Replacing the Concorde with a new supersonic aircraft is not just a dream. Work was already well under way among academia, industry and government agencies while the Concorde was still flying, and it continues 10 years later.
All of their efforts, however, face what is still very much an open question: is it possible to build a transport aircraft that can fly faster than the speed of sound, yet also be profitable to operate, and comply with all current and even some future regulations governing emissions of fuel and – above all – noise?
To develop and introduce into service such an aircraft before 2020 now seems most improbable.
However, several projects under way or still pending could – within the 2020 timeframe – answer the basic question of whether a profitable and compliant supersonic jet is possible.
Arguably, the most critical barrier that must be overcome is the taming of the sonic boom. Like all civilian aircraft, the Concorde was prohibited from exceeding Mach 1.0 overland, lest the double crack of the supersonic shock wave startle people or cause damage.
New technology suggests the sonic boom can be muffled or even eliminated, but international regulators still need persuading. So-called “quiet boom” design techniques have been demonstrated on a heavily modified, NASA-owned Boeing F-15A, but that is not quite enough to change policy.
The ICAO’s working group for aircraft noise has developed a roadmap for revising the regulation that effectively bans supersonic flight by civilian aircraft overland. According to emailed responses to questions put to NASA’s aeronautics research branch, the roadmap “assumes that community overflight response testing will be required to provide the scientific data” to support the new regulation.
What is needed to enable such a test is a new research aircraft. Last April, a NASA official confirmed to Flight International that the agency was considering funding such an initiative, following the completion of the five-year Environmentally Responsible Aviation (ERA) project in fiscal year 2015.
NASA’s proposal involves commissioning a private company to build a specially designed research aircraft roughly the size of a business jet. It would perform the survey of community noise levels required under the ICAO roadmap.
As this article went to press, the US budget for fiscal year 2014 was still mired in a deadlock over implementing health care insurance reforms and raising the debt ceiling. The outlook for the following year’s budget looks almost as difficult, but NASA is still considering the proposal to fund the supersonic research aircraft.
“A low boom research aircraft, which would replicate the supersonic acoustics of a larger aircraft, would be key to developing this data at a reasonable cost,” the agency says.
If private companies are unwilling or unable to invest the tens or even hundreds of millions of dollars needed to build the aircraft and conduct the noise survey, NASA’s proposal may be the only option for persuading the ICAO and other regulatory agencies to change the regulations banning supersonic flights overland.
Established aircraft manufacturers such as Gulfstream and Sukhoi have been working on supersonic business jet concepts for more than two decades. Meanwhile, start-up companies, including Aerion, are also working on the problem.
The noise generated by the supersonic shockwave at M1.0 is not the only challenge. If NASA launches the low boom research aircraft project, that problem could be easier to solve than the issue of noise on take-off.
Last February, the ICAO proposed Chapter 14 noise standards that would require manufacturers to lower aircraft noise by 7dB after 2020 compared to the current regulations, which are called Chapter 4. The ICAO also has indicated that supersonic aircraft will be required to meet the same standards.
The new standards could make it even harder to develop a supersonic aircraft. Aerion decided to redesign their aircraft after the ICAO standards were published. The original Pratt & Whitney JT8D engines would have to be heavily modified to meet the noise requirements on take-off, and the cost of making the changes was too high.
Supersonic engines work by accelerating a comparatively small amount of air very quickly through the combustion cycle, which usually calls for a low bypass ratio. Noise reduction is partly a function of increasing the bypass ratio, so a supersonic engine usually generates a lot more noise than a subsonic aircraft, even at low speeds on take-off.
“If not the [biggest], it’s certainly one of the biggest design challenges,” says Richard Tracy, Aerion’s chief technology officer. “The engine compromises to provide good supersonic performance and meet the then-operable community noise requirements – that’s a great challenge.”
Aerion is in discussion with multiple engine companies, however, and remains confident that an aircraft and engine combination can be configured to meet the Chapter 14 noise standards.
The US military is developing technologies that could help solve Aerion’s problem with take-off noise. Since 2006, the Air Force Research Laboratory has funded a combined-cycle engine programme, which varies the bypass ratio between take-off and cruise modes. GE Aviation has been developing the technology under the AFRL’s adaptable versatile engine (ADVENT) programme.
“Probably the best, the optimised solution for a supersonic aircraft is an engine that has a different configuration on take-off than it has during cruise conditions,” says Shawn O’Day, GE’s manager of business aviation marketing.
GE Aviation has had several conversations with Aerion about the possibility of powering the supersonic aircraft.
“We already have engines ready for them,” says Brad Mottier, vice-president and general manager of GE Aviation’s business and general aviation division.
The Passport 20 engine is one example, he says. Launched to power the Bombardier Global 7000 and 8000 large-cabin business jets, the engine features a 5.6:1 bypass ratio. It would have to be adapted for combined-cycle and to achieve supersonic speed without afterburner to meet Aerion’s needs, he says.
“The question is, can you make a business case?” O’Day says. “The engine is going to be very expensive as an engine like that would be. So can you sell enough of them at the right price to justify that kind of an investment?”
As Aerion redesigns its supersonic aircraft around a new engine, the company is also taking a new look at the market for the product.
The company has launched its first market survey in over a decade to determine customer interest in cabin size and propulsion systems. The survey is expected to be completed by year-end, but Aerion officials are already leaning at making some major changes to the original aircraft beyond the engine change.
“The current configuration tapers at the front end,” says Brian Barents, vice-chairman of Aerion. “We would be looking at a wider configuration with a longer constant section as part of the study.”
Another element of the study is considering the maximum range of the aircraft. The high-speed cruise range of the original aircraft was about 4,250nm (7,870km) at a speed of M1.4, or enough to fly most transatlantic routes. By adding another 600nm or so to the range, the Aerion jet could reach Tokyo from Los Angeles.
One part of Aerion’s strategy that will not change with the configuration is the company’s unique approach to overland supersonic flight. While other companies, such as Gulfstream, are waiting for the NASA-sponsored project to supply data for changing the regulation, Aerion wants to skirt the issue completely.
Aerion is proposing to fly its aircraft slightly below the speed of sound overland. In the original configuration, the aircraft has slightly more range at M0.95 than M1.4.
“There isn’t a defined, acceptable boom level and the prospect of it happening and getting set and embedded in the regulatory structure in the next five or six or eight years is slim to none,” Tracy says. “It just doesn’t make any sense in our view to compromise the design for a low boom that may or may not be acceptable.”
There is, however, another option. The supersonic shockwave is known to dissipate before it reaches the ground at speeds generally below M1.2. The US policy states that all supersonic flight is prohibited, even if the boom can’t be detected on the ground. The US Federal Aviation Administration’s office of energy and the environment has assured Aerion that it would approve overflights of the aircraft at the lower Mach speeds, Tracy says.
“If you have demonstrated flight with no boom, they will approve flights above Mach 1,” he says. “There is a path to the Mach 1.15 cut-off flight over the USA.”