A new dawn for supersonic air travel?

United Airlines, one of the world's biggest carriers, has just ordered up to 50 supersonic airliners — there hasn't been news like this since the 1960s.

Back then, the top airlines were racing to ink deals with the Anglo-French Concorde and/or the American Boeing SST project. Lufthansa put its name down in the order books on both sides of the Atlantic for aircraft capable of flying at twice the speed of sound. At the time it was assumed that at some point in the 1970s, almost all long-haul passenger flights would be operated by supersonic airliners.

But that never happened. After billions of dollars had been spent, the Boeing project was unceremoniously dropped in 1971. The Europeans pushed Concorde through, also at immense expense, as it was a priority for prestige reasons. However, it turned out to be a bad financial decision. Air France and British Airways flew just 14 Concordes in heavily subsidized operations that finally ended in 2003.

The main reasons for the end of the first supersonic era were fundamental hurdles mostly to do with physics. Supersonic flight produces a sonic boom (due to the aircraft being faster than the speed of sound (Mach 1). As a result of the noise, supersonic flight over land is not allowed. Moreover, supersonic airliners emit extreme noise on takeoff due their aerodynamics. They also guzzle considerably more fuel than ordinary airliners. That wasn't sustainable then and certainly wouldn't be today.

Feeling supersonic

When United, the world's fourth-largest airline, announced its intention in early June to buy 15 Overture airliners from the startup Boom Supersonic and commit to 35 further options, the news caused a sensation. "A move that facilitates a leap forward in returning supersonic speeds to aviation," the jubilant companies said in a statement.

However, there was one significant caveat. The deal would be confirmed "once Overture meets United's demanding safety, operating and sustainability requirements." Nonetheless, to underscore the seriousness of its purchase intentions, United has already paid an undisclosed sum to Boom.

The timeline foresees that the aircraft's first flight will take off in 2026 and that it will be carrying fare-paying passengers for United by 2029 with Mach 1.7 speed, significantly slower than Concorde, which reached Mach 2.02. Still, Overture could potentially fly from Frankfurt to New York in just four hours, quicker than Concorde could, due to Overture's superior range.

The new airliner would seat 50 to 60 passengers — making it smaller than Concorde, which had capacity for around 100 travelers. However, it would emit three to five times more CO2 than a subsonic flight on the same route, and according to estimates by independent researchers with the International Council on Clean Transportation (ICCT), five to seven times the fuel per passenger.

Fuel is a key issue. Boom has committed to make Overture the world's first airliner capable of operating with 100% synthetic, sustainable aviation fuels (SAF). But there is a severe shortage of SAF and production is currently far more expensive than that of kerosene. The 15 Overtures of United alone could need double the amount of SAF than will exist in the whole of the EU by the end of the decade, according to ICCT estimates. As this would contradict the claim of sustainable supersonic flight, critics are already accusing the project of greenwashing.

Making overtures

Whether an Overture model will ever take off at all is still a moot point. Industry experts have been critical of the deal. Richard Aboulafia, an aerospace consultant with Teal Group, points to the huge value in free advertising with such soft order announcements, not costing the participants anything.

"United continues to position itself as environmentally friendly, an idea that now looks a bit ridiculous," Aboulafia told the Financial Times, "but then again since this plane is completely notional, it really doesn't matter much." Bernd Liebhardt, supersonic expert at the German Aerospace Centre DLR in Hamburg, foresees a "significant public credibility boost for Boom."

Whether or not the startup from Denver, Colorado, can fulfil the expectations will soon be clear. At the end of this year or in early 2022, the already existing single-seat XB-1 test aircraft is supposed to take off to validate and optimize the concept for Overture.

United's venture is all the more remarkable given that another US company with ambitious supersonic plans recently ceased operations after one and a half decades of research and development, spending around €1 billion ($1.21 billion) in the process.

Aerion Supersonic had initially planned to launch a twelve-seat supersonic business jet, evolving into airliners later, and had Boeing as a strong partner. But when huge investments of up to €3 billion were needed to actually start production, there was a lack of funding. The same could happen to Boom, Richard Aboulafia warned. "If Aerion can't do it with a promising business case, who the hell can? " he told CNN. On the same network, Boom CEO Blake Scholl had already declared: "Either we fail or we change the world."