Boom Overture: Will Supersonic Passenger Travel Return?

 

Beyond Concorde: Supersonic Airflow on Modern Airliners

Here's a quick quiz for you. When was the last time that an aircraft carrying fare-paying passengers flew through the air faster than the speed of sound? Well, I'm sure you're all thinking of Concorde, which last flew in 2003, and you'd be right. But did you know that the last time you flew on an airliner, there was a very good chance that the aircraft you were flying in actually had supersonic air flowing over certain parts of the airframe. And that's because all modern airliners have something called a supercritical wing. Now, on a traditional wing, the top surface is highly cambered or curved, which results in air speeding up as it flows over it due to a phenomenon called Bernoulli's conservation of energy. Now, this is a vital part of how a wing generates lift. But as the aircraft fly closer to the speed of sound, this very vital part of the lift process actually begins to limit us. Now, imagine an aircraft flying sub sonically, well below the speed of sound, let's say Mach 0.7 or 70% of the speed of sound, it'll be logical to think that a subsonic wing would be flying quite happily. But due to Bernoulli's principle, the air flying over the top of the wing could be accelerated to supersonic speeds, and on a wing designed to fly sub sonically, that's a problem.

The Supercritical Wing: Delaying Shockwaves and Enabling Higher Cruise Speeds

Now, the speed at which this happens is known as the critical Mach number, or MCRIT. And once MCRIT is exceeded, large shockwaves can begin to form, rapidly increasing drag and disrupting the airflow behind the wing, destroying the lift being generated, similar to how a low-speed stall destroys lift. Now, to delay the onset of these lift-destroying shockwaves, the supercritical wing was developed. This design basically flattens the top surface of the wing, which delays and reduces the shockwave formation, enabling higher cruise speeds. And sometimes you can actually see this phenomenon out of your window. This happens because the aircraft is flying in what is known as the transonic speed regime, where some parts of the aircraft, like the upper surface of the wing, accelerate the air to supersonic speeds. Now, the airflow forward of that shockwave, as you look through your window at the wing from above, is supersonic, that's slowing back down to subsonic speeds afterwards. And that's pretty cool, and definitely worth looking out for during your next flight.

Boom Supersonic's XB-1: A Step Towards Resuming Supersonic Passenger Travel

But, of course, this is not what we mean when we talk about supersonic flight. If we want all of the plane to fly faster than the speed of sound, then obviously the last airliner to do so was Concorde before it was retired in 2003. And I find it really sad to think that it's been over two decades since the beauty of Concorde graced our skies above us. But if Blake Scholl, the CEO of Boom Supersonic, has his way, the company will make supersonic passenger travel possible again in just a few years. And to that end, Boom took a key step in achieving this on the 28th of January this year, when the XB-1 experimental testbed broke the sound barrier. This was the 12th and final flight of this aircraft, and it was a long time coming. Boom rolled out this plane back in 2020, describing it as a one-third scale test aircraft meant to test the layout of Overture, which is the name that Boom uses to describe its future supersonic passenger jet.

XB-1's Design: A Blend of Old and New Technology

Now the XB-1 has something called an ogival delta wing, which is a similar design to that of Concorde. And it's the same wing design that Boom originally planned for the Overture. Now, while that wing design is excellent for high-speed supersonic flight, it's not very efficient when flying at the lower speeds required for takeoff and landing. So in order to generate sufficient lift during the approach, Concorde had to fly with a very high angle of attack. This meant that Concorde's long nose was pointing more towards the sky than it was the runway. And without any modifications, it would make it impossible for the pilots to see the runway. And that's not good. So to overcome this problem, Concorde's designers came up with a novel but costly solution, a droop-nose system that can move out of the way. Now, whilst this was a very innovative solution to the problem, it also added a high degree of complexity and weight, which not only delayed the certification process, but also required costly maintenance. So to avoid having to install a similar system to the XB-1, its designers incorporated an augmented vision system called the FLVS or the Forward Looking Vision System. This system includes two redundant cameras, a multi-function display, a data acquisition system, and an inertial navigation system which provides pilots with everything they need to fly safe approaches. Now, while this is an innovative solution to a problem, this plane itself is a bit of a mix of old and new. For example, the wing and front half of the fuselage are made of modern composites, whereas the rear structure is aluminum which houses the engines, which themselves aren't exactly modern. These are General Electric J85s, basically a 1950s design for small fighter jets and military trainers. And Boom actually uses a T-38 aircraft with two of the same engines as a chase plane for the XB-1.

