Ever since the supersonic Concorde was grounded in 2003, airline passengers — at least, —have yearned for the return of faster-than-sound transit. Today, a trip from New York to London takes seven hours in the air; the Concorde could do it in in 1976. In an era where the wealthy have to , and precision manufacturing is easier than ever thanks to computer-aided design, the skies seem ripe for a .

And indeed, there’s a that all promise to deliver the spiritual successor to the Concorde very soon. American Airlines and United have both put deposits down on one company’s supersonic jet — the Boom Supersonic Overture — that promises to be in passenger service by 2029.

Let’s not forget, however, that faster-than-sound travel should be a solved science; the first supersonic transit (SST) broke the sound barrier 52 years ago. So why must the wheel be reinvented? To delve into the future of SSTs, we first have to revisit the past.

We Had Supersonic Planes Decades Ago, So Where Are They Now?

The history of SSTs is really a tale of two airplanes. In the West, transatlantic passengers could ride the famous British Aircraft Corporation/Aerospatiale Concorde; in the Soviet Union, there was the oft-forgotten Tupolev Tu-144. The Tupolev was actually the first to kick off the supersonic era; in June of 1969, it became the first commercial airplane to break the sound barrier, beating the Concorde by a few months. By 1970, it was the first commercial jet . It boasted over 200,000 pounds of thrust from its four afterburner-equipped jet engines and a top speed of Mach 2.15 (), making it the fastest commercial jet to ever fly. The Concorde, while slightly less impressive on paper, was the first to be put into , and had a much longer service life. Unfortunately, both planes proved impractical for commercial aviation.

On the Soviet side, failures were apparent nearly immediately. After a spectacular delayed its rollout, the Tupolev TU-144 finally entered passenger service in 1978, only to be pulled from state carrier Aeroflot’s commercial fleet roughly six months later, after just 55 flights with passengers aboard.

In characteristic Soviet manner, the development of the Tu-144 was rushed to meet arbitrary deadlines meant to commemorate various anniversaries of the Communist Revolution. As a result, the Tu-144 was incredibly unreliable, racking up in just 181 total hours of flight time. Its underlying design was inherently flawed, as its design did not include . As a result, and were extremely difficult to repair.

It was also uncomfortable; passengers reported being unable to speak to each other while cruising, thanks to the deafening noise of the always-on afterburners. There weren’t many passengers to complain, though: Aeroflot only sold (despite the Tu-144’s capacity of 144 passengers) on once-weekly trips on a single route, to minimize casualties if the plane crashed. And indeed, a second fatal crash on a test flight of a was the final nail in the Soviet superjet’s coffin.

The British/French Concorde initially appeared much more promising. The Western jet was developed over a longer time frame, with of the Tupolev. It entered service in 1976 and stayed in transatlantic use until 2003. During this time, the Concorde only suffered and was widely considered one of the . It was also vastly quieter and more comfortable for passengers than the Tupolev, with the afterburners only used for short periods rather than for the entire flight.

Unfortunately, the Concorde suffered from many of the same economic problems as the Tupolev. Both planes were incredibly expensive to develop; the Soviets on the Tu-144 project, and the British and French governments subsidized Concorde development to the tune of $3 billion. By the time the Concorde was launched in 1976, its from original estimates; as a result, demand collapsed. At one point, 18 different air carriers had expressed interest in ordering more than 100 planes combined. When the Concorde went into service, only 14 were ordered, split evenly between Air France and British Airways.

Both the Concorde and the Tupolev Tu-144 were frighteningly inefficient in the air. The Tu-144 consumed a staggering in supersonic flight. The Concorde was somewhat more efficient, consuming . (For comparison, an Airbus A380, the largest commercial aircraft currently in use with a , consumes .) Initially, this high fuel consumption wasn’t a problem — development on both planes began in the early ‘60s, when fuel was cheap. But by the mid-’70s when both began passenger service, oil prices had skyrocketed. New widebody aircraft with lower per-passenger fuel costs had entered service, and the operational profitability of supersonic aircraft tanked.

Thanks to the Concorde’s extreme thirst and high up-front cost, Air France reportedly on its supersonic transatlantic flights, despite ticket prices averaging over $12,000 by the late ‘90s. Those high prices meant most flights were only half full, making them even less profitable. The Concorde was also barred by the FAA from flying lucrative , due to the noisy and potentially destructive sonic boom it created at cruising speed. As a result, the Concorde required hundreds of millions of dollars of government subsidies over its lifespan to keep flying, and when it was discontinued, no one was clamoring for another money-hungry replacement.

The Next Generation

The first generation of supersonic aircraft was basically doomed from the get-go, with ridiculously high price tags, limited routes, and staggering fuel costs. These failures have not stopped a host of recent startups eager to bring back supersonic travel, and they’ve all proposed some form of tailless delta-wing aircraft, updated to help solve the issues that plagued the Concorde and Tu-144.

