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Gulfstream Says SSBJ 'Isn't There Yet'
A realistic SSBJ proposal is still 10 years away from entering service, and some of the people making the running right now are not facing up to the fundamental issues that have dogged progress to date.
That's the opinion of Pres Henne, Gulfstream's svp, programs, engineering and test, who says, "we are continuing to research the environmental, cost and technical problems [for SSBJs] and the solutions are not there yet. Our design is capable of reaching Mach 1.8, but unless you can fly supersonic over land then you're not in the game.
"Until the regulations change to allow supersonic flight over land we just aren't going to make a significant investment in launching a product. We have shown that we can reduce the supersonic boom to a noise level that's less than that of a car door slamming—but until we can convince the regulators, there's nowhere to go. The rule change is the real key in our minds—not to some of the other guys it seems."
The issue of transonic noise and shockwaves is a complex one. While the noise from a cruising aircraft is now seen as manageable, says Henne, there are still difficulties in limiting the so-called 'focus' and 'defocus' sonic booms produced when the aircraft accelerates and decelerates through Mach 1. These conversion points can experience a noise level that may be twice that of the cruise portion of the flight. However, Gulfstream's work to minimize cruise noise makes the focus/defocus problems more tractable, Henne believes.
The issue of noise and overland flight affects every element of future SSBJ operations, according to Henne. "Time is the biggest driver for SSBJ design. The magic figure is to achieve a 40-50% saving in transcontinental travel times. Even flying at Mach 1.8 we may only be able to cut that time by about 35% because of the stop/start penalty at the end of each trip. The other thing is cost—you are going to pay a lot of money for a supersonic aircraft but if you're going to spend a lot of time at subsonic speeds then what's the point? You might as well buy a Citation X or something."
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