XB-1's Supersonic Flights and the Breakthrough of "Boomless Cruise"

The first supersonic flight lasted just over half an hour, and the XB-1 and its pilot broke the sound barrier three times, reaching a speed of Mach 1.1 before coming back in for a safe landing. Then a couple of weeks later, on the 10th of Feb, Boom's XB-1 took off again for its second supersonic flight. This one lasted a little bit longer, and the aircraft flew slightly faster at Mach 1.12. But all of the XB-1's flights took place in special airspace over the Mojave Desert, where NASA has special testing equipment for just such flights. And during the XB-1's second supersonic flight, Boom and NASA arranged for a special photo to be taken. This process is called a Schlieren photography. And if the aircraft flies at a certain angle in relation to the Sun, then it becomes possible to see the way the air density changes around the aircraft as shock waves form, similar to shock waves that we saw over the Airbus A320 wing, but on a much bigger scale. More importantly perhaps, Boom announced that neither the XB-1's supersonic flights actually featured a sonic boom that reached the ground. And that's really a big deal. During testing, the aircraft flew high enough and a low enough supersonic speed, and in the right atmospheric conditions that no sonic boom reached the ground, something that NASA's special array of microphones in the desert was able to confirm. Boom calls this boomless cruise, and it can occur when the aircraft flies between Mach 1 and Mach 1.3, but again in the right altitude and atmospheric conditions. In very simple terms, as the shock waves travel downwards, the warmer, denser air causes these shock waves to diffract, bending them back upwards away from the ground. But believe it or not, this isn't actually a new idea. The phenomena is called Mach cutoff, and NASA has been studying it and the conditions it can happen in for quite some time now with their own test aircraft, which is why they already had those special microphones in place.

Overture's Features and Future Outlook

Now as it stands at the moment, commercial supersonic flights overland are not permitted. And even if they were, Boom currently intends to optimize Overture for cruise speeds of Mach 1.7, much too fast for boomless flight to be possible. But if these rules change to permit overland supersonic flights when they're quite enough, then Boom's Overture could still take advantage of this, gaining a big speed advantage over conventional airliners, and more operational flexibility than Concorde ever had. Crucially though, the focus is now shifting from the XB-1 to the Overture passenger jet, because Boom doesn't intend to fly the XB-1 again. The plane has done its job now, and Boom plans to display it in the lobby of its headquarters in Denver. Now the Overture will be utilizing a wide variety of new tech, bringing supersonic flight to the 21st century. Its flight deck will incorporate modern avionics with 17-inch touch screens, active side sticks that allow pilots to be aware of each other's inputs, which, in my view, is a distinct advantage over the Airbus system, plus the enhanced flight vision system, which will basically be a wearable head-up display to increase situation awareness, especially in low-visibility conditions. Also somewhat unusually, perhaps, the Overture will be able to receive what Boom calls over-the-air upgrades, allowing new system features and improvements to arrive on the entire fleet regularly and quickly, similar to how modern cars receive software updates. So what's next? How quickly can we expect the development of Boom Overture to advance from now on? Well, Boom remains quite ambitious with its development timeline, but ultimately, this is a question of money. And the company has already secured funding from many interested players, which I'll come back to in a bit.

