Quiet Supersonic

How science is turning down the volume on sonic booms.

Supersonic Jet

Supersonic Jet

Imagine, high above you, the sleek silhouette of an airplane with a shape more like that of some futuristic spacecraft from a Hollywood blockbuster. You can clearly make out the long, lance-like nose, steeply swept wing and powerful engines as they blast this 21st-century private jet through the stratosphere at twice the speed of Concorde. Inside, well-heeled passengers are whisked in supreme, supersonic comfort toward their destination half a world away. Rather than a bone-rattling sonic boom, the sound you hear is a mere burble as the jet streaks overhead, vanishing beyond the horizon as quickly as it appeared.

The idea sounds like pure fantasy — and yet this is precisely the dream of a handful of bright entrepreneurs, and even many scientists and engineers at NASA and companies such as Boeing and Lockheed Martin, who say a quiet supersonic transport capable of flying from New York to Paris in less than two hours might be closer to reality than any of us could have imagined.

If such an incredible airplane could be brought to market, it would represent the ultimate in luxury travel — and a potentially potent business tool, too. After all, at its essence a private jet is really little more than a time machine, albeit one with a comfortable interior and a wet bar. And a supersonic private jet would be the ideal such conveyance, allowing its owner to move from point A to point B in the minimum amount of time.

When it comes to future supersonic jet design, there are two schools of thought. The first holds that a civil supersonic transport should be able to fly much faster than the speed of sound over the ocean and the uninhabited parts of the earth, then slow down to high-subsonic speed for the portions of its journey over populated regions. The second school — and the one that is quickly winning the upper hand in the debate — says that for supersonic travel to make economic sense, such an airplane should be capable of flying at speeds well above Mach 1 during its entire time aloft, other than takeoff and landing.

Anatomy of a Sonic Boom
Before we discuss the ways in which researchers hope to quiet the noise caused by sonic booms, let's tackle the question of what happens when an airplane breaks the sound barrier.

When an object passes through the air, it creates a pressure wave ahead of it and behind it — similar to the bow and stern waves of a boat. As the speed of the object increases, these waves travel at the speed of sound and are increasingly forced together, or compressed. Because the waves can’t get out of the way of each other, they eventually merge into a single shock wave at the speed of sound — the critical speed known as Mach 1, measured as approximately 667 knots at sea level and a temperature of 20 degrees C (68 degrees F).

If you listen as a fighter jet flies overhead at subsonic speed, you can hear the roar from the jet engines and even the turbulence over its body as it passes through the air. Because the fighter is traveling slower than the speed of sound, all of that raw sound reaches your ears ahead of the fighter. As the jet speeds up and goes “transonic” (the realm of speed very near the speed of sound), sound waves and the air around the airplane start to “pile up” in front of the fighter. The forces pulling the air around the fast-moving jet at this point are comparable to the actual air pressure around it, creating more drag and more turbulence than in deeply subsonic flight, with noise levels increasing many decibels above what they would be at, say, two-thirds the speed of sound.

Once our theoretical fighter jet reaches the speed of sound, its bow wave of pressure becomes a shock wave — the disturbance from air passing over the wings and around the fighter’s body travels faster than sound itself. At this point, new air that comes in contact with the fighter receives no physical information about the jet’s arrival until the shock wave passes over it. Behind this shock wave is air that is traveling with the jet. Sound waves travel forward and backward along the shock. A good fraction of this sound energy from the airplane arrives at our ears pretty much all at once, making the “snap-boom” sound we hear.

Taming the Sonic Boom
For readily understandable reasons, international governing bodies prohibit aircraft from traveling at supersonic speed over populated landmasses. But what if technology could mitigate the sound from an airplane traveling above Mach 1 by essentially turning down the volume of a sonic boom? If we're ever going to realize the benefits of flying people and cargo over land at super-fast speeds, that's exactly what today's research needs to accomplish, experts say.

How do we get there? One leading concept centers on the idea of attaching a long spike to the nose of our supersonic airplane; the spike is intended to keep the shock being created at the front of the aircraft separate from the shock at the rear so they don’t combine to create a big boom. The thinking is that smaller booms will weaken as they travel down to the ground, and if the airplane is flying high enough, by the time these small booms reach the ground, they’ll sound like almost nothing at all.

