Concorde is the world's only passenger aircraft capable of breaking the sound barrier. It may forever retain that honour.

Even before the crash of an Air France Concorde at Gonesse, the future of supersonic civil aviation looked in doubt.

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In the 1960s, with man racing for the Moon, the enthusiasm for "white hot" technology was at its height. Airlines and aerospace companies were far from immune.

Higher speeds topped the list of priorities for many air operators, beating both extra cabin space and better fuel efficiency.

Even as Boeing developed the 747 jumbo jet, its director of engineering, Joe Sutter, said many experts felt "supersonic transport (SST) was going to take all the business".

Jumbo rival

The 747, with its 400 seats to Concorde's 100, has instead become one of the most familiar sights at airports worldwide.

However, the United States had supersonic ambitions of its own. When Boeing's 2707 project was finally shelved in 1971, the Federal Aviation Authority had spent far more than Concorde's Anglo-French development budget of £1.5bn.

They had only a plywood model to show for their investment.

Tony Benn: Championed Concorde

The Soviet Union's attempt to take civilian passengers beyond the sound barrier proved even more abortive.

The Tupolev 144 "Concordski" went into a fatal dive at the 1973 Paris Airshow. All aboard were killed and the Soviet programme stalled with it.

The tragedy left the British Aircraft Corporation, Aerospatiale and taxpayers either side of the Channel alone in the supersonic race.

MP Tony Benn, the former Labour minister for technology who oversaw the development of Concorde, says the project was a triumph of innovation and co-operation.

"It was something to be proud of."

Slight praise

Malcolm MacKinnon, Boeing's programme manager of High Speed Civil Transport, agrees Concorde should be admired, but within limits.

"Concorde is a shining example of brilliant engineering, but the economics of the aeroplane are awful."

"Concordski": Grounded by USSR

Though a product of the 1960s, the aircraft did not enter full service until 1976 - the height of the fuel crisis.

As oil prices increased almost tenfold and with Concorde's four Rolls Royce engines guzzling 5,638 gallons of fuel an hour, high speed lost its sheen.

Its heavy fuel consumption, coupled with small tanks, also meant Concorde was unable to enter the increasingly lucrative trans-Pacific market.

More for less

Rather than spending £50m on a Concorde, major airlines opted for subsonic jets, with their lower operating costs, more numerous seats and larger cargo holds.

The handful of Concordes operated by British Airways and Air France were virtually donated to the national carriers.

British Airways says its fleet turns a profit. The standard fare for a return trip from London to New York is in the region of £6,000 ($8,300).

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Hard times were not the only thing conspiring against supersonic air travel. The rise of environmentalism also added to Concorde's woes, says Tim Johnson of the Aviation Environment Federation.

Aside from guzzling gas, Concorde flies much higher than normal jets and the nitrogen oxide expelled from its engines breaks down ozone, contributing to the ozone hole.

"Then there's noise pollution - it's much noisier, and has a special exemption from noise pollution guidelines," says Mr Johnson.

Any successor to Concorde in Europe would have to meet today's more exacting environmental standards.

Green skies

"Could you bring down the environmental impact? The likelihood is that you can't, that's the penalty of supersonic travel," says Mr Johnson.

A new generation of quiet, clean engines capable of exceeding Mach 2 would not come cheap.

Aviation experts estimate a supersonic jet with the passenger capacity and range to attract paying customers would cost £12bn to design and require international co-operation.

Airbus: Syper capacity, not supersonic

However, major players such as Airbus Industries are committed to developing the A3XX super jumbo, with up to 600 seats.

"High Speed Civil Transport" departments at Nasa and Boeing continue to conduct research into fast passenger aircraft. However, the word "supersonic" is shunned as being too dramatic.

Even if the will, money and orders (for at least several hundred airliners) were to be forthcoming tomorrow, the next generation of SST aircraft would not grace our skies for perhaps more than a decade.

Today's Concordes are likely to be collecting dust in museums before the next step is taken in supersonic flight.