Evolution of Overture's Design and Engine Strategy: From Rolls-Royce to Symphony

But let's start with something that was a pressing problem for Boom the last time we talked about them, engines. In the summer of 2022, Boom effectively parted ways with Rolls-Royce, who had been a strategic partner for the company for around two years. Now, the purpose of that partnership was to explore the possibility of using Rolls-Royce engines in Overture. But Rolls-Royce later stated that the job they'd actually been contracted to do was to help Boom with studies about the engines that such an aircraft would need. To start with, Boom's Overture had a three-engine layout, one under each wing and a third one in the rear of the fuselage. As a result of those studies, Boom revised its design to feature four underwing engines and a new type of wing. The company moved away from the Ogival Delta wing that Concorde and the XB-1 had for a more conventional delta-wing design, along with horizontal tail surfaces. The configuration is actually really similar to late versions of Boeing's ill-fated 2707 SST from the late 1960s, although Boeing's design was much bigger. It would have been a wide-body with seats for well over 200 passengers. But that's not all that changed. Boom's new aircraft also got slower. Originally, Boom wanted the Overture to cruise at Mach 2.2, which was slightly faster than Concorde, but the company now says that the new version will cruise at Mach 1.7, which is still pretty rapid. Also, this newer Overture is bigger. The original was big enough for around 55 passengers, but the new one will have room for 66 to 80 passengers. And that growth has had some interesting implications for those engines. Originally, each of the three engines of the first Overture was to have a thrust rating of between 15,000 and 20,000 of thrust. And to put that into perspective, the thrust rating on the 737 MAX LEAP engines that I fly are rated at 27,000 pounds. So you may ask, why does a supersonic aircraft have significantly less thrust than that of a subsonic airliner? Well, we've got to remember here that the 737 MAX is two to three times the size of the Overture and has two engines as opposed to four. Boom says that each of the four engines of the new version will now generate 35,000 pounds of thrust, more than doubling the total installed thrust of the aircraft despite the lower cruise speed. You also may ask, why does Boom need a new engine? Can't they use an existing jet engine instead? Well, modern airliner engines designed for subsonic speeds are known as high-bypass turbo fans. And by bypass, we mean the part of the airflow, well, actually most of it, doesn't actually go through the core of the engine. Instead, after passing through the big fan that we see at the front of the engine, almost 90% of the air goes around a central core and out of the rear of the engine, meaning that the fan at the front is effectively acting like a giant propeller. The other 10% goes through the central core of the engine where it gets compressed, ignited and shot out of the rear of the engine, which provides an element of thrust as well as spinning the turbine blades, which are connected to the compressor blades at the front of the engine via a shaft and, in turn, spin those blades. Broadly speaking though, a high bypass ratio allows turbofans to increase their efficiency, which is what we all want. But when flying faster than the speed of sound, the large frontal area of these big engines becomes a problem. That's why even very modern jet engines for military supersonic jets are low or medium bypass turbofans, and that's exactly what Overture needs. But with Rolls-Royce out the picture and no interest from the likes of General Electric, Pratt & Whitney and Safran to design and build an engine for Overture, Boom decided to take matters into their own hands and develop their own engine. They named this Symphony. Now this doesn't mean that Boom won't have partners with engine-making experience, they do. And these partners do make components for many existing jet engine manufacturers for commercial and military applications, while others have experience in maintaining such engines. But by taking the lead in this project means that Boom will need to shoulder the cost of developing this engine on top of what needs to be spent to develop the rest of the aircraft itself.

Funding and Infrastructure: The Overture Superfactory and North Carolina's Support

And that brings us neatly to money. How much does Boom have and what will they actually need to get Overture into service? Well, Boom estimate that they will need between $6 billion and $8 billion to develop the aircraft. And seasoned analysts like Richard Aboulafia raise the estimate to at least 10 billion, which frankly, still seems quite conservative given that a modern subsonic airliner can cost twice as much to develop. Also, Aboulafia made that estimate back in 2020 when Boom was hoping to get an engine from a supplier. According to The Air Current, developing an all new engine for a project like this would realistically add at least $1.5 billion to the total development cost. So how much money has Boom managed to secure so far? Well, by the time the XB-1 broke the sound barrier for the first time, the company had managed to secure around $600 million. And this money includes deposits from launch customer airlines, contributions from various investors like OpenAI's Sam Altman and LinkedIn co-founder Reed Hoffman, plus support from potential customers like the US Air Force in a collaboration with military contractor, Northrop Grumman. The company has also received support in different ways. To assemble the Overture, Boom will use a purpose-built site in Piedmont Triad International Airport in North Carolina. Boom calls this the Overture Superfactory. The construction of the main building was completed in June 2024 and the work was made possible in large part thanks to the funding from the state of North Carolina. In essence, the state believes that the jobs that Boom will create in the region and the taxes they expect to collect from the company and those who work for it make it worthwhile to fund part of the project. The relevant paperwork lists approvals for $56.75 million for the construction of the hangars, $15 million for related work around the airport, plus $35 million to improve the road access to the airport specifically for this project, and that's a total of $106.75 million.

