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Supersonic without the boom

The effort to build an airplane with a quieter sonic boom could let ordinary passengers break the sound barrier again.

Andrew Hoyle/CNET
Yara Shahidi Winter 2018 cover

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Mark Mann

It hasn't flown in 15 years, but Concorde Alpha Foxtrot still looks like something from far in the future.

You'll notice its striking and almost sensuous lines the moment you enter its museum home in Bristol, England. A sharply pointed nose gently widens to a slender fuselage and broad delta-shaped wings that dip slightly to the floor. Farther back, streamlined nacelles hide the massive engines and a raked tail fin towers above a rear end that tapers to another fine point. Though it was made in an era of slide rules and blueprints, Alpha Foxtrot remains a stunner.

If you've always regretted (hello!) not flying on the world's only supersonic airliner to regularly carry passengers, all you can do today is sit in Alpha Foxtrot, or one of the other handful of Concordes in museums around the world, and watch the cabin Machmeter trip over Mach 1 (Champagne not included).

Yes, you're just playing pretend, but keep your seat belt fastened. Several companies are now working to bring the dream of commercial faster-than-sound travel back to the flying public. The list includes startups and established aerospace firms like Lockheed Martin, but all have the same goal: design a supersonic airliner that's cheaper, quieter and friendlier to the environment than Concorde while minimizing the troublesome effects of a sonic boom. It's that last point that's fraught with the most complications, and each company is going about it in different ways... but change is in the air, or at least sitting on the runway.

Lockheed Martin Skunk Works X-Plane

The Lockheed Martin Skunk Works' X-plane design will cruise at 55,000 feet, Mach 1.4, and will generate a gentle, supersonic heartbeat instead of a sonic boom.

Lockheed Martin

Unlocking a low boom

It's been almost half a century since Lockheed built a passenger airliner, the cutting-edge (for the time) L-1011, but the company is now mulling a return to the commercial sector. Through a partnership with NASA announced in April, it's building a test airplane designed to fly with a quieter sonic boom. Scheduled for a first flight in 2021, the Low-Boom Flight Demonstrator (also known as the LBFD or the X-59 QueSST) could be the first step to the return of supersonic Champagne.

Peter Iosifidis, Lockheed's LBFD program manager, says mitigating the effects of a sonic boom over land is critical to making supersonic travel economically viable. Only with a quieter boom would governments in the United States and other countries consider changing legislation that limits or outright bans commercial supersonic flight over land. Such regulations restricted Concorde to flying only between New York and Paris and London, robbing it of a worldwide customer base.

"If you don't have a low-sonic-boom airplane and you're only traveling fast over the ocean, you're limiting your pair cities for travel," he says. "The business model to service only a third of the world ultimately doesn't work… you have to satisfy the entire global community."

To alleviate noise concerns, both Lockheed Martin and NASA promise the LBFD's sonic boom will be much quieter than the thunderous noise Concorde produced, sounding instead, they insist, more like a car door slamming. Instead of the two sharp pressure pulses that formed Concorde's sonic boom, an LBFD observer on the ground would discern only a very gradual pressure rise, if they noticed anything at all.

"There are other things that could come into play like atmospheric conditions and background noise," Iosifidis says. "It's possible you might not even distinguish [the sound] as a sonic boom versus literally a car door slamming."

The key to making the LBFD a "low-boom" aircraft is its shape. The long pointed nose, the sharply swept wings and the shape of the canards (small wings positioned forward of the main wings) ensure that the individual pressure waves that the airplane produces at speeds faster than Mach 1 never converge and cause a traditional sonic boom. Otherwise, the LBFD won't greatly differ from other supersonic aircraft. It won't be made of specialized materials and it will use an existing General Electric F414 engine.

"A low sonic boom is directly attributable to the shape of an aircraft," Iosifidis says. "If the airplane was shorter, we would not be able to separate those shocks like we need to."

