Revival of supersonic air travel is a subject which has largely been relegated to ‘silly season’ status in the two decades since withdrawal of the BAC-Aerospatiale Concorde fleet.
While no supersonic airliner programme has been successful – at least economically, technological achievements notwithstanding – the lingering appeal is illustrated by the regular appearance of ‘son of Concorde’ articles promising the return of high-speed transport.
Hence the stir when a research paper emerged earlier this year from China’s Shanghai aircraft design and research institute, part of aerospace firm Comac, detailing a long-range supersonic aircraft proposal.
It attributes Concorde’s failure to its relatively short range and limitation to transatlantic routes – a consequence of its notorious sonic boom. One of Concorde’s legacies is the US FAA’s continuing prohibition on civil aircraft exceeding Mach 1 in its airspace.
“At present, the sonic boom pollution problem that plagues supersonic passenger aircraft still exists,” says the Comac paper.
“The next generation of supersonic passenger aircraft should expand demand as much as possible, and should have low sonic boom flight capabilities over land, and long range characteristics to expand the number of routes.”
It sets out several design criteria for the proposal including a maximum take-off weight of 79t, a range of 11,000km (5,900nm), and capacity for 48 seats.
The aircraft would typically accelerate from around 300kt after take-off to M0.95, then transition to the supersonic regime at an altitude of 11,000m (36,000ft), reaching M1.2. For the long-range cruise at 16,000m it would increase its speed to M1.6-1.7.
It would be 61.1m in length and a 68.8° low-set swept wing, rather than a delta, with a 22.5m span. Along with the main wing the jet would have a forward canard and a V-tail.
Powered by two rear-mounted engines the jet would avoid high take-off noise through the absence of afterburners, although this would require larger engine flow rates and lower exhaust speeds.
Fuel would be housed in seven tanks, two in each main wing, a central fuselage tank, plus two smaller tanks, located forward of the canard and aft of the engines. The fuel would be concentrated forward to enhance stability at low speed, while the centre-of-gravity would shift aft to improve cruise aerodynamics.
But the design carries a sonic penalty. The paper says the wing produces a strong compression wave at the mid-fuselage, which can merge with the canard compression wave, while the tail has its own strong shockwave. This means the perceived sonic boom on the ground can reach more than 105dB.
The paper explores options to refine the design and bring the noise level down. It proposes a reverse curvature of the mid-fuselage to help weaken the fore and aft shockwaves, reducing the boom to about 93dB.
But it further modifies the design with a so-called “silent cone” nose-spike, as well as a “bulge” in the aft tail cone – in lieu of changes which would risk tail-strike – and claims these would further limit the sonic boom to less than 84dB.
Without afterburners, the proposed design would need to accelerate to the transonic regime at about 11,000m. According to the paper, the atmospheric attenuation effect on the sonic boom is “relatively weak” but it can use atmospheric refraction “to achieve sonic boom-free flight”.
This ‘boomless cruise’ principle is not new, having been studied even before Concorde entered service. As the sonic shockwave transits from high-altitude cold air to lower warm air – where the speed of sound is faster – the wavefront is refracted.
At a certain height the wavefront is deflected to such an extent that it does not reach the ground, a phenomenon known as Mach cut-off.
Using refraction to neutralise the sonic boom is central to a US supersonic civil aircraft programme which has progressed beyond academic papers.
Boom Supersonic intends to exploit Mach cut-off with its Overture airliner to enable quiet passenger flights beyond the sound barrier.
Sonic boom or not, the US FAA currently bans all civil flight at speeds in excess of M1 – a rule it adopted in 1973, as Concorde and other supersonic transport aircraft were emerging. The rule was amended in 1977 specifically to protect against noise from Concorde, whose sonic boom could still reach US coastal areas even while it was outside US territory.
Although Concorde services ended in 2003, the fascination with high-speed travel has not. As the aerospace industry looked closely potential ‘boomless’ supersonic flight, the FAA sought legal clarification in 2015 over the wording of its rule, to determine whether it left room for Mach cut-off to be considered a means of compliance.
But the response from the FAA’s office of the chief counsel was clear. Such a conclusion “ignores the express language” about the outright ban on M1 flight, and required a “significant misreading” of the amendment – which was intended to expand protection, “not suggest a means of skirting it”.
“Mach cut-off may someday be prove itself out to be a viable operational profile,” the office added. “But even then, its use over land in the [USA] would require an exemption from [the M1 ban] or a change in the rule language itself.”
Boom Supersonic chief Blake Scholl describers the regulation as “absurd”, adding that it has had “severe and lasting consequences” including a “stifling” of US aviation innovation.
“Supersonic passenger travel could have started with small supersonic aircraft, such as business jets designed for coast-to-coast travel,” he says. “The supersonic ban effectively made this product illegal in the USA.”
But after more than 50 years, the prohibition – which is based on limiting speed, rather than specifically limiting noise – might be heading for an overhaul long-awaited by those who believe breaking the sound barrier does not automatically mean breaking residential windows.
Legislation known as the Supersonic Aviation Modernisation Act was introduced into both houses of the US Congress – the Senate and the House of Representatives – in mid-May. The bills request that the FAA revises the ban on exceeding M1, under the condition that sonic booms do not reach land.
And in early June, US president Donald Trump instructed the FAA to take steps to repeal the prohibition within six months, with an executive order to rescind the “outdated and overly-restrictive” regulation.
It requires the FAA to issue a notice of proposed rulemaking within 18 months – and a final order within two years – establishing a noise certification standard for supersonic aircraft.
