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An Overview of Supersonic Flight

An Overview of Supersonic Flight

Supersonic flight has come a long way since the first manned flight to exceed Mach 1 in 1947, and to mark the 60th anniversary of that milestone, Aviation Week & Space Technology looks at what comes next.

Sixty years ago this week, the clear air over the Mojave Desert resounded to the crack of the first ever sonic boom created by a manned aircraft. High overhead in the Bell Aircraft XS-1 experimental aircraft, U.S. Air Force Capt. Charles (Chuck) Yeager noted how his primitive Mach-meter, having reached an indication of Mach 1.02 “momentarily stopped and then jumped to 1.06 and this hesitation was assumed to be caused by the effect of shock waves on the static source.”

While Yeager wrestled with the aerodynamic shocks created by his Reaction Motors XLR11 rocket-powered XS-1, it was the reaction of the world at large which caught the Air Force off guard when news of the previously secret achievement was first broken by Aviation Week in its Dec. 22, 1947, edition. The breaking of the “sound barrier,” as the Mach 1 milestone was popularly dubbed by the world’s press, represented a pivotal milestone to the burgeoning post-war aviation industry. It also set the stage for a fiercely competitive era of international speed record-breaking that culminated with developments such as the Mach 2 British Aerospace/Aerospatiale Concorde supersonic transport (SST) and Mach 3.3 Lockheed SR-71 reconnaissance aircraft.

Yet, despite the early promise of supersonic developments and the routine Mach 1-plus capabilities of frontline combat aircraft today, it could be argued the wider potential benefits of faster-than-sound travel and military utility have failed to materialize. The SR-71 and Concorde, both built in limited numbers as befitting their exotic nature, are now relegated to museums.

While spy satellites, U-2Rs and other assets succeeded the SR-71, the grounding of Concorde, and its shorter-lived counterpart the Tupolev Tu-144, deprived the public of its fastest-ever mode of “mass” transport. The touchdown of the last Concorde at Filton, England, in November 2003, effectively represents the first time in human history that progress in travel time has gone into reverse. Up to that moment the speed of transport, and with it social progress, had steadily increased with every technological step since humans began riding on horseback.

So what is the true value of speed, and is it sufficient to warrant the expense that inevitably comes with development and deployment of supersonic technology? In the commercial world, pundits would point to the abandoned U.S. SST, NASA High-Speed Research program, the grounded Concorde and even Boeing’s Sonic Cruiser as evidence that it is not worth it. In each case, the cost of making these ambitious transports economically viable and just as critical, environmentally acceptable, has proved too great to bear.

There is no getting around the laws of physics that dictate more engine power is needed to overcome the massive drag rise associated with transonic flight. Going beyond Mach 2 becomes even more expensive as skin friction and compression effects raise air temperatures beyond 200F, requiring costly materials and complex systems. Even without environmental concerns, technical challenges alone have left the road to supersonic transport littered with the remains of dead SST projects.

However, speed continues to hold its attractions in the civil aviation world, particularly to the executive and corporate markets, and even against a rising environmental headwind, the emergence of the supersonic business jet (SSBJ) appears to be gaining traction (see p. 50).

The issues of airport noise, sonic boom and high-altitude emissions are also less with a smaller aircraft and the development of a viable SSBJ could certainly help reduce the risk attached to certificating a second-generation supersonic airliner—should any new project get beyond the study stage. Also, innovations such as sonic boom mitigation using advanced shaping techniques honed as part of Defense Advanced Research Projects Agency’s (Darpa’s) Quiet Supersonic Platform program, and the NASA-Gulfstream Quiet Spike extendable nose probe (see p. 52) have brought designers closer than ever to the goal of boom-less flight.

“Success on a small vehicle will be a stepping-stone to larger vehicles,” says Preston Henne, Gulfstream Aerospace’s senior vice president for programs, engineering and test. “Today’s subsonic air transport system grew from small vehicles and affluent customers—history will repeat itself with supersonics.” Henne argues that industry prematurely created a large SST, and that the sonic boom problem—which makes it illegal for supersonic over-flights of the U.S.—is an unfortunate Concorde legacy.

Gulfstream’s position is that supersonic flight over land is a “must” for a successful SSBJ design although others, such as Aerion, disagree. Either way, Henne believes that thanks to Quiet Spike and other developments, “assessments of new technologies indicate such a design is now within reach for a small vehicle.”

Military exploitation of supersonic propulsion and airframe technology for sustained Mach 1-plus flight during long-range strike missions is receiving renewed interest in the U.S., despite USAF’s recent decision to target a subsonic aircraft for its next-generation bomber. To go beyond this, the groundwork for new-generation combat variable-cycle engines has begun under Advent—a part of the Air Force-led Vaate research and development program (p. 58).

Propulsion technology is seen as a key reason why “long-haul” military supersonic dreams have consistently lagged behind reality. “The only one that was really good at sustaining it, and in a relatively affordable way, was the Concorde engine,” says Steve Komadina, program manager for Northrop Grumman’s supersonic tailless air vehicle study, referring to the SST’s Rolls-Royce/Snecma Olympus 593 engines. In their final production form for Concorde the 593 was rated at 32,000.-lb.-thrust dry and 38,050.-lb. thrust with afterburner. Over the life of the Olympus project, the Concorde engine fleet accumulated 930,000 flight hours, more than 500,000 of which were supersonic.

Now with Advent, a growing U.S. Defense Dept. interest in long-range supersonic capability, and related systems and aerodynamic studies into high-speed concepts, Komadina believes the impetus is there to reach reality.

In this supersonic special we review the latest developments in the SSBJ arena and the ambitious supersonic transport research plans of JAXA, the Japanese aeronautical research agency. We also talk to USAF’s chief scientist on the long-term importance of supersonic capability, examine key high-speed airframe and weapons projects in the U.S. and Europe, and review the Air Force Research Laboratory Advent engine program that might help deliver some of these performance goals.

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