While these engines may be quiet enough, questions of durability remain. With the exception of the Concorde’s earth-shaking, fuel-ravenous Olympus engines, no civilian jet engine has ever demonstrated robustness at sustained supersonic speeds. Indeed, even most military engines have upper thermal limits, as they typically are thrown into afterburner for only a few minutes at a time. However, the Air Force’s F-22A Raptor is equipped with Pratt & Whitney F119 engines and has the ability to supercruise—to fly at supersonic speeds for long periods of time—without the noise and high fuel consumption of an afterburner.
Other than its smaller size, the Aerion, unlike the QSST, contains no specific boom suppression or reduction technology. Because regulations prohibit supersonic flight over land, a potential competitor of Aerion’s questions the aircraft’s market appeal. “High subsonic overland does not make sense” for a supersonic business jet, says Gulfstream’s Henne.
“Our market studies were very explicit that the model in question would not be able to fly supersonically over the U.S.,” says Tracy. “In fact, the majority [of those surveyed in the Aerion market study] would purchase the non-supersonic overland model,” even if a quiet supersonic aircraft that could fly over land were to become available five years down the road.
In Tracy’s opinion, “it is doubtful that any definitive regulation [regarding supersonic flight over land] will be forthcoming” before a supersonic bizjet model is built—another reason Aerion isn’t focusing on low-boom shaping. “There is a drag penalty associated with aircraft shaping to decrease sonic boom levels,” says Tracy. “[And that] translates into larger engines, higher fuel burn…and increased costs.”
To the untrained eye, the Aerion resembles an F-104 with an even more pointed nose. It relies on natural laminar flow to achieve speed and economy. It is not surprising that Richard Tracy, who owns Reno Aeronautical Corporation, is one of the company’s principals. During the 1980s, Tracy teamed with Stanford University aerodynamicist Ilon Kroo and Jim Chase, a former colleague of bizjet legend Bill Lear, on different configurations for a supersonic transport in their “free time.”
“I had this passion since graduate school days to see supersonic become a practical mode of flight,” Tracy says. By 1987, the group was on the verge of giving up. “Nothing looked like a significant breakthrough,” says Tracy. Then one night he was awakened by the thought “Why not look at laminar flow?” The idea had been in his subconscious since his days at the von Karman Institute in Belgium in 1959. To Tracy, the advantages of a supersonic natural laminar flow wing were abundantly clear. He was confident that others would see so too, and that he would have no trouble securing research funding to prove the concept. He couldn’t have been more wrong. For 14 years he was politely received, often more than once, by all the major business jet and airliner manufacturers, a variety of other companies large and small, and a broad cross-section of individual investors. They offered praise and encouragement, but no funds. Neither did NASA, until the late 1990s, when DARPA came through with funding for the F-15 experiment.
Serendipity struck when Tracy’s work came to the attention of Robert Bass, a billionaire with a passion for aviation and technology. Bass hired Michael Henderson, who had managed Boeing’s NASA-funded High Speed Civil Transport program during the 1980s, to do further research on the work of Tracy’s group. Henderson reported back favorably and Bass invested.
When it comes to the supersonic bizjet, SAI and Aerion are not the only games in town. At NBAA in 2005 Gulfstream focused on its new Supersonic Acoustic Signature Simulator II. The mobile audio booth, housed in a white trailer, enables listeners to experience the loud double-bang of the Concorde followed by the far quieter “Gulfstream whisper,” and replicates the sounds in environments ranging from noisy city streets to a playground filled with children. The “whisper” is the sound a supersonic bizjet—traveling at Mach 1.8—would make if it were fitted with a special spike for reducing the sonic boom. The spike, for which Gulfstream received a patent in 1994, would extend from the nose of the airplane during supersonic flight, and retract during subsonic flight.
The simulator is Gulfstream’s bid to take the idea of changing regulations regarding supersonic flights over land directly to the people who will be making those changes: Everyone from environmentalists to the Federal Aviation Administration has been invited to step into the trailer and hear, or not hear, Gulfstream’s “whisper.”
Dassault also continues to work quietly on its designs, and NASA remains a prime funder of supersonic research that someday could be applied to a supersonic bizjet. Its pallet of programs includes the Ultra-Efficient Engine Technology program, which focuses on reduced emissions, lower noise, and higher-efficiency designs.