Supersonic boom or environmental bust?

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Concorde is famous for being radical, beautiful, expensive, fast and LOUD! Heathrow's loudest known recording of Concorde's take-off was measured at 126 dB. Long and repeated exposure to noise greater than 85dB can damage our hearing, and the louder the noise the less exposure is needed. So, given the environmental awareness and commitments that exist today, it's perhaps surprising to report no fewer than four potential new supersonic aircraft in development: the Aerion AS2, Boom, Spike S-512 and the NASA X-plane.

These new entrants are primarily targeting the bizjet markets, with only Boom anticipating carrying more than 18 passengers (in fact targeting 55). They differ in other ways too. They are all using modern and more environmentally friendly materials and technologies, but their maximum speeds and associated noise footprints range from subsonic to 2.2 Mach and low boom to conventional boom.

This is where it gets complicated …. a sonic boom is created when aircraft fly faster than the speed of sound (known as Mach 1), approximately 1,235 km/h (767 mph) at sea level and 20 °C (68 °F). It varies according to height and temperature, and chases the aircraft wherever it goes, approximately one mile laterally for every 1,000 ft of height (although the volume dissipates with lateral distance). However, new technologies can mitigate its impact, giving us so called 'low boom' and 'Mach cut off'.

Environmental and Regulatory Challenges

The biggest barriers facing this new generation of supersonic aircraft are environmental and regulatory. Believe it or not, supersonic aircraft are currently exempt from CO2 regulations, have weaker NOx standards than their subsonic cousins and are subject to no noise standards at all.

Environmental issues include airport noise reduction, sonic boom suppression and the need to reduce high altitude emissions (the higher gases are emitted, the more damaging they are to the environment).

From the regulatory point of view the challenge is to come up with landing and take-off standards, an en-route standard for low boom/Mach cut-off aircraft, and emission standards … all whilst aircraft developments are rapidly progressing.

An ICAO taskforce is making progress with identifying certification measurement and is now working on selecting an appropriate noise metric for use in a Standard that assesses sonic boom noise. ICAO acknowledges that a supersonic airline certification could occur between 2020-2025, but there is no firm commitment to when Standards will be agreed and published, and in the meantime aircraft design and investment proceeds (or not) at risk.

For its part, the FAA has previously stated in respect to airport noise: "any future supersonic airplane should produce no greater noise impact on a community than a subsonic airplane." The FAA is pushing ahead with new rules for noise certification of supersonic aircraft and a rule to clarify the procedures to obtain authorization for flight testing. There are some in the US who are putting forward the idea that supersonic aircraft should be allowed to make more noise on take-off and landing than subsonic aircraft; in a similar fashion to the current rules that allow heavier aircraft and those with 4 engines to make more noise than a lighter 2 engine aircraft. The FAA anticipates issuing its proposed rules in 2019.

The message from Europe is that the current (subsonic) noise limits should be seen as a "guideline" for developing landing and take-off rules. With the social acceptability of aircraft noise decreasing in Europe it would appear difficult to see how a more relaxed noise standard for supersonic jets would ever be favoured. It has been reported that Europe does not expect new standards "before 2025 or later" because of insufficient data on the social attitudes and health impacts of sonic booms.

Boom or bust?

All of which leads us to question where this is heading. Will we see a boom in a new generation of supersonic aircraft, primarily targeting the business market? Or will the environmental and regulatory challenges prove too great?

There is clearly the risk of a clash between ICAO, the FAA and the EU over the setting of international noise and emission standards for supersonic aircraft, and yet these will be key to unlocking a new era of supersonic travel.

In the meantime, the Chinese have a hypersonic aircraft (travelling at >Mach 5) that they have wind-tunnel tested. Watch this space!

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Nick Boud
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