“Everybody talks about the weather,” Mark Twain once famously quipped, “but nobody ever does anything about it.”
The sound barrier amounts to roughly the same thing. Not the sound barrier that was once thought an insurmountable obstruction, the one that caused piston-driven airplanes to shed wings and props, the hotly pursued goal of Mach-plus flight. No, almost as soon as Chuck Yeager’s bulbous little Bell X-1 poked its nose through the speed of sound, another problem presented itself–the sound of speed.
As soon as airplanes capable of carrying men began routinely puncturing Mach 1, a new noise was heard throughout the land–the sonic boom. Its sheer power and far-reaching range soon made it clear that supersonic flight, especially over populated areas, would never be routine. As for commercial supersonic flight, the continuous wave of rolling thunder it generated at Mach-plus speeds forever confined such operations to oceanic airspace.
Today, 27 years after the introduction of commercial supersonic flight, researchers from Northrop Grumman, Lockheed Martin and General Electric–with funding from the Defense Advanced Research Projects Agency (DARPA)–are looking into ways to quiet supersonic flight, albeit with development of a hushed supersonic bomber (not a civil transport) initially in mind.
Under what’s known as the Quiet Supersonic Program (QSP), the goal is a strike aircraft with a 6,000-nm unrefueled range and an mtow of 100,000 pounds. The real challenge lies in the QSP’s third program requirement– production of a sonic boom with a force of less than 0.3 pounds per square foot. Compared with Concorde’s average of two pounds per square foot, that would make the passage of a QSP aircraft at Mach-plus speed roughly equivalent to a light summer breeze.
Already in flight test is Northrop Grumman’s shaped sonic-boom demonstrator (SSBD), a heavily modified Northrop F-5E fighter bearing a four-foot extension to its forward fuselage, giving the aircraft a pronounced duckbill look reminiscent of the 1960s-vintage Sabreliner business jet. The SSBD has already completed preliminary test flights up to Mach 1.1. With its basic handling characteristics confirmed (the slightly unstable-as-modified F-5E won’t have to do much more in flight than go supersonic in a straight line), the aircraft has been moved to Palmdale, Calif., where further flight tests will be conducted at the NASA Dryden Flight Research Center/Edwards AFB.
Head-to-head comparison flight tests will begin this fall, with the SSBD aircraft following an unmodified F-5E through the sound-monitoring area. Both aircraft will fly at speeds of Mach 1.3 to 1.4 and their sounds will be compared. “It’s essential to remember here that what we’re doing is seeking ways to shape and thereby reduce the sonic boom, not eliminate it,” said program manager Charles Boccadoro.
Bizjet Builders Involved
Ever eager to obtain the edge that would put their products head and shoulders above everyone else’s and define that ever-elusive new market, some business-jet builders are assisting with QSP testing. While NASA plans to fly its McDonnell Douglas F-15 testbed aircraft close to the QSP at low altitude to ascertain supersonic shock wave readings, Gulfstream has volunteered a G550 for measurements at 20,000 feet, while Raytheon plans to do the same with one of its Premier Is for the 10,000-foot portion of the tests. Given present levels of program funding, eight full-up test flights are scheduled.
Program participants stress that the QSP is just the first stage in the development of quiet supersonic cruise technology that could branch out to embrace more exotic methods of achieving quiet high-speed flight, such as aircraft shaping (similar to the droop-nose technique used to optimize cockpit visibility during low-speed flight in Concorde, only more dramatic), and plasma, heat or particle injection into the engines.
As for a practical timetable for development of this new technology, understand that the U.S. Air Force has announced plans for a new bomber to be operational by 2035. (With overhauls and upgrades, the venerable Boeing B-52, already being flown by crews less than half their aircraft’s age, is expected to be kept in service until then, when some of that fleet will be 80 years old.) While Congress is already financing small-scale R & D programs such as the QSP, robust funding for the eventual long-term future supersonic quiet cruise bomber is not expected before 2010.