TCS Daily

The Concorde's Sonic Boomlet

By Rand Simberg - October 24, 2003 12:00 AM

Shortly after this article is published, the Concorde -- the world's first (and perhaps, but probably not, last) supersonic airliner -- will make its last flight.


Like the Apollo moon program, it was a creature of the early space age -- a harbinger of the world of the future, in which we would have flying cars, and Mars colonies, and meals and French lessons in a pill, and it was a triumphant testament to an era of both national pride, and national hubris.


Also like Apollo, it was a program before its time, and ultimately unsustainable.


Simple Economics


The Concorde never made any economic sense, at least from a business accounting standpoint. It couldn't repay even a portion of its massive development costs. However, because the French-British government consortium that developed it absorbed those up-front expenditures, it managed to make an operational profit for British Airways and Air France for almost three decades. It may be that even then it didn't make a profit, but had sufficient value to both airlines as a prestige item useful for marketing to allow it to continue.


That suspension of rational business economics comes to an end this month, because the price that people are willing to pay for a three-hour trans-Atlantic trip doesn't justify the increasing maintenance or replacement costs as the small fleet of aircraft, now quite long in the tooth, goes into its dotage. They were particularly unwilling to pay such a price after the disastrous crash in 2000 that killed all on board, and was a result of an intrinsic design flaw that allowed blown tires to be swallowed by hazardously located engine intakes.


None of this is to say, of course, that there's no demand for supersonic passenger travel. Various business jet companies have been looking at options for developing small supersonic vehicles for years, so far without success. People, and not just businesspeople, would certainly like to get from point A to point B much faster - getting from the east coast to the west coast in two hours, say, or cutting trans-Pacific flight time of twelve to fourteen hours to five or six. And they'd like to do so at costs comparable to current ticket prices.


What's the Problem?


So in the case of supersonic transports and the Concorde, the problem with it wasn't that there was a lack of market. Its limitations come in the form of a technology problem; but it may be one that's solvable.


The military aircraft technology on which Concorde was based doesn't allow overflight of land, because of the sonic boom problem, or long-range operations, due to the supersonic wave-drag problem. Even military aircraft don't fly supersonic for long -- they can't afford the fuel, and their ground signature is unacceptable in peacetime -- it's only allowable in wartime in an environment in which sonic booms, and their associated broken windows, are the least of the worries of those on the ground below them. That's not an acceptable mode of operation for a commercial airline.


What is the source of these constraints? It's our continuing ignorance of the fundamentals of supersonic aerodynamics.


In the twenties, the German aerodynamicist Ludwig Prandtl laid out the fundamental principles for subsonic aerodynamics, including the all-important circulation theory, which applied the basic physical concept of conservation of angular momentum to the principle of lift. This laid the foundation for much of modern aerodynamics, making possible the rapid progress and sophistication of aircraft designs from the golden age of the late twenties and thirties, through the war, extending to today's modern subsonic airliners.


Prandtl also developed the first theory for supersonic flow, but it was sadly deficient. It didn't, in fact, extend his critical subsonic circulation-flow theory into that realm, though some believe that this can be done, at least based on more recent theory and results. As a result, we've been plowing through the atmosphere supersonically, in a more or less controlled manner, wasting fuel and shattering glass on the ground, since Yeager's flight in 1947, and we've characterized the supersonic effects of aircraft of all manner of shapes to the nth degree, and managed to design a commercial aircraft that could fly supersonically. But we haven't really developed a true understanding of the supersonic flow regime.


Why is this a problem?


Because the reason that supersonic flight is currently commercially impractical (and only barely practical even militarily) is because of the shock waves formed by aircraft designed on the current theory. These generate a tremendous amount of drag, increasing fuel consumption by a factor of at least two, and often much more, relative to subsonic flight. They also take the energy they thus steal from the engines and convert it into the ground disturbance that restricts overflight of land. If their formation could be eliminated, or at least substantially reduced, supersonic flight could in fact become as practical and routine as subsonic flight.


Conventional wisdom is that shock waves are an inevitable consequence of supersonic flight, but there is actually no law of nature that requires them. In fact, shock-free supersonic bullets have been designed for use by military snipers (for the purpose of keeping misses quiet -- much of the noise of a conventional bullet whizzing past the ear is a small sonic boom). So we know that it's possible to travel supersonically without shock. The trick is to do it while also generating lift.


Working Towards Supersonic Flight


There is a small company in South Pasadena, California, called Vehicle Research Corporation (VRC), that believes this can in fact be done, and holds some patents on the process (in the interests of disclosure, I note that I have a small bit of equity in this firm). They believe -- with a lot of analysis, and some computational fluid dynamics (CFD) simulations that validate it at least to a degree -- that shock waves are nature's inefficient way of balancing the wing circulation, required for lift, in supersonic flight. Thus, with more clever designs, introducing a small amount of energy into the flow to provide this necessary countercirculation, the formation of the shock waves can be greatly reduced, if not entirely eliminated, to the point at which they become inconsequential.


If they're right, it would mean that aircraft could fly at Mach 2 or 3 for not a lot more fuel than they currently fly at Mach 0.9. It means that they could have far greater range, and fly lower in the atmosphere (one of the concerns about supersonic transports was that to maximize fuel economy, they had to fly so high that they became a concern from an ozone standpoint). Engine design requirements would be greatly eased, allowing more conventional placement (eliminating the possibility of a repeat of the Concorde disaster in 2000). Wings of such aircraft could have much larger aspect ratios (similar to subsonic airliners), further reducing induced drag, and providing much better takeoff and landing performance, eliminating the need for afterburners and swing wings, vastly increasing safety and decreasing cost..


They could fly from Los Angeles, or even New York, to Hong Kong or Sydney, and do it in a half or a third of the time of a 747. They could fly from the east to west coast in two hours. There would, in fact, be no more restrictions on where they could fly than currently apply to subsonic air transports. And they could potentially do it in wide-bodied aircraft, rather than the slender wasp-waist designs such as Concorde.


In short, they could revolutionize air transport far beyond the dreams of any previous supersonic program.


So, are they right? Many knowledgeable people, including the former Chief Engineer of Northrop, who have looked at it think they're on to something. My own opinion is that the physics seems sound, and they're not obviously wrong. The question is whether or not practical aircraft designs can be developed from their theories, and that's one that won't be answered until the theory is further developed and quantified with additional CFD, analysis for which, to date, they've been unable to find funding. Should they do so, however, either from the government or private sources, it may finally usher in a new, practical supersonic era, perhaps rendering today's subsonic jets obsolete, and firmly ending any false nostalgia for the first unsuccessful supersonic transport.


Rand Simberg is a recovering aerospace engineer and a consultant in space commercialization, space tourism and Internet security. He offers occasionally biting commentary about infinity and beyond at his web log, Transterrestrial Musings.


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