A new generation of supersonic commercial aircraft could have high environmental consequences, says ICCT study
(graphic: Boom Technology)
Tue 17 July 2018 – The last commercial supersonic flight took place 15 years ago with the final grounding of the Anglo-French Concorde due to poor economics, unviable fuel efficiency and noise bans. However, the revival of passenger supersonic transport aircraft (SST) may be just seven years away as three US start-up companies develop a new generation of civil aircraft to fill a void in a market they believe still exists. Aerion and Spike, aimed at business jet operators, and Boom, with a commercial airliner capable of carrying up to 55 passengers, have the backing of a number of major aerospace manufacturers. Boom claims to have options from five airlines, including Japan Airlines and Virgin. Although the three say they can overcome the fuel and noise drawbacks of Concorde, a new paper by ICCT expects them to exceed existing international standards on aircraft pollution, noise and CO2 emissions.
International standards for subsonic aircraft are set by ICAO and the UN agency’s environmental committee CAEP has been developing a standard for future supersonic aircraft for some time now. Discussions centre on sonic boom measurement and establishing technical flight test procedures for supersonic noise certification. These would be in addition to the certification requirements for the subsonic local airport conditions and applicable maximum noise levels. ICAO says progress has been made and anticipates certification of a supersonic aeroplane could occur in the 2020-2025 timeframe.
However, others see a clash between the United States and major European countries over SST noise standards holding back the process. An early setback in the 1970s for the Concorde was a decision to prohibit supersonic commercial aircraft from flying over the US because of the sonic boom it created when travelling faster than the speed of sound and the public nuisance it was said to cause. In a reversal, it is the FAA that is now pushing for a new supersonic international standard that is not considered stringent enough by Europe. Although an internationally-agreed standard is the preferred route, the FAA told Reuters that if there is a delay in adoption then it would establish its own regulations for domestic certification.
With experts gathered from NASA and Richard Branson’s Virgin Galactic space project, Boom says advances in aerodynamics, engine technology and lightweight composite materials will allow its SST to cruise at speeds up to Mach 2.2 (1,451mph) – compared to Concorde’s Mach 2.04 – and “at least 30 times quieter” than Concorde. Spike claims to have developed a patent-pending ‘Quiet Supersonic Flight Technology’ that will stop a sonic boom reaching the ground.
An analysis by the US-based International Council on Clean Transportation (ICCT), however, suggests these near-term commercial SSTs are unlikely to meet existing ICAO efficiency and pollution standards set for subsonic commercial air fleet unless newly designed, ‘clean-sheet’ engines were used. If based on an existing derivative engine, estimates ICCT, a SST could exceed nitrogen oxides (NOx) and CO2 limits by 40% and 70% respectively. ICCT modelling showed it burned five to seven times as much fuel per passenger as comparable subsonic aircraft. In a best-case scenario, it burned three times as much fuel per business-class passenger relative to subsonic aircraft; in the worst case, it burned nine times as much fuel relative to an economy-class passenger.
The modelled supersonic aircraft also exceeded allowable landing and take-off (LTO) NOx limits for subsonic aircraft by 38% in the most likely configuration and CO2 metric value limits by 52% to 115%, with a most likely exceedance of 67%. A best case advanced clean-sheet engine was estimated to comply with the latest subsonic NOx standards. A qualitative noise analysis by ICCT suggests that near-term SST designs are unlikely to meet ICAO’s 2018 LTO subsonic noise standard.
Because little information has been disclosed from the three SST companies about the environmental performance of their designs, ICCT says it has relied on publicly available data, expert engineering judgement and an open-source conceptual aircraft design tool for what it describes as a preliminary analysis. Other environmental factors such as sonic boom, particulate matter and stratospheric water vapour have not been addressed in this study.
The work, it says, is meant to inform policymakers’ thinking about future standards for new supersonic designs until such time that more detailed data is made available.
A question for policymakers, therefore, is whether they should apply existing subsonic environmental standards to SSTs or adopt new standards specifically for SSTs based upon the performance of poorer performing derivative engines that would allow for increased air pollution, noise and CO2 emissions relative to new commercial aircraft.
“There are reasons to be worried about the environmental impact of reintroducing supersonic aircraft,” said Daniel Rutherford, co-author of the study and Aviation Program Director at ICCT. “A modest first step is for manufacturers to commit to meeting existing standards for new aircraft.”
Boom’s founder and CEO Blake Scholl will be presenting a manufacturer’s perspective on supersonic travel at the Farnborough Air Show tomorrow, 18 July.