marvelling at how quickly they could be conveyed to their destinations—and then wondering if their journey time could be reduced even further. The jets of the 1960s, however, always came up against the “sound barrier” that prevented them from zooming any faster. But aviation experts felt it was only a matter of time before the barrier would be decisively breached. After all it had become routine for military jets to fly at twice the speed of sound, so why not airliners?

On August 21, 1961, a Douglas DC-8 did cross the threshold during a flight to collect data on a new design for the wing. Reaching Mach 1.012 (1,062 km/h), while in a controlled dive through 41,000 ft (12,497 m), the DC-8 became the first airliner to ‘go supersonic’—albeit for a few seconds.

The Race Hots Up
The battle to build a true supersonic commercial jet began in earnest. First off the blocks were the Europeans. In November 1962, Britain and France signed the Anglo-French Supersonic Aircraft agreement which would eventually lead to development of the world’s first supersonic airliner, christened Concorde. This set alarm bells ringing in Washington. Would Concorde replace all other long range designs? Would American stalwarts, like Boeing, Lockheed and Douglas Aircraft, that were virtually supreme in world airliner sales, now be outmanoeuvred by the Europeans?

In June 1963, President John F. Kennedy announced government support for design and development of an American supersonic transport (SST). The following year, tests began over Oklahoma City to see how the public would react to being subjected to a daily dose of sonic booms. But on March 24, 1971, the US Congress voted to end further funding for the SST, also called the Boeing 2707. Though over $1 billion of taxpayers’ money had been spent on the ambitious project, not a single plane was ever built. Concerns about the SST’s enormous cost and vocal public protests about sonic booms led to its ignominious demise.

Meanwhile, as happened so many times during the Cold War, the other major player, the Soviet Union was feverishly at work behind the scenes, quietly determined to win the race. It did so, to a limited extent. In December 1968, the Soviet Tupolev Tu-144 became the first supersonic transport aircraft to take to the air. It breached the sound barrier on June 15, 1969, and 10 days later became the first commercial transport to exceed Mach two. The plane’s close resemblance to Concorde led to allegations (never proven) that its design was obtained via industrial espionage.

Derisively nicknamed ‘Concordski’ by the western media and initially limited to freight service, it never entered extended production—just 16 were manufactured—and did not fare well operationally due to a variety of reasons. It was hampered by high fuel requirements, which restricted its range, and suffered from severe stability problems. In a public display at the Paris Air Show on June 3, 1973, a Tu-144 entered an uncontrolled downward manoeuvre. Attempting to pull out of the dive, it broke up and crashed, destroying 15 houses and killing all six crew on board, besides eight people on the ground.

On November 1, 1977, an Aeroflot Tu-144 flew from Moscow to Alma-Ata in Kazakhstan, marking its first passenger flight. However, a Tu-144D experienced an in-flight failure during a pre-delivery test, and crash landed with crew fatalities on May 23, 1978. After notching up just 55 scheduled flights over seven months, the last passenger flight of the Tu-144 took place on June 1, 1978.

Concorde Supreme
What of the world’s only successful supersonic airliner? Working together, the British and French built Concorde, a high-performance passenger jet. Concorde was a four-engine, ogival, delta-winged aircraft. It was the first airliner to have an analogue fly-bywire flight control system. Its distinctive drooping nose was a compromise between a streamlined design, to increase aerodynamic efficiency, and the need for adequate pilot view during taxi, take-off, and landing. It had an average cruise speed of Mach 2.02 (approximately 2,140 km/h) with a maximum cruise altitude of 60,000 ft (18,300 m). Concorde first went supersonic on October 1, 1970.

Following a long development period lasting nearly 15 years and after swallowing huge sums of government money, Concorde entered commercial service simultaneously with Air France and British Airways on January 21, 1976. It soon became evident that operational restrictions would be placed on the aircraft, forcing pilots to slow to subsonic speeds over land. This limited its access mainly to coastal cities. The hullabaloo over SSTs in America resulted in New York banning the plane outright, thus depriving Concorde of the lucrative London-New York route, and crippling its economic prospects. The ban was later rescinded.

Other countries, such as India and Malaysia, ruled out Concorde supersonic over-flights due to sonic boom concerns. Consequently, on many routes it had to cruise much of the time at inefficient subsonic speeds. When unfettered, Concorde’s operational performance was truly amazing. It took just 3 hours 47 minutes to fly over 4,000 nautical miles (7,400 km) from Miami to London, with 70 passengers on board. It was even able to outrace the rotation of the earth. On westbound journeys, therefore, it was possible to arrive at a local time earlier than the local time at which the flight had departed. British Airways profitably used the slogan: “Arrive before you leave”.

Just 20 Concordes were built, six for development and 14 for commercial service. On July 25, 2000, an Air France Concorde crashed during take off in Gonesse, France, killing all 109 passengers and crew, and four people on the ground. Investigations suggested the accident was probably caused by a thin metal strip on the runway, which blew a tyre, which in turn ruptured the airliner’s fuel tanks. It was Concorde’s only fatal incident. In June 2003, Air France retired its last Concorde and British Airways followed suit in October 2003. Concorde’s final flight was on November 26, 2003.

Two Negatives Don’t Make A Positive
There are no SSTs in commercial service today. Why did such a promising concept not succeed? A supersonic aircraft is subject to great stresses and temperatures and needs a heavier structure to minimise flexing. It also requires a stronger and heavier structure because the fuselage must be pressurised to a greater differential than subsonic aircraft, which do not operate at the high altitudes necessary for supersonic flight. Consequently, the empty weight per seat of Concorde was more than three times that of a Boeing 747. Both Concorde and the Boeing 747 used approximately the same amount of fuel to cover the same distance, but the 747 could carry more than four times as many passengers.

Concorde just did not make economic sense. Yet another major problem was the high noise levels. Airlines potentially value faster aircraft, because they can perform more flights per day, which allows for higher return on investment. However, Concorde’s high noise levels around airports and time zone issues meant that only a single return trip could be made per day. Since SSTs emit sonic booms at supersonic speeds, and are not usually particularly efficient at subsonic speeds, this limits the routes that the airliners can be used on, drastically reducing its attractiveness for most airlines.

Around the 1960s, when environmental concerns also emerged, the SST was seen as particularly offensive due to its sonic boom and the potential for its engine exhaust to damage the ozone layer.

Another Concorde?
Within a decade or so, SSTs were sidelined. When first conceived, these were intended to compete with longrange airliners of about 100 passenger capacity, such as the Boeing 707. However, with newer aircraft like the Boeing 747 carrying four times that number, the speed and fuel advantages of the SST concept were completely negated. Subsonic airliners also improved as Boeing finally faced tough competition from a European grouping, Airbus Industrie, which was formally established in December 1970. Boeing was a formidable company, but Airbus developed a reputation for innovation and for responding to customer needs.