Supersonic aircraft: Twice as nice, or double the trouble?

The climate impacts of aviation are increasingly coming into focus. Following sharp traffic reductions due to the COVID-19 pandemic, greenhouse gas (GHG) emissions from airlines are expected to exceed 2019 levels this year. Recognizing the need to reduce emissions in line with the Paris Agreement, in 2022 aircraft manufacturers agreed to support the aviation industry’s goal of net-zero carbon dioxide (CO₂) emissions in 2050.

Achieving that goal will require a gargantuan effort by stakeholders including airlines, fuel providers, and aircraft manufacturers, and an estimated US$4 trillion in new investments in clean fuels and planes for international flights alone. Moreover, according to our research, starting around 2035, all new subsonic aircraft would need to be zero-emission throughout their operating lifetimes.

That’s sobering, especially given how rare new aircraft types are today. Beyond work to certify Boeing’s widebody 777X, none of the “big three” western commercial manufacturers (Airbus, Boeing, and Embraer) have announced plans to certify new aircraft types before 2035. And only new types hold the potential for substantial improvements in fuel efficiency and, potentially, the shift to zero-carbon fuels like hydrogen.

But one manufacturer, Boom Supersonic, seems unbothered by the huge effort that’s needed. Boom just completed the eighth test flight of its one-third scale XB-1 demonstration aircraft. Boom aims to break the sound barrier by the end of the year, en route to bringing its Mach Number 1.7 aircraft, Overture, into service by 2029. They argue that supersonic aircraft “are twice as fast so it’s also going to decarbonise twice as fast.” But how does that square with flight physics and a net-zero carbon budget?

Due to technological advances since Concorde was developed in the 1960s, it’s expected that Overture will be more fuel efficient than the jet-fuel-guzzling Concorde aircraft that flew in the 1980s and 1990s. But subsonic aircraft have improved over time, too, and today’s advanced widebody aircraft like the Boeing 787 and Airbus A350 burn 30% less per seat km than the B747-400 aircraft that shared the skies with Concorde.

Due to Overture’s high speed, small size and therefore poor economy of scale, small payload capacity, and limited range (it requires refueling stops on longer flights), it’ll inevitably burn more fuel per seat than competing subsonic widebodies. According to Boom, a seat on Overture will consume two to three times more fuel than business class seating on today’s widebodies, and seven to 10 times more fuel than an economy seat. Because CO₂ emissions scale with fossil fuel burn, supersonics will have a disproportionately high climate impact.

Boom points to the promise of sustainable aviation fuels (SAFs) to address these emissions. SAFs are alternative jet fuels produced from biological or renewable feedstocks that can have lower life-cycle emissions than conventional jet fuel. But SAFs remain scarce (just 0.2% of fuel supply in 2023), expensive (generally quoted as two to five times higher than fossil jet fuel costs), and they come with sustainability concerns of their own.

So, if Overture successfully makes it to market, what would that mean for a net-zero carbon budget? I ran the numbers using the approach from last summer’s paper with Supraja Kumar, namely, to estimate the GHG emissions associated with new aircraft deliveries throughout their operating lifetimes. I started by assuming that Boom will deliver 1,000 of its Overture aircraft by 2050; this is 33 aircraft produced annually at its Overture Superfactory in Greensboro, North Carolina, starting in 2029, and then a doubling of annual production via a second production line in 2042.

Drawing on previous modeling that we did with MIT, I assume that an average Overture flight burns 44 tonnes of jet fuel over a typical flight of 3,800 km and operates at an average speed of 1,350 km per hour (approximately MN 1.27 at 50,000 ft). That’s 75% of design cruise speed, which is the average for subsonic aircraft in our recent paper, after accounting for time spent in the slower takeoff, climb, descent, and landing phases of flight.

Each Overture is assumed to operate as a typical widebody subsonic aircraft, meaning that it flies an average of 2,900 hours per year over its typical lifetime of 25–30 years. I ran two cases to bound the range of uncertainty on future SAF use: a business-as-usual (No SAF) case and a Maximum SAF case that assumes ReFuelEU Aviation levels of SAF globally through 2050. That’s equivalent to the European Union’s ambitious SAF mandate, which will require 70% SAF supply at EU airports in 2050, but implemented globally. This is an aggressive assumption, given that SAF mandates are still in their infancy outside of Europe.

The results are frankly startling. Beginning in 2029, 1 year of Overture deliveries would emit between 113 and 155 million tonnes (Mt) of CO₂ over their lifetimes in the Maximum and No SAF cases, respectively. To put those numbers in perspective, in 2022, the entire country of Chile emitted 137 Mt of GHGs. That means Overture deliveries alone would be responsible for a GHG footprint equal to the fifth largest economy in South America, home to nearly 20 million people. Lifetime emissions from a year of deliveries are estimated to rise to between 128 Mt (Max SAF) and 310 Mt (No SAF) CO₂ by 2050. That’s equivalent to about 40% of the emissions from Airbus deliveries in 2023, and would be from a manufacturer building less than one-tenth as many aircraft.

How do these emissions relate to aviation’s remaining net-zero carbon budget? The figure below shows cumulative lifetime emissions from Overture deliveries under the No SAF (blue) and Maximum SAF (red) cases. Absolute CO₂ in billion tonnes (Gt) is shown at left and the share of aviation’s remaining net-zero budget is shown at right. I estimate that Overture deliveries through 2050 would emit between 2.4 and 4.8 Gt of CO₂ over their lifetimes. That’s between about one-quarter and one-half of aviation’s remaining 9.4 Gt net-zero carbon budget.

Figure. Cumulative CO₂ emissions (left) and share of a net-zero aviation carbon budget (right) from Overture aircraft by delivery year and scenario

This means that a single startup manufacturer could consume at least one-quarter of aviation’s remaining net-zero carbon budget through 2050. That leaves much less available for conventional aircraft to sip the SAF that Overture would gulp. And remember, if Boom’s estimate of Overture’s fuel burn per seat is correct, one gallon of SAF burned in a subsonic plane would provide 6.6 times more passenger kilometers of travel than on Overture.

So, in our carbon-constrained world, there’s reason to think the return of supersonic aircraft will be double the trouble, not twice as nice.