Overture's Economics and Timelines: Speed, Range, and Certification Challenges

Now, for a facility like this, Boom would need to have a steady demand for Overtures from customers around the globe, and that means that the economics and features of the aircraft need to make it a practical proposition for as many routes around the world as possible, which brings us back to those updated specifications of Overture and the choice to use four 35,000-pound turbofan engines, and one potential issue here though is range. The Overture will be able to fly a bit further than Concorde, around 4,250 nautical miles instead of Concorde's 3,900. That means that the Trans-Atlantic routes that Concorde flew will be no problem for Overture, but Boom also hopes to sell the aircraft to airlines that will perform Trans-Pacific routes, which won't be possible without a fuel stop along the way. Boom argues that the plane will still be significantly faster than conventional airliners. For example, flying from Seattle to Tokyo in a regular airliner takes around nine hours, but Overture will be able to fly the same route in half that time, even with a 30-minute fuel stop. Boom also claims that it's designed Overture for quick refueling to make this process easier. But weirdly, many analysts believe that this speed is actually too slow for the economics of the aircraft to work. There's a reason why the original Overture design was planned to cruise at Mach 2.2. As explained by Scott Hamilton in Leeham News, the speed was necessary not just to please the passengers, but for practical and operational reasons too. At Mach 2.2, an aircraft and its flight crew would be able to fly two legs in a day, for example a return flight between London and JFK, which is pretty insane. For many routes, reasonable arrival and departure times at airports with different time zones also became trickier to organize if the flights aren't quick enough. In practice, designing the aircraft for Mach 1.7 meant that the engines could be simpler. Unlike the complicated intakes of many military jets, the intakes of the Overture's Symphony will be fixed and simpler than those at the XB-1 testbed was originally meant to test. Fuel efficiency is another issue here. Boom says that the Overture will be able to use 100% sustainable aviation fuel, or SAF, from its introduction into service. But with SAF being quite scarce and very costly, many question the use of SAF in supersonic applications. Finally, there's the project's timeline. Despite mounting delays, Boom still claims that it will fly passengers in the Overture before the end of the decade. And that seems quite difficult, especially given the delays of certification of other aircraft that we've seen recently. Engine certification timelines are even tighter. John Ostrower of The Air Current points out that engines from established manufacturers like GE, Rolls-Royce and Pratt & Whitney can take four years to develop, even when they're basically updated versions of existing engines. All new engines like the CFM LEAP needed five years from launch to service entry, despite using the same technology from GE's wide body engines. Boom launched the Symphony engine at the end of 2022 for a 2029 entry into service, which seems quite optimistic, given their inexperience and the unusual nature of the engine's application.

The Enduring Challenge of Supersonic Flight and Boom's Ambition

Frankly though, the daunting task that Boom face today really emphasized how ahead of its time Concorde was when it entered into service all of those years ago. On paper, at least, the decades since then offer many opportunities for improvements, from more efficient engines, composites, lighter, stronger metal alloys, better aerodynamics and yet matching Concorde's speed is still proving difficult. Now, I don't really want to end this on a negative note though. Is this an ambitious project? Absolutely. But you have to be ambitious, creative and maybe a little crazy to take on something as monumentally challenging as this. And to be fair, Concorde's development timeline and its costs didn't exactly go to plan either. You need to be a good storyteller to get the right funding and support for a project like this. And you need to do so in creative ways, which brings us back to where we started this story with the XB-1 supersonic testbed. You see, even when the Overtures still had three engines and a different wing, many questioned the need to make a one-off testbed like the XB-1, suggesting that this plane was as much a test vehicle as it was an investment vehicle. And when asked about this, Blake Scholl, the CEO of Boom, didn't deny it, telling Forbes, "You're right. It has many purposes." It opens eyes to what's possible and what his team is capable of. Now, we miss Concorde, so we hope that Boom really can prove its critics wrong.