Study into quieting sonic booms started in earnest at NASA about 15 years ago. The first practical demonstrations of boom-mitigating technology occurred during test flights of a Northrup F-5E with a modified nose and belly that some say gave the airplane the shape of a duck. NASA and Northrup Grumman measured about 1,300 recordings during the so-called shaped-sonic-boom demonstration program in 2003, with encouraging results — the quirky shape of the customized F-5 cut its boom by about a third.

More recently, Gulfstream partnered with NASA’s Dryden Flight Research Center in California to test a concept known as the “Quiet Spike.” The project centered on testing a telescoping, 24-foot, lance-like spike mounted in the nose of a NASA F-15B research test airplane. The composite spike was designed to create three small shock waves that would travel parallel to each other all the way to the surface, thereby producing less noise than the typical shock waves that build up at the nose of a supersonic airplane.

At the conclusion of the tests in 2007, Gulfstream proclaimed the Quiet Spike trials a success, even going as far as to say that the demonstrations moved the company “one step closer” to the development of an SSBJ. But the trials proved only that the Quiet Spike could successfully extend and retract as the engineers hoped, not that they made any difference in the supersonic noise signature. That’s because Gulfstream and NASA didn’t specifically test whether the apparatus mitigated the sonic boom — and, in fact, NASA scientists say it couldn’t have done so because an otherwise stock F-15 is in no way designed to be quiet at supersonic speeds.

SSBJ Players
Today, a small number of startup firms and government research projects are seeking to advance the SSBJ discussion. One of the most well-known is Aerion, the Reno, Nevada-based technology company founded by Texas billionaire Robert Bass. Aerion has been doing research into the design for a 12-passenger SSBJ that would be capable of flying at nearly twice the speed of sound. But rather than building an airplane on its own, Aerion's goal is to partner with one or more established OEMs, which would be tasked with certifying and producing the airplane.

Aerion has never focused specifically on quiet supersonic technology, preferring instead to refine its patented natural laminar flow wing technology, which it says can significantly reduce drag. Because of the performance gains, Aerion foresees its SSBJ being powered to Mach 1.6 by a pair of off-the-shelf Pratt & Whitney JT8D turbofans, the same engines that have powered commercial airliners, including the Boeing 727 and the McDonnell Douglas DC-9.

The company is now conducting flight tests with NASA at Dryden on an F-15 test airplane, with a goal of determining future required airfoil manufacturing standards.

Far more ambitious plans have been revealed by HyperMach Aerospace, which announced at last year’s Paris Air Show its plans for a 20-seat SSBJ dubbed the SonicStar. The company claims the airplane will be capable of flying at Mach 4 from New York to Sydney in four hours with a “minimal” sonic boom. How might this be possible? Well, when you’re a company with the word “hype” in its name, you can pretty much make any claims you want to, but here’s what HyperMach says: The sonic boom of the SonicStar jet will be minimized by an electromagnetically induced plasma wave that “absorbs” supersonic pressure waves (don’t laugh too hard — it’s theoretically possible, and NASA is looking into this technology). The airplane itself would be constructed of alloyed titanium, “nano-carbon” composites and thermoplastics — all required materials because of the extreme heat any flight at Mach 4 would generate.

The other key enabling technology of the SonicStar is its H-Magjet 4000-X series engines, under development by Portland, Maine-based SonicBlue, a sister company. Not a lot of information about this particular engine is available, although we uncovered this interesting nugget: Oil capacity of the engines will be zero because they will employ “magnetic levitation turbine technology” — and therefore will be totally frictionless.

HyperMach says the SonicStar is scheduled to fly in 2021, with certification possible by 2025. Price has been set at $180 million in 2011 dollars.

Supersonic Breakthroughs
Boeing and Lockheed Martin both are participants in NASA's N+3 supersonic research program, intended to field a commercially viable replacement for Concorde. Two competing proposals are currently under study. Boeing's Icon II design would carry 120 passengers in a two-class, single-aisle configuration and be capable of cruising between Mach 1.6 and Mach 1.8, with a range of about 5,000 nautical miles.

Lockheed Martin’s Supersonic Green Machine is focusing on the possibility for achieving overland flight with dramatically reduced sonic boom through the use of an “inverted-V” engine-under-wing configuration. Lockheed Martin says it also is investigating other “revolutionary” technologies to help it achieve range, payload and environmental goals.

Clearly, bringing any of the proposed cutting-edge technologies to the design of a future SSBJ will be a challenge. Still, if a future supersonic transport can incorporate even a fraction of the technologies that are being dreamed up in labs and flight testing today, the world of tomorrow could become a much smaller place.