It would be difficult to describe the prospects for future supersonic civil transports as anything other than bleak, and getting bleaker, even as the enabling technology is advancing. Notwithstanding the promise of a supersonic corporate jet getting off the ground, there seems little realistic likelihood of even premium airline passengers being able to keep up their supersonic travel habit once the BAC/Aerospatiale Concorde is retired in, say, 15 years' time. If there is to be a future alternative to subsonic long-haul travel, it is much more likely on current readings to be of the hypersonic variety which, of course, could also make it even less likely to happen.

So, with billions of dollars being, and having been, spent on high-speed civil transport (HSCT) research, why have the prospects of a commercial realisation of that research suddenly become so bleak? Curiously, the damage appears to have been done as much by the proponents of HSCT technology as much as by its traditional opponents. The arguments of the latter are well-rehearsed, but gaining in strength with time. They fall into two main areas of concern: economics and the environment.

For example, Sir Ralph Robins of Rolls-Royce recently suggested that developing a new supersonic airliner would entail non-recurring costs of around $30 billion, against a potential market for as few as 100 aircraft. That is a sum that he says could only be met by governments.

Leaving development and acquisition costs aside, the airlines that could reasonably be expected to exploit a new supersonic airliner show little enthusiasm for it. An HSCT could only be viable if it carried something like 300 passengers over distances of 8,000km (5,000 miles). That viability could only be achieved, however, by raiding all the first and business-class passengers who currently make subsonic long-haul operations viable, to fill those 300 seats.

Environmentally, the long-term effect of emissions from high-altitude supersonic operations is still largely unknown, but suspected to be perhaps even more harmful than that from lower-level, subsonic flights. The effect of noise is more quantifiable - especially at take-off - and is, indeed, the rock on which current efforts in the USA seem to have been grounded.

HSCT researchers have, recognised that the assumption of Stage III noise targets, against which their efforts were to be measured, are unrealistic when the subsonic world is already moving to a more-demanding Stage IV. They have equally correctly decided that Stage IV performance is effectively unattainable at economic cost with technology available either currently or in the near future.

Hence the attraction of the further-out hypersonic aircraft, drawing perhaps on the technology of the X-33 spaceplane project.

A non-air-breathing vehicle cruising far above the atmosphere would at least be immune from the complications of supersonic boom - though how to get it quietly from the ground to extra-atmospheric cruise seems a daunting challenge.

What is the alternative, then? A small(ish) supersonic business jet is attainable as a goal using extensions of existing technology, and is probably more viable economically in a market that has shown an extraordinary elasticity in pricing in recent years.

Ironically, it could also be the business-jet community that provides the answer for the commercial airline sector as well, however, through its exploitation of the transonic. Current airliners typically cruise at around Mach 0.8 - 0.85: business jets like the Cessna Citation X cruise far faster without the use of rocket science or unrealistic amounts of fuel. The extension of that experience to the next generation of airliners could give the airlines real, boom-free, benefits at a far lower cost in monetary and public-image terms than searching for a supersonic solution.

What that needs, though, is for the aerospace community to have the courage to say that the road to the supersonic airliner, for all its dazzling promise 30 years ago, was a blind alley, and to call a halt to the HCST effort now. The resources that will be wasted in failing to find an environmentally and economically valid HSCT solution could better be used elsewhere.

Source: Flight International