Civil Supersonics

Fourteen years have passed since British Airways and Air France retired their 13 Concordes, but now there are multiple indications of a supersonic revival. And the activity appears to be more advanced in the field of business jets than in the airliner sector.

Aerion continues to be the most enduring player in supersonic business jets (SSBJ), and the company’s AS2 design now has three engines (originally two), the involvement of Airbus and an agreement with GE Aviation to explore the supply of those engines. Spike Aerospace started flying subsonic scale models of its S-512 Mach 1.5 business jet last month and expects to begin trials of a manned two-thirds-scale supersonic demonstrator in 2019. HyperMach hopes to have an unmanned scale model of its Mach 5 business jet flying within two years. And Boom Technology is working on a 55-seat Mach 2.2 airliner that it plans also to offer as a private SSBJ.

Overcoming the Sonic Boom

The sonic boom produced by a supersonic aircraft has long shaped regulations that prohibit civil supersonic flight over land, presenting aircraft designers with no choice but to remain below Mach 1 over land and exceed the speed of sound only over water. But research into reducing the severity of sonic booms reaching the ground is advancing.

“We call it sonic boom shaping because it shapes the acoustical signal of the shockwave that reaches the ground,” said Peter Coen, project manager for the commercial supersonic technology project at NASA Langley. “All aircraft flying to date create what is called an N-wave sonic boom. You get a large pressure impulse, a gradual decrease to below the local atmospheric pressure and then another impulse back to the atmospheric level. You hear bang-bang, and you don’t hear the gradual pressure change in the center of the signal.

“The reason you have the two impulses is that the air doesn’t know the airplane is coming, so the pressure changes instantly. Near the airplane there are shockwaves on the canopy, the wing, the nose, the engine nacelles and the tail, and they’re all different strengths, randomly spaced along the airplane. Because they have different strengths they travel at slightly different speeds, so as that wave travels away from the airplane the energy coalesces into the two spikes.

“You want to shape the airplane so that the shockwaves formed near the airplane have a specific pattern in which they are relatively consistently spaced and of the same strength. If they’re the same strength, they travel at about the same speed and they tend not to coalesce. Or, if they do, they coalesce into a pattern that you want on the ground—instead of a spike, a series of small pressure changes. With a gradual pressure change, your ear does not hear it and the disturbing crack or bang is gone.”

Aircraft designers’ predicament is not made any easier by the absence of hard numbers in the regulations, which do not provide actual figures for decibels or strength of pressure wave that the FAA would deem acceptable for overland supersonic flight. This is what is driving NASA’s low-boom flight demonstration program—it will produce measurable evidence of the reductions made possible by boom shaping.

A Low-boom Quest

To test this, NASA and Lockheed Martin are designing a new airplane called Quest, for quiet supersonic technology. It’s a single-seat experimental airplane, some 100 feet long and with an mtow of 25,000 pounds, shaped to produce this low-boom signal. “For the dive testing we have been using an F/A-18, instrumented to repeat a dive profile precisely but not modified in any way to shape the boom,” said Coen. “We start the dive subsonic at 50,000 feet and allow the airplane to achieve supersonic just briefly in a steep dive so that the signal comes off the top of the airplane at about 45,000 feet at a shallow angle and travels a long distance through the atmosphere before it reaches the ground.

“In a small area of the ground, you end up with a signal that is kind of rounded and has a low peak pressure, the sort of signal we’re looking for with our low-boom design. The signal hits the ground about 30 miles from the aircraft. Right below the aircraft you get an intense, focused boom. We’ve done all this testing at NASA Armstrong [the former Edwards AFB] and the test community we’ve used is the base housing community—a small area surrounded by desert. The people we’re exposing to this sound depend for their livelihood on aircraft that regularly make sonic booms. So we don’t really get true data that we could give to the FAA to prove that we’ve achieved the right level, but we can use it to test the procedures and fine-tune the surveys and how we record the noise in the community.”

The low-boom flight demonstrator is slated to fly in 2021, at which time researchers can do tests in other, average communities anywhere in the U.S. Lockheed is doing the preliminary design of the aircraft for NASA, and that phase ended this summer. “We will then have a new competition for the actual detail design and development of the aircraft. It’s a clean-sheet airframe that uses a lot of existing components, mostly from existing military aircraft. The Lockheed design uses F-16 landing gear and a GE F414 engine. The canopy and cockpit area is the aft cockpit of a T-38. To maintain the desired supersonic shape, you can’t have a forward-vision window, so we’re going to use a synthetic external vision display to give the pilot essentially VFR-equivalent vision.