How do these modern companies intend to succeed where two government-backed efforts failed? All the companies working on SSTs today promise much better fuel efficiency, thanks to new medium-bypass jet engines that don’t require thirsty afterburners. They all intend to use 100-percent sustainable aviation fuel (SAF), which — in theory — reduces overall carbon emissions compared with conventional jet fuel. (SAF, which still is for use in passenger planes unless it’s mixed 50/50 with conventional fuel, is a for jet fuel. Proponents of SAF say it can reduce carbon output by up to 80 percent compared to conventional Jet A fuel.)

The long-time leader in modern SSTs was , which was developing a low-boom business jet called the AS2, targeting Mach 1.4 speeds. Since Aerion’s inception in 2004, the company invested over of supersonic aircraft, produced a , broke ground on a $300 million headquarters complex in Florida, and received a in orders.

Aerion had partnered with Airbus, then Lockheed Martin, then Boeing (which reportedly invested several hundred million dollars for a 40-percent stake) as it searched for a viable airframe design. a medium-bypass supersonic-capable jet engine for the AS2. Aerion, like most other players in the space, was hoping to earn FAA certification by 2024, with deliveries promised by 2029.

. On May 21, 2021, of operation, laying off its entire staff of 160 employees. The company simply ; it folded without so much as a full-size mockup to show for its efforts.

What Makes Today’s SST Concepts Different From the Concorde?

Aerion’s sudden and dramatic failure raises the question: How do any of today’s new startups hope to succeed in supersonic flight? Almost all of them intend to offer revenue service flights by 2029. None of them have actually demonstrated the ability to fly at all, much less at the speed of sound.

The complexities of supersonic flight notwithstanding, achieving FAA certification by 2029 hardly seems workable. The and 200,000 hours of technical work to be type-certified by the FAA. That’s for a relatively conventional subsonic aircraft. SST development is made more complicated by the fact that supersonic passenger flight has been banned domestically since 1973, and for faster-than-sound craft. Any new SST manufacturer would likely need to partner with the FAA to help determine what, if any, new rules must be created specifically to deal with supersonic passenger planes.

I asked the FAA if anyone has reached this phase; a public affairs representative told me one supersonic jet manufacturer has indicated it’s ready to discuss type certification, but the FAA could not divulge the name of the company.

The majority of these companies hope to create a type of aircraft that’s never been seen before. Three of the four remaining supersonic startups promise their jets will be “low-boom” designs — able, in theory, to fly over populated land without disturbing residents on the ground. This would open up thousands of potential routes, making supersonic flight vastly more economically viable.

Just one hitch: No low-boom jet has ever actually flown. Lockheed Martin’s single-seat X-59 prototype, the first low-boom plane to ever reach the testing phase, is after six years of development. This plane uses a as well as an with zero forward visibility, all in the name of achieving a quieter sonic boom. It’s unclear how the techniques learned from the X-59 will apply to commercial passenger airplanes.

Further complicating matters is the state of the jet-engine market. In aircraft development, jet engines are often supplied by a third party. (For example, the Concorde engines specifically developed for supersonic flight.) Today, there are no engines in production that meet the criteria for efficient supersonic flight on a passenger-sized aircraft.

GE was working on a medium-bypass , and was scheduled to flight-test it as early as 2023, but that program ended shortly after Aerion shut down. Rolls-Royce has from supersonic engine development, saying that commercial supersonic flight is “not a present business priority.” Safran Aircraft Engines earlier this year that supersonic engine development isn’t part of its current strategy. The CEO of CFM International has said in public investor forums that the company is not interested in supersonic engine development. No one seems to want to build engines for the next generation of supersonic passenger planes.

And there’s a final problem that none of these supersonic startups seem to have answered. Even if the FAA approves flights using 100-percent sustainable aviation fuel, supersonic flight will still consume massive amounts of that fuel. Right now, SAF only accounts for 0.5 percent of all aviation fuel consumed per year; scaling up will require dedicating huge swaths of farmland to SAF production. It’s possible that a switch to SAF would have minimal environmental benefit: studies have shown that converting farmland to grow corn for ethanol, and using nitrogen fertilizers to promote growth, could actually .

Even if those problems can somehow be avoided, the that supersonic jets using SAF “could actually exacerbate the medium-term climate impacts” of supersonic planes, due to the extreme heat generated by supersonic travel, being released at the extremely high altitudes where SSTs would fly.

So it’s clear there’s an uphill battle, but that hasn’t dissuaded the new crop of SST startups from trying to bring back the Concorde concept for the 21st century. Next week, we’re going to look at the four major startups vying to solve all these problems — and how close they may actually be to solving them.