“Supersonic flight without an audible sonic boom should obviously be allowed,” says Scholl. “The ban on supersonic has held back progress for more than half a century.”
Boom has, however, already achieved civil supersonic flight in US airspace, with the delta-winged XB-1 research aircraft created to support development of Overture.
It obtained FAA authorisation to fly the three-engined jet over a Mojave test range in California, carrying out a series of 11 test sorties before taking the jet through the sound barrier at around 35,000ft during two further flights on 28 January and 10 February this year.
Those two flights were the last to be conducted by the XB-1. But the jet, which reached M1.12, crucially demonstrated Mach cut-off, with microphone arrays beneath the flightpath showing the sonic shockwave did not reach the ground.
“Data collected during XB-1’s multiple supersonic runs allowed Boom to validate sonic boom models and improve the algorithms that predict operating in Mach cut-off,” says the company.
The tests were similar to those performed by US aeronautical agency NASA with a Boeing F/A-18B, which carried out passes near Edwards air force base in November 2012. The jet flew at 34,400-39,300ft, at speeds of M1.12 to M1.17, to investigate cut-off thresholds under a project designated ‘FaINT’.
Boom had planned to operate Overture at M0.94 in continental airspace, and only accelerate to M1.7 over water. But XB-1, it says, indicates that Overture could operate supersonically – and quietly – over land.
“This opens up even more route possibilities for Overture, which already offered a meaningful speed-up on over 600 routes around the world,” claims Boom.
NASA and Lockheed Martin are pursuing quiet supersonic flight through a programme called ‘Quesst’, which centres on a low-boom design designated the X-59 – a research aircraft whose technology and profile are intended to suppress the sonic boom.
But Overture is not intended to be a low-boom aircraft. Cruising at M1.7 over water, it will produce a sonic shockwave. Overture will rely on Mach cut-off to make its case for operating supersonically over land.
Demonstrating Mach cut-off for a few minutes over the California desert is one thing; building an 80-passenger airliner which can efficiently utilise the principle is another.
Mach cut-off flight presents multiple challenges. To be effective the aircraft must be flying in the transonic regime, at lower supersonic speeds of around M1.1-1.3, because this broadens the trailing Mach shock-cone and makes it more susceptible to refraction.
But German aerospace centre DLR, in a 2022 paper, says the transonic realm features shockwave instability and causes complex stresses which need consideration in aircraft design.
This realm also generates pronounced peaks of drag, which forces a trade-off between transonic speed and higher fuel consumption.
“As aerodynamic drag increases closer to speed of sound, there is a modest increase in fuel burn at low supersonic speeds,” says Boom. “Even with this drag penalty, Overture still has plenty of range to fly the longest transcontinental routes.”
One of the primary obstacles with Mach cut-off is that the phenomenon is heavily dependent on atmospheric conditions.
Analysis of trajectories along the same routes, but using weather data 6h apart, shows relatively large differences in cut-off Mach numbers, the DLR paper states.
“This indicates that shorter-term atmospheric variability probably has a significant impact on sonic-boom propagation,” it says, adding that “particularly accurate” weather forecasts will be required.
“It further appears plausible that cruise speeds cannot be planned beforehand and will need recurrent updating during missions.”
Boom Supersonic says Overture will use an “advanced autopilot” which will use current weather conditions and software algorithms to “automatically select the optimal speed” to achieve ‘boomless’ cruise.
There is also a need to consider integration with the air traffic management system if, while flying in the Mach cut-off regime, the aircraft will be operating at altitudes similar to those occupied by subsonic traffic – potentially requiring new procedures or greater airspace capacity.
While the sonic shockwave can be refracted in the atmosphere, the region below the cut-off altitude – known as the ‘shadow zone’ – features a residual effect, known as evanescent waves. These could still be perceived on the ground, and a viable commercial operating model will need to ensure that any such noise effect remains acceptable.
Measuring and assessing shadow-zone acoustics was a central part of the NASA F/A-18 experiments carried out during the ‘FaINT’ programme. But civil supersonic operations will need to establish margins for Mach cut-off flight.
Like the Anglo-French teams which developed Concorde more than six decades ago, Boom Supersonic is confident that it can overcome the technological challenges associated with producing a modern, efficient supersonic airliner.
It points out that its personnel built and flew the XB-1 – the first independently-developed supersonic jet – and is “all-in” on producing Overture and its crucial Symphony propulsion system.
“That includes freezing the design on Overture,” it adds. Roll-out is intended for 2027 and passenger service in 2029.
Symphony development “continues to progress rapidly”, says Boom. It passed a key technical review in March, enabling manufacturing launch.
The engines will feature “enhanced transonic performance” compared with commercially derived engines, the company claims, allowing Overture to “efficiently transition” to supersonic speeds at altitudes above 30,000ft.
“Multiple parts have already entered the manufacturing phase and engine assembly will start later this year,” adds Boom, which disclosed in April that it would carry out engine testing – set to begin at the end of this year – at the Colorado Air and Space Port in Watkins.
Chinese aerospace firm Comac’s own supersonic aircraft proposal, combining design features with Mach cut-off operation to produce quiet high-speed transport, is currently an academic curiosity – but it rattled Boom enough for it to warn over China’s technological pace.
“The supersonic race has begun, and it is critical that the US wins,” says Boom.
“China is already producing clones of Western-made airliners, and recently announced its own supersonic passenger aircraft. Unless we invent and build the next generation of aircraft in the US, our leadership in next-generation aerospace technology will pass from America to Asia.”