The nose on the LBFD is so pointy that the cockpit won't have a natural field of vision out the front. So instead of a windscreen, it will use a monitor called an external vision system that NASA will provide. (Concorde's long nose presented a similar problem, but its designers got around if by letting the nose drop during landing to let pilots see the runway.) 

Peter Coen, project manager for NASA's Commercial Supersonic Technology Project, says NASA's mission in the partnership is to design the technology that will make the return of supersonic airliners possible. Who will build the final passenger-carrying aircraft using the technology is unimportant, at least for now.

"Our goal is to get to the point where a supersonic airliner could fly any route efficiently and while being an environmental good neighbor," Coen says. "We're putting the technology out there that the aerospace industry can use to make successful products that get beyond Concorde's limitations."

NASA reinvigorated its sonic boom reduction efforts in 2006 using aeronautical design and mathematics research that had steadily advanced since the 1960s. An early step was to create 20-inch-long models of LBFD designs and predict the strength of the boom reaching the ground.

"What we really wanted was a ground signal that gave you the lowest possible noise," Coen says. "We've found the best way to do that… is to control the strength and position of each shock wave so that they're relatively similar in strength."

Lockheed Martin and NASA will begin construction of the first test aircraft next year at Lockheed's Skunk Works facility in Palmdale, California. Skunk Works produced some of the US Air Force's most secretive aircraft, including the U-2, the F-117 Nighthawk and the SR-71 Blackbird (the L-1011 was built at an adjacent facility).

After initial test flights to prove the LBFD's airworthiness, Lockheed and NASA will start flying supersonically over populated areas to make sure the aircraft is performing acoustically the way it was designed and gauge public response to the low booms. Those tests are currently scheduled to begin in 2022 from nearby Edwards Air Force Base.

"We really want to get as broad a response as we can," Coen says. "We want to understand what the effects of repeated exposures are as well as individual exposures."

SNAPPA

The problem with booms

sonic boom is the shock wave that results when an aircraft flies faster than the speed of sound. If you're going to go supersonic, there's no way around it.

During flight an airplane pushes air molecules out of the way, creating a series of pressure waves that move in all directions at the speed of sound (Mach 1). As you near supersonic speeds, these waves can no longer get ahead of your aircraft and start to compress together, forming a shock wave with a cone shape. It's rather like what happens when a boat moves through water: At slow-enough speeds, the waves that the bow produces can stay ahead of it. But once the boat starts to move faster, the waves can't get ahead and form a wake.

When an aircraft breaks the sound barrier, the shock wave cone radiates out and back from the aircraft down to the ground. The shock wave itself takes the shape of an "N" with a sudden rise in air pressure followed by a sudden drop in pressure and a return to normal. A person on the ground hears the air pressure changes as two sharp bangs, which form the sonic boom. The boom's intensity increases the closer an aircraft is to the ground and the bigger the aircraft is.

Now playing: Watch this: A Concorde gets a new home
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One common misconception is that an aircraft produces a sonic boom only at the moment when it breaks the sound barrier. Rather, a sonic boom is a continuous effect the entire time an aircraft is flying supersonically. So, if you were to fly from New York to Los Angeles supersonically, people across the country under the flight path would hear the boom when you pass overhead.

As sonic booms can annoy people, disturb wildlife, break glass and damage older structures, commercial supersonic flight was almost doomed from the start. Things started particularly poorly in the United States when the Federal Aviation Administration conducted a series of tests over Oklahoma City in 1964 to observe the effects of sonic booms on populated areas. Over a period of six months Air Force planes produced 1,253 sonic booms during daylight hours.

Though residents were amenable to the tests at first, complaints and damage reports exploded after the first two months and the program ended early. Opposition to sonic booms, particularly from environmentalists, only grew from then on, leading Concorde to face worldwide protests when it started test flights in 1969 and passenger flights seven years later. 

spike-s-512

Designed as a business jet, the 18-passenger S-512 is scheduled to fly in 2019.

