American Airlines Orders 20 Supersonic Jets—Is It Really Sustainable?

There are two types of people in the world: those who think we shouldn't be flying due to the carbon footprint of air travel and those who can't wait to fly on the Boom Supersonic Overture Jet. The latter promises to be fast and guilt-free, as it runs on sustainable aviation fuel (SAF). What's not to love?

Now, American Airlines placed a non-refundable deposit with Boom Supersonic for 20 planes, with options for 40 more. For the record, these planes are still a work in progress and years away from taking flight. The carrier is the Colorado-based startup's second U.S. client: United Airlines put down a deposit last year for 15 supersonic jets.

I have expressed some skepticism about Boom before. But Blake Scholl, Boom's founder and CEO, said in the past that "the pursuit of ever-faster travel speed is really a moral imperative" and supersonic travel "is important to preserve mankind’s ability to flourish on our planet." I approve of mankind flourishing, so here we are again.

Some analysts wonder if Boom can actually deliver, noting how complex airplanes are and how even giants like Boeing have trouble getting them to stay up in the air. Engines are obviously a critical component, and they take a long time to develop: The Concorde's incredibly powerful Olympus engines first flew in 1950, almost 20 years before the plane's first flight.

The Boom Overture was recently redesigned from three engines to four hanging under the wing, which some experts question. Aerospace consultant Richard Aboulafia told AP that four-engine planes “are that much worse from every standpoint, from economics to emissions” and that “nobody wants more engines, the answer is fewer engines.”

The real problem for Treehugger types has always been the claim of "sustainability," which relies on the promise that the plane will fly on SAF. Dan Rutherford of the International Council on Clean Transportation expressed concerns to Treehugger about whether there could ever be enough of the and went into greater detail in a recent post, "Zero Cheers For the Supersoynic Renaissance." (I thought there was a typo in the headline, but "supersoynic" is a clever coinage describing supersonic planes running on soy-based SAF.)

Rutherford noted that one-third of the corn and soybeans grown in the U.S. goes into making ethanol and biodiesel, and calculated how much more would have to be grown just for United Airlines' 15 planes. Each would burn 24 million gallons a year, totaling 360 million gallons, which is equivalent to 6% of all U.S. production and more than all the soybeans grown in South Dakota.

But what is even more disconcerting is the fact that the soy SAF isn't much cleaner than regular fossil jet fuel. Burning it still emits carbon dioxide—whether it is carbon-neutral or not depends on its production.

Rutherford wrote:

"... emission savings depends on the amount of CO2 captured from the atmosphere during crop production—net of all emissions associated with growing those crops and converting them to biofuel. According to life-cycle emission accounting rules developed by the UN, SAFs derived from US soy reduce life-cycle GHG emissions by about one-quarter (27%) compared to conventional “Jet A” fuel."

Given that flying supersoynic uses so much more fuel per seat-kilometer than conventional flight, it will still put out five times as much CO2 as a subsonic plane. Rutherford concluded by reiterating a point we often make about growing food for people, not cars or in this case planes:

"The potential for additional price increases as crops are diverted to fuel production is real and worrisome. The idea that the U.S. government would subsidize the feeding of supersonic jets, rather than people, beggars belief. There are ways to avoid the problem, including not providing tax credits for crop-based SAFs or SAFs used in supersonic aircraft. But step one is to just say no to the supersoynic renaissance."

There are other SAFs besides those made from soybeans. They can be made from animal fats, which is not going to generate enough supply to keep an airline aloft. They can also be made of "electrofuels," which could be made from CO2 and green hydrogen made using renewable electricity, using the Fischer–Tropsch process that Germany relied on for fuel in the Second World War.

A March 2022 study prepared by the International Council on Clean Transportation estimated the average cost to be $8.80 a gallon at present, coming down to $4.00 per gallon in 2050. Given today's fuel prices, that doesn't sound so bad. But the supply of green hydrogen is pretty negligible now and, as noted previously, its best and highest use is for industrial processes, not fuel, and the volume needed for supersonic planes is huge.

In a previous post, commenters didn't appreciate my sarcasm when I wrote, "With Boom, it is a whole new world of green sustainable flight." So this time, I will conclude unequivocally: This ain't gonna fly—at least in any way that can be described as "sustainable."