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Thursday, October 22, 2020

How to Design a Supersonic Plane for the (Fairly Rich) Masses

Then there’s the issue of Concorde’s infamous racket on takeoff and the ear-splitting sonic booms that followed it when it flew above the sound barrier. The XB-1 prototype won’t be particularly quiet—it will use old-school but reliable and easily serviced GE J85-15 military engines originally developed in the 1950s—but it will be flying over open desert in California, so noise won’t be an issue. The final Overture, however, could be as quiet as a comparably sized commercial airplane on takeoff, if the new engines Rolls-Royce is developing come through as anticipated. Aboulafia notes that even though propulsion engineering has come a long way in the 50 years since Concorde debuted, there really isn’t a comparable engine for Overture at the moment. Most aircraft engines are either compact and powerful for fighter jets, or large and efficient for commercial airliners. The engines for Overture will need to fit inside the fuselage and rocket the airplane to supersonic velocities without the fuel-gulping and noisy afterburners that Concorde required, yet still meet noise and emissions standards.

As for the notorious sonic booms that can rattle windows and wake up babies, Boom will skirt the issue by having Overture avoid overland routing while at supersonic speeds—and in any case, its shape should generate a muffled boom that won’t be nearly as pronounced as that of the Concorde or the average fighter jet, Scholl says.

Supersonic flight poses materials challenges as well. The craft are exposed to far greater heat and stress than conventional aircraft, and aluminum, for example, loses strength at high temperatures. Carbon fiber, however, retains both its shape and strength, giving designers more latitude in shaping the wing and fuselage so as to minimize airflow disturbances and reduce drag.

For the aft fuselage, engineers used titanium, to better withstand the high impact forces during landing, estimated to be 112,000 pounds of force at each wheel, and better support the weight of the three engines. The XB-1 also relies on a material called Ultem 9085, a thermoplastic that can be 3D-printed into strong, lightweight, and fire-resistant parts, the company says. The ability to print hundreds of spacers, ducts, brackets, and more at the hangar saved significant time and money.

Advanced as they are, those materials are well known within the industry. Where things are more unknown is the fussy, often spooky aerodynamics of supersonic flight. At high speed, air vortices coming off the nose can interact with vortices coming off the wing and tail, affecting how the airplane behaves, so engineers have to tune the aerodynamics to avoid these collisions. And since the wing won’t mechanically sweep forward and back to optimize for both low-speed and high-speed performance—think the F-14 in Top Gun—configuring the wing’s delta shape primarily for high-speed flight impacts its stability when it flies more slowly during takeoff and landing. So engineers developed a hybrid fly-by-wire system that uses conventional hydraulic mechanical linkages for the controls, supplemented with electrical actuators to help improve stability, and they prescribed a higher nose angle in order to maximize airflow under the wing at low speed. Since that could cause a tail strike on landing, they opted for taller landing gear. Of course, that higher nose angle also limits the pilots’ forward visibility. Concorde addressed this with a drooping nose mechanism, but Boom didn’t like the mechanical complexity of that system, so it’s opting for an augmented-reality-based camera system in the nose gear to help pilots see during takeoffs and landings.

Flight tests of the subscale XB-1 will also show how the plane deals with a phenomenon known as “Mach tuck.” Here, as an aircraft approaches supersonic speed, the nose tends to dip down as shock waves, migrating rearward as speed increases, create pressure differentials that increase lift at the back of the wing, destabilizing it. “We have predictions for this, but it’s really critical that we go collect data on a real aircraft so we can fully counteract that,” says chief engineer Greg Krauland, who worked at rocket maker SpaceX and aerospace innovator Scaled Composites before joining Boom. “There might be implications for how the flight controls are programmed, so these are the kinds of issues we’ll look at as we push the XB-1 from subsonic testing through transonic and then up to supersonic.”

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