The NASA effort is focused on a longer-term goal of airliner-type transportation, but there is significant overlap in the technologies for business jets. NASA still has a no-funds-exchanged agreement with Gulfstream under which they share information about understanding the community response to low-noise supersonics.

A few years ago, Gulfstream took out patents on a nose spike designed to mitigate the boom. “They haven’t shown anything relative to that in a while,” observed Coen. “The breakthrough that we came up with by shaping really just happened in 2009 to 2010. We tunnel-tested it in 2011.”

Gulfstream was the first to exploit this with its Quiet Spike, but NASA started working with Lockheed and Boeing and realized it could also be done with a more conventional nose shape and thus less mechanical complication. “Gulfstream has not shown any designs publicly, and I’m guessing they’re also looking at other approaches,” Coen noted.

“Unfortunately, we don’t have anything new to report in terms of supersonic,” a Gulfstream spokeswoman told AIN. “We have a small team dedicated to researching sonic-boom mitigation and working with other organizations to remove the ban on flying supersonically over land.”

Powerplant Hurdles

Even if an SSBJ flies supersonic only over water, as Aerion proposes for its design, the aircraft still has to meet engine noise requirements for takeoff and landing. “Right now the NOx emissions requirement applies only to takeoff and landing,” said Coen, “so a Concorde-type turbojet is not going to cut it. Our vision at NASA is a little farther out, but we are looking at engine cycle designs, nozzle and inlet configurations that meet at least the existing regs and, longer term, the proposed Chapter 14 stringency.

“That’s an important area of our work that the business jet folks need to pay attention to. In conjunction with our subsonic research efforts, one of the big areas of focus is on the reduction of emissions and fuel consumption. We’ve developed some interesting concepts for the combustor, which should reduce emissions even at altitude to a point that is compatible with preserving the ozone layer in the upper atmosphere.

“This sort of technology should be available to the likes of GE and Rolls if they’re providing the power for supersonic business jets. GE and Rolls do seem to be the two engine OEMs most involved in possibly powering an SSBJ today. Pratt & Whitney is out there as the biggest manufacturer of supersonic engines now, but it doesn’t seem to be showing anything related to commercial aviation.”

GE Aviation has been working with Aerion “for some time now and doing studies around different configurations and options,” according to Shawn O’Day, marketing general manager for GE Aviation business and general aviation and integrated systems. The agreement with Aerion—announced at EBACE in May—“involves working together to define a configuration that makes sense for the program.”

At this stage, scant detail about the agreement has been released. “We don’t have a timeline drawn out,” he said. “We’ve been talking for years, and this is the next step. Talks will get more serious about what the next stage is. It’s tough to paint a picture here because a lot of what we face now is diving in to see what these next steps are.” For now, engineering teams at Aerion and GE currently are working jointly in an engine definition phase.

Asked if its Passport is a strong candidate to serve as the core of a supersonic engine, O’Day said, “We’re not disclosing that at this point. We’re using existing capability and technology to address the need. Aerion requires somewhere in the region of 17,000 pounds from each engine for takeoff, but really what you’re designing for is high-altitude thrust.

“Unlike [those on] a traditional airplane, the thrust levers on an SSBJ go forward for cruise at altitude. The engine is actually operating at a higher power at altitude, and it needs a lot of air flowing through the core. Bypass ratio is lower than on a traditional business jet engine, so the core has a lower-bypass fan attached to it and is bigger than what you’d normally have. Technically it’s still a turbofan rather than a turbojet.”

The other engine manufacturer publicly interested in getting on board the next generation of supersonic civil aircraft is Rolls-Royce. Dr. Dean Roberts, market analysis executive for business aviation at Rolls-Royce, said, “We have supersonic civil aircraft experience, and can draw on our fighter experience as well.” He noted that until regulatory hurdles can be overcome to allow supersonic flight over land, the “halfway house” will be a hybrid SSBJ. The company has done detailed analysis, Roberts said, that showed the hybrid approach is “quite an attractive proposition—looking at the routes you could fly there are very clear benefits.”