Environmental performance of emerging supersonic transport aircraft
Turbulence ahead for the supersonic renaissance
The UN’s International Civil Aviation Organization (ICAO), which sets internal standards and practices governing civil aviation worldwide, wrapped up its 11th triannual environmental committee (CAEP/11) meeting last Friday in Montreal. Among other issues, CAEP considered what type of noise standards should apply to new supersonic aircraft. Three U.S.-based startups are working to develop new supersonic transport (SST) aircraft, including a 55-seat commercial jet. The Trump administration, which favors permissive standards for SSTs, has clashed with Europe about the aircraft’s potential noise impacts. To date, regulators were faced with two choices: either to develop new SST standards that would allow those aircraft to produce more noise, air, and climate pollution than new subsonic designs, or to apply existing subsonic standards to SSTs.
So, what exactly happened in Montreal? ICAO’s general press release on the meeting can be found here. Overall, ICAO declined to establish an international landing and takeoff (LTO) noise standard specific to supersonic aircraft (the first approach). Instead, it will conduct a comprehensive review of the likely increased noise, air, and climate pollution from supersonics and consider how the existing (Chapter 14 in UN parlance, Stage 5 in the U.S.) LTO noise requirements for subsonic aircraft can be applied to supersonics. In other words, the UN has selected the latter approach by declining to set weaker international environmental rules for SST aircraft. It’s now up to the U.S. to figure out how to set a domestic LTO noise standard for SSTs on its own, as required under the 2018 FAA Reauthorization Act.
As we discussed here, it’s harder for supersonic aircraft to meet subsonic noise standards because they require high thrust, low bypass ratio engines that are louder in landing and takeoff. In theory, the U.S. can go home and set a weak domestic noise standard for SSTs to promote the industry. However, the reality is more complicated. Due to a ban on supersonic operations of civil aircraft over land in most parts of the world, the vast majority of flights for the foreseeable future will be overwater and, therefore, international. It is doubtful that demand for U.S. domestic flights alone will be enough to finance the re-introduction of SSTs that don’t comply with ICAO’s presumptive Chapter 14 noise standard.
The figure below, drawn from our recent assessment of potential commercial SST routes, maps out the likely distribution of about 5000 SST flights that could link 500 airport pairs, a goal laid out by Boom Supersonic last year. This network is meant to support sales of 2000 commercial SSTs.
The figure shows the major routes that commercial SSTs could serve in 2035 under an “unconstrained” scenario with no overland flight restrictions or airport noise or congestion constraints. Only 650 (13%) of the hypothetical 5000 flights are domestic; among those, the overwhelming majority (590, or about 12%) are in the U.S. At most, 190 aircraft could be used on U.S. domestic routes. However, the vast majority of those flights would be overland and therefore banned under current U.S. law. So, the market for a commercial supersonic that cannot operate internationally, nor fly overland, might be on the order of several aircraft, not 2000. That’s nowhere near large enough to payback the development costs, which Boom has pegged at $6 billion, despite using a “derivative” engine based upon existing technologies.
It’s now clear that the route to any international market passes through existing UN noise limits for subsonic aircraft. So, what does this decision mean for manufacturers? Short-term, it shouldn’t impact Aerion and Spike, which have both committed to meeting Chapter 14 noise requirements with their smaller business jets. For Boom, which is aiming to develop a larger (55 seat) and faster (Mach number 2.2) airliner, tough choices lay ahead. Boom’s public statements to date suggests it plans to comply with Chapter 3 (1977) subsonic noise standards. But, that won’t be nearly enough.
So where might Boom go from here? Two approaches seem possible. First, Boom could try to convince an engine manufacturer to develop a new, and more expensive, advanced clean sheet engine with lower noise. Alternatively, it could significantly reduce the design speed of its aircraft to reduce its LTO noise footprint, a tradeoff that Aerion has already made. The former approach would further inflate the development costs of its aircraft – note that to date Boom has raised about $141 million, or about 2% of the $6 billion it needs to develop the cheaper aircraft from derivative technology. The latter approach would require a fundamental change to a business model built largely around being faster than the MN 2.0 Concorde. Either tactic will make Boom’s goal of providing fast, cheap supersonic service to the masses that much harder to achieve.
Clearly, turbulent times are ahead for the “inevitable” return of commercial supersonics.