Environmental performance of emerging supersonic transport aircraft
Kirby your enthusiasm about the supposed supersonic revival
It’s been a herky-jerky several weeks for those who study supersonic aviation. On May 21, Aerion Corporation, which has been working to develop a supersonic business jet since 2003, abruptly ceased operations, citing the difficulty of raising capital. But on June 3, United Airlines CEO Scott Kirby announced a deal to purchase 15 of Boom Supersonic’s “Overture” aircraft, with options for up to 35 more, if delivered as promised by 2029.
The United-Boom partnership leads some aviation watchers to re-assert their confidence in an “inevitable” renaissance in supersonic jet travel. But the Aerion shutdown may be the more appropriate parable. Let’s cool our afterburners and look at the facts: for a host of reasons, supersonic aircraft are unlikely to take off, and are a distraction from more rational efforts to create a sustainable aviation sector. Among other challenges, development of supersonic aircraft faces:
- A shortage of capital—Aerion found the funding challenge to be insurmountable, and Boom has raised only about 1% of the $20 billion that may be needed. Recall that development of the Concorde flown by Air France and British Airways late in the last century was underwritten heavily by government funding, which is absent here.
- No identified engine—Boom has an agreement with Rolls Royce to explore options for engines, but no concrete design has been announced. Building a supersonic engine is incredibly expensive, especially since Boom’s goal is for a medium bypass turbofan without afterburners, which would be a completely new design.
- Little interest among airlines—Aircraft sellers need buyers, and beyond United, only Japan Airlines currently has options on the Boom offering. United’s real commitment is also unclear since we don’t know if any money actually changed hands.
- A price-sensitive market segment—Making the economics work for airlines is a huge challenge since most commercial fliers are very price-sensitive. Even Aerion, which aimed to build a business jet (a lesser challenge than a full-fledged commercial aircraft) and was further along in developing key partnerships than Boom is, could not make the economics work.
- Unslakable thirst for fuel—By all accounts, on average a fast commercial supersonic will burn at least five times more fuel per passenger than a subsonic aircraft (Figure 1). Even before considering climate impacts, that’s a huge fuel bill for airlines to absorb.
- Unavailable fuels—Sustainable aviation fuels, which Boom commits to using, are expensive and woefully underdeveloped. The 15 supersonics ordered by United could consume the EU’s entire 2030 synthetic jet fuel supply—twice over—if used on transatlantic routes (Figure 2). Nor is supply increasing as forecast: in 2019 jet aircraft used just one quarter of one percent of the International Air Transport Association’s goal for sustainable fuels use by 2020.
- Noise challenges—The Concorde’s sonic booms led to restrictions on its use over land, which contributed to its economic non-viability. Sonic boom remains a serious challenge today. If United and other operators respect current overland flight restrictions, the market size for supersonics shrinks by 80 to 95 percent compared to the reference design. This implies a market much smaller than the 2000 planes that Boom aims to sell. And if overland flights are allowed and that market were achieved, sonic booms could be experienced as often as every five minutes in parts of Europe and North America.
- Other environmental impacts—The atmospheric impacts of supersonic emissions extends well beyond their large carbon dioxide (CO2) emissions. Research from MIT suggests that non-CO2 climate impacts from fast supersonics can be up to 20 times more damaging than subsonics because of their high cruise altitude, which increases the residence time of emissions significantly.
- Societal opposition—Many constituencies oppose the return of supersonics, including EU countries, airports, and civil society groups. These groups would resist development of international standards to enable supersonics, without which financing becomes a serious challenge.
So, in summary, do I think commercial SSTs will succeed? Nope. Would the environmental impact be large if they did? Yup. Is this all a huge distraction from the real challenge of decarbonizing aviation? Completely.
It’s especially a distraction for United Airlines CEO Scott Kirby, who has called climate change “the defining issue of our generation” and has called for United to zero out all its greenhouse emissions by 2050. If Mr. Kirby really believes this, it would be wiser to kick supersonics to the curb than to support their development.