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A Supersonic Laminar Flow Control model of the F-16XL takes a trip through the wind tunnel at NASA's Langley Research Center in Virginia. (NASA Langley Research Center)

Mach 1 for Millionaires

Briefcase-toting suits who travel in bizjets-those will be the next pioneers in supersonic flight.

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The same year the Citation X made its debut, Boeing and General Electric announced the launch of the Boeing Business Jet, a modified 737 airliner outfitted with longer-range fuel tanks and winglets. Customers would buy the BBJ “green,” or unfinished, for around $33 million, then spend whatever they wanted on custom paint and interiors. And spend they did, with the average BBJ finished price heading well north of $50 million. Prior to the BBJ, the top-end market for corporate jets ran in the $40 million range. Of course, there were well known exceptions to even this level of excess: The Saudi royal family had been outfitting 747s and Lockheed L-1011 jumbo jets as flying palaces for years, and at nine-figure prices few could fathom. But the market was shocked by the BBJ’s level of success—100 have been delivered through mid-2005—and it wasn’t long before Airbus rushed to market with a competing aircraft. The BBJ demonstrated that, within the corporate jet market, there were customers in a rarefied niche who would spend almost anything if it meant they would be flying a much larger aircraft.

But even with this seeming disregard for price and the confirmed need for speed among bizjet buyers, supersonic has long been the third rail of commercial aviation. For decades, analysts and manufacturers have viewed a supersonic aircraft as noisy, financially risky, and politically toxic. In the United States, supersonic flight over land by any non-military aircraft is, in fact, illegal (Federal Aviation Regulation 91.817). Many other countries have adopted similar prohibitions.

Beginning in 1958 and largely in response to the supersonic Anglo-French Concorde airliner, the United States sank more than $1 billion into civilian supersonic research, capping it with the very public cancellation of its supersonic transport program in 1972. For those who opposed it, the SST embodied all things wrong with technology. In his 1970 anti-SST tome, William A. Shurcliff, director of the Citizens League Against the Sonic Boom, perfectly captured the histrionics of SST opponents when he wrote: “If overland supersonic flight is permitted, 500,000,000 persons in America, Europe, and Asia may be jolted every hour, day and night by sonic booms from hit-and-run SSTs. People working, relaxing, sleeping will be banged repeatedly, without apology. Surgeons performing delicate operations will be startled, and their instinctive reflex reaction may cause permanent harm to the patients…. Aviation, instead of being man’s servant, would be his scourge.”

The abrupt death of the American SST program, coupled with the slow bleed outs of the Soviet Tu-144 and Anglo-French Concorde programs, did not end research into supersonic transport, but did slow it down. In the United States, funding for such research dropped from about $100 million a year to $13 million under the guise of NASA’s Supersonic Cruise Research program. But even as the anti-SST drumbeat was reaching its crescendo, science was getting results that would make the protests irrelevant. It just wasn’t happening fast enough.

In 1964 NASA scientist F. Edward McLean hypothesized that changing an aircraft’s shape could minimize its sonic boom. Seven years later, two scientists at Cornell University, Richard Seebass and Albert George, published an algorithm “for defining the minimizing equivalent area distribution based on flight Mach number and altitude, and the aircraft’s length and weight.” In other words, it was not only possible to mitigate an aircraft’s sonic boom by altering its shape, you could also use a mathematical model to predict the boom signature of any given shape. NASA validated Seebass-George during 1972 wind tunnel testing for regimes at Mach 1.5 and 2.7. (The model was more accurate at the lower Mach number.) However, it would be more than 30 years later, on August 27, 2003, that these theories were tested on an actual aircraft by the Defense Advanced Research Projects Agency’s Shaped Sonic Boom Demonstration, part of DARPA’s Quiet Supersonic Platform program (see “The Boom Stops Here,” Oct./Nov. 2005).

Predicting and quieting sonic booms are only part of the new science driving the development of the supersonic business jet. The other involves supersonic natural laminar flow.

An aircraft experiences a certain amount of drag from skin friction as air moves across the wing and gives rise to turbulent “boundary layers.” Laminar flow describes air immediately next to the wing, which flows in a series of smooth layers, free of turbulence, resulting in less aerodynamic drag on the wings and improving range, speed, and fuel economy. It is virtually impossible to achieve extensive laminar flow on a subsonic aircraft. Supersonic aircraft offer more possibilities, but their potential for laminar flow is often defeated by design factors, like a highly swept wing, which invariably creates turbulence and drag.

NASA experiments in 1995 and1996 used the delta wing on an F-16XL fighter jet, modified with a power-suction “glove,” to improve laminar flow. According to laminar flow expert and aerodynamicist Richard Tracy, a normal F-16 wing “has too much sweep to support laminar flow and has a slightly blunted [wing] leading edge…and thus a very high wave drag at its maximum supersonic speed.”

While suction gloves seek to improve laminar flow on swept-wing designs, natural laminar flow relies on a wing’s shape alone, without help from other devices. Greater wing sweep produces greater turbulence, thus a natural laminar flow wing has very little sweep, a design that helps stabilize airflow.

The first widely known aircraft to take advantage of this principle was Lockheed’s F-104 Starfighter, a Mach 2 interceptor developed in the 1950s. In 2000, NASA and DARPA teamed up with the Reno Aeronautical Corporation to demonstrate a three- by four-foot natural laminar flow test article mounted to the belly of an F-15B fighter and flown up to Mach 2 at 45,000 feet. The demonstration validated the theory and led NASA to speculate that natural laminar flow had the potential to enable supersonic aircraft to produce “economies comparable to, and in some cases better than, subsonic aircraft in the same role.”

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