CIVIL SUPERSONIC Concorde is a museum piece, but the allure Aerion continues to be the most enduring player, of speed could spell success for one or more and the company’s AS2 design now has three of these projects. engines (originally two), the involvement of Air- bus and an agreement (loose and non-exclusive, by Nigel Moll but signed) with GE Aviation to explore the supply Fourteen years have passed since British Airways of those engines. Spike Aerospace expects to fly a and Air France retired their 13 Concordes, and for subsonic scale model of the design for the S-512 the first time in the history of human flight, air trav- Mach 1.5 business jet this summer, to explore low- elers have had to settle for flying more slowly than speed handling, followed by a manned two-thirds- they used to. But now, more so than at any time scale supersonic demonstrator “one-and-a-half to since Concorde’s thunderous Olympus afterburn- two years from now.” Boom Technology is working ing turbojets fell silent, there are multiple indi- on a 55-seat Mach 2.2 airliner that it plans also to cations of a supersonic revival, and the activity offer as a private SSBJ. NASA and Lockheed Martin appears to be more advanced in the field of busi- are encouraged by their research into reducing the ness jets than in the airliner sector. severity of sonic booms on the surface of the planet. www.ainonline.com © 2017 AIN Publications. All Rights Reserved. For Reprints go to Shaping the boom create what is called an N-wave sonic boom: if The sonic boom produced by a supersonic air- you plot the pressure distribution that you mea- craft has long shaped regulations that prohibit sure on the ground, it looks like the letter N. You civil supersonic flight over land, presenting air- get a large pressure impulse, a gradual decrease craft designers with no choice but to remain to below the local atmospheric pressure and then below Mach 1 over land and exceed the speed another impulse back to the atmospheric level. www.ainonline.com of sound (760 mph at sea level) only over water. You hear bang-bang, and you don’t hear the But research over the past decade or so is bear- gradual pressure change in the center of the sig- ing fruit toward revealing how to shape the air- nal. The reason you have the two impulses is that craft to muffle the sonic boom. the air doesn’t know the airplane is coming, so “We call it sonic boom shaping because it shapes the pressure changes instantly. Near the airplane the acoustical signal of the shockwave that reaches there are shockwaves on the canopy, the wing, the the ground,” says Peter Coen, project manager nose, the engine nacelles and the tail, and they’re for the commercial supersonic technology proj- all different strengths, randomly spaced along the ect at NASA Langley. “All aircraft flying to date airplane. Because they have different strengths © 2017 AIN Publications. All Rights Reserved. For Reprints go to AVIATION INTERNATIONAL NEWS • JULY 2017 Half a century ago, engineers designed Concorde with slide rules. Today, they have an arsenal of computing tools to solve the Anglo- French SST’s shortcomings: boom noise, engine noise and fuel thirst. they travel at slightly different speeds, so as that disturbing crack or bang is gone. You’d hear it as wave travels away from the airplane the energy a double thump.” coalesces into the two spikes; as that signal trav- Aircraft designers’ predicament is not made els through the atmosphere it gets attenuated (the any easier by the absence of hard numbers in the pressure gets lower) but it still maintains the N regs, which do not provide actual figures for deci- shape until the sonic boom reaches the ground. bels or strength of pressure wave that the FAA “You want to shape the airplane so that the would deem acceptable for overland supersonic. shockwaves formed near the airplane have a spe- This is what is driving NASA’s low-boom flight cific pattern in which they are relatively consis- demonstration program: it will produce measur- tently spaced and of the same strength. If they’re able evidence of the reductions made possible by www.ainonline.com the same strength they travel at about the same boom shaping. speed and they tend not to coalesce. Or if they In addition to shaping airframes, the researchers do they coalesce into a pattern that you want on have also been studying how people respond to the ground: instead of a spike, a series of small different impulsive sounds such as sonic booms, pressure changes. The atmosphere has worked on indoors and outdoors. “Using simulators, large them and in effect rounded them a bit, so what rooms with large speakers attached to the walls, as we’re aiming for is something that looks more well as flight-test work where we fly the airplane like a sine wave than an N shape. With a gradual in a dive maneuver to produce in a very small area pressure change, your ear does not hear it and the the signal we’re looking for, we have an idea that © 2017 AIN Publications. All Rights Reserved. For Reprints go to AVIATION INTERNATIONAL NEWS • JULY 2017 at somewhere between 60 and 70 dBA the noise and how we record the noise in the community.” begins to fade into the background, which might The low-boom flight demonstrator is slated to be acceptable. The proof is when we leave the lab fly in 2021, at which time researchers can do tests and take the dive test off range and see how the in other, average communities anywhere in the average person, going about their business on the U.S. Lockheed is doing the preliminary design of average day, responds to this sound.” the aircraft for NASA, and that phase ends this To test this theory, NASA and Lockheed Mar- summer. “We will then have a new competition tin are designing a new airplane called the Quest, for the actual detail design and development of for quiet supersonic technology. It’s a single-seat the aircraft. It’s a clean-sheet airframe that uses a Experimental airplane, some 100 feet long and lot of existing components, mostly from existing with an mtow of 25,000 pounds, shaped to pro- military aircraft. The Lockheed design uses F-16 duce this low-boom signal. “For the dive testing landing gear and a GE F414 engine. The canopy we have been using an F/A-18, instrumented to and cockpit area is the aft cockpit of a T-38. To repeat a dive profile precisely but not modified in maintain the desired supersonic shape, you can’t any way to shape the boom,” says Coen. “We start have a forward-vision window, so we’re going to the dive subsonic at 50,000 feet and allow the air- use a synthetic external vision display to give the plane to achieve supersonic just briefly in a steep pilot essentially VFR-equivalent vision.” dive so that the signal comes off the top of the air- Is Lockheed’s end goal some sort of airliner? “I plane at about 45,000 feet at a shallow angle and can’t say for certain,” ventures Coen. “My guess travels a long distance through the atmosphere is they want to position themselves to be a tech- before it reaches the ground. nology provider. I don’t see them getting back “In a small area of the ground, you end up with into the airliner business. They might partner a signal that is kind of rounded and has a low with somebody to do an airliner design. Lock- peak pressure, the sort of signal we’re looking heed is not involved with Aerion or any other for with our low-boom design. The signal hits the SSBJ project. A few years ago they were part- ground about 30 miles from the aircraft. Right nered with SAI [Supersonic Aerospace Interna- below the aircraft you get an intense, focused tional] on the design of an SSBJ, but that design boom. This maneuver is effective as a research has languished. The NASA effort is focused on a tool as long as you’ve got a big desert to fly over— longer-term goal of airliner-type transportation. we’ve done all this testing at NASA Armstrong But there is significant overlap in the technolo- [the former Edwards AFB] and the test commu- gies we’re working for business jets too. We still www.ainonline.com nity we’ve used is the base housing community— have a no-funds-exchanged agreement with Gulf- a small area surrounded by desert. The people stream under which we share information about we’re exposing to this sound depend for their understanding the community response to low- livelihood on aircraft that regularly make sonic noise supersonics.” booms. They’re not anti-sonic boom—for them A few years ago Gulfstream took out patents it’s the sound of freedom. We don’t really get true on a nose spike designed to mitigate the boom. data that we could give to the FAA to prove that “They haven’t shown anything relative to that in we’ve achieved the right level, but we can use it a while,” observes Coen. “The breakthrough that to test the procedures and fine-tune the surveys we came up with by shaping really just happened © 2017 AIN Publications.