Spike Aerospace

A business boom

Other companies working to bring back supersonic flight are taking different approaches. Though Boston-based Spike Aerospace also is developing a low-boom aircraft, its S-512 is designed as an 18-passenger business jet.

Vik Kachoria, Spike's president and CEO, says his company chose a smaller jet for two reasons: Business travelers will be more willing to pay to fly supersonically than leisure travelers and it's less of a design challenge. And like with the LBFD, the shape of the S-512's components plays a part.

"It is really, really difficult to [mitigate a sonic boom], especially in a large airliner-sized aircraft," Kachoria says. "The solution, then, for us, is to create a smaller business jet. A smaller shape, shorter length and lighter weight are all significant factors in reducing that boom."

Scheduled to fly in 2019, the S-512 should have a boom no louder than 75 decibels on the ground, or just a bit louder than normal conversation (Concorde's sonic boom was around 105 decibels on the ground).

"A [traditional] sonic boom can be a little disturbing, especially if you aren't expecting it," Kachoria says. "At an air show, you're ready for it so it's kind of fun."

aerion-as2

Aerion's AS2 will exploit a "Boomless Cruise" phenomenon where shock waves will be refracted upward away from the ground.

Aerion Supersonic

Back across the country in Reno, Nevada, Aerion Supersonic also is opting for a business jet, but one that does not incorporate specific low-boom features. Instead, the AS2 will exploit a phenomenon called "Mach cutoff speed," which will enable it to fly at Mach 1.2 without a sonic boom reaching the ground.

During a Mach cutoff flight, which Aerion refers to as "Boomless Cruise," the shock waves will be refracted upward away from the ground as they enter warmer layers of the atmosphere below cruising altitude. The company says this approach means the AS2's success doesn't depend on the still-unproven low-boom technology.

"The key to Aerion's efficiency is its pioneering wing technology to reduce drag and fuel consumption," says Brian Barents, Aerion's executive chairman and CEO.

Aerion is working with Lockheed on the design of the AS2, which is scheduled for a first flight in 2023. That's also about when a Colorado startup, Boom Technology, says it will fly a supersonic 55-passenger aircraft with a sonic boom 30 times quieter than Concorde's.

Boom refused multiple requests for an interview or to disclose more details on its plans.

aero-bristol-concordejpg

OK, Alpha Foxtrot had deafening engines, she guzzled fuel and spewed emissions at ozone-damaging altitudes, but she's still a beauty.

Kent German/CNET

Before the boom

But even a low-boom aircraft faces a big obstacle in the United States: Since 1973, the Federal Aviation Administration has banned civil supersonic flights over the United States. Getting the FAA to lift the ban would be a two-step process. The agency would need to define an acceptable noise level for a sonic boom, and each company would have to prove that its aircraft doesn't exceed it.

NASA's Coen says he'd like to see the FAA be more aligned with the International Civil Aviation Organization (ICAO) regulations on sonic booms that most countries outside the US follow. Rather than banning them outright, the ICAO prohibits noisy booms from reaching the ground.

"We want to replace the prohibition with a certification standard more like there is for takeoff and landing noise now," he says. "Essentially, 'If the airplane is quieter than this, and you can prove it,' it gets certified."

Though the FAA has yet to discuss any changes to its policy, in its 2018 FAA reauthorization bill Congress directed the agency to take the lead in bringing back civil supersonic aircraft.

But even if that process takes 10 years, Iosifidis says it's vital that the federal government follow through if the US aerospace industry is to remain competitive.

"It's really the government's responsibility to take the initiative to overturn that ban and open up the domestic aerospace industry to a new commercial market of supersonic travel," he says. "If we don't do that, it's just a matter of time before another country does." 

This story appears in the Fall 2018 edition of CNET Magazine. For other magazine stories, click here.

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