Joe Wilding was the co-founder of Boom Supersonic, an independent company attempting to build a supersonic transport aircraft. They have already unveiled a small scale demonstrator, the XB-1 ‘Baby Boom’ (left). I met Joe in a field full of tanks in the middle of the English countryside, within minutes we were talking about supersonic inlet designs, here was a man I wanted to hear more from. I asked Joe to take us for a fast flight through 10 supersonic transport aircraft projects.
Here they are, in rough chronological order with a recurring crazy idea at the end.
We will start this list with the first and only successful supersonic transport in history. Concorde really was revolutionary, it was years ahead of its time. Its uniqueness begins with its origins. Being a joint venture between Britain and France, it was their flagship project during the (aero)space race of the 1960s. It’s remarkable the project went anywhere given that most previous Anglo-French joint ventures had been wars.
Many new technologies were developed and certified on Concorde. The engines were afterburning turbojets with the reliability and longevity to be usable in airline service. To this day the Olympus 593 is the largest aerospace jet engine core ever built. The engines were fed by two-dimensional, variable geometry inlets. These inlets slowed the incoming air to subsonic speeds with incredible efficiency. They also were stable under all conditions through a combination of an active and passive control system, which prevented potentially deadly engine “unstarts”. Concorde could supercruise without afterburners. This is a feat modern fighters can just barely pull off, and even that is for a brief duration.
The aircraft was the first to certify a full authority fly-by-wire flight control system. It also had a complete manual backup in case the new-technology system should fail. Although the aircraft had stability augmentation to ease pilot workload, it was statically stable and could be hand flown with the augmentation system disabled.
Concorde used fuel transfer to shift the aircraft center of gravity at supersonic speeds, thus reducing trim drag to near-zero.
Concorde was certified to fly up to 60,000 feet, where a rapid decompression would instantly kill all on board. To prevent this the aircraft contained very small passenger windows, which provided a survivable cabin environment with the complete failure of up to two of the cabin windows.
Heat dissipation is a big problem for supersonic aircraft. At Mach 2 (twice the speed of sound), the aircraft heat soaks around 100°C. This temperature rise is due to the compression of the air at high speed and exponentially increases at higher Mach number. At Mach 3the temperature increases to 275°C. Concorde mitigated this thermal load with a complex environmental control system that transferred excess heat to the jet fuel before it was burned in the engines. Even with this system, cabin windows were hot to the touch in flight.
It is most remarkable that all of this was accomplished before the age of digital computers. Concorde was completely designed using slide rules, drafting tables, and physical testing. Yet Concorde was certified and flew successfully for 27 years. Ticket prices were high, partially due to its fuel economy, but also due to maintenance costs and a limited fleet size. Despite this, Concorde was profitable on certain routes including New York to London.
2. Tupolev Tu-144 ‘Concordski‘
You can’t talk about Concorde without referencing the Soviet Tupolev Tu-144, which was given the belittling nickname ‘Concordski’. Many believe this aircraft was a reverse-engineered Concorde, copied from data obtained through industrial espionage. Though rather larger than Concorde, the general performance and configuration would seem to support this assumption. However, a lot of differences exist between the two aircraft, pointing to it being far from a carbon copy. The Tu-144 had a different wing planform and airfoils. It used a retractable canard for low speed control. The landing gear arrangement is significantly different, being housed within the engine nacelles. The cooling system, inlets, and engines also diverge from the Concorde design. The Tu-144 required afterburners for long range cruise, where Concorde used them only for takeoff and transonic acceleration. The final piece of data is that the original Tu-144 actually flew before Concorde did. Granted, it flew only a few months before, and much data still could have been obtained and used.
Ultimately the Tu-144 was not a successful aircraft in service. Its mission performance limited its range to under 2,500 nautical miles. This is sufficient for travel across the wide expanses of Russia, but insufficient for most trans-oceanic flights. Additionally the aircraft suffered from reliability issues and was used only briefly for passenger flights.
3. Early American Concepts
Seeing what Europe and the Soviets accomplished with supersonic transports, what was the American aerospace up to during this time? The answer is a lot. Sort of. An immense amount of funded research and proposed programs were launched in the US in the 1960s. Unfortunately, none of them ever turned into a flying prototype. American aerospace was anxious to develop a supersonic transport during this time, not wanting to be left behind in the modern airliner race. Unfortunately for these programmes, the US had another little project underway that severely limited the funds and talent that could be applied to supersonic aircraft design: the Apollo program and sending humans to the moon. That said, the US government did significantly fund a series of programmes through the 60s, 70s, and 80s in support of an American Supersonic Transport. Programmes such as Supersonic Commercial Aircraft Technology (SCAT), Supersonic Commercial Aircraft Research (SCAR), and High-Speed Civil Transport (HSCT), despite their questionable acronyms, did produce a lot of valuable research, wind tunnel testing, design tools, and even a prototype engine in the GE-4. A plethora of designs and configurations evolved during this period including swept wings, delta wings, swing wings, canards, tailless, and three-surface aircraft. Much of the research and evolution paralleled developments in the military supersonic realm with new findings being shared in both directions..
The most likely reason a flying prototype evaded these teams can be traced to over-ambition. Boeing, Lockheed, and North American were all working on credible designs that could have potentially been developed into a commercial product. However, most of these were designed for Mach 3, trans-Pacific range, and 300+ seats. Concorde was being developed in parallel, and its design goals were much more practical (Mach 2 and 100 seats). As the Concorde programme began to look more real, the American teams decided they needed to differentiate their potential products and thus completely eclipse Concorde upon entry into service. Unfortunately it ended up being an aircraft too extreme. Between the technical challenges of Mach 3, and the scale and cost of an aircraft with a weight approaching 1,000,000 pounds, there just wasn’t enough funding for a project of this scale. NASA (and some of the aerospace companies) still have teams with direct lineage to these decades-old programs working on the possibilities of a large future commercial supersonic transport. Only time will tell if there will ever be an appetite for one of these mammoth cruisers.
It should be noted that the Soviet Union also had larger conceptual programs in work during this same time. Most notable is the Tupolev Tu-244, a much larger and faster sibling of the Tu-144.
4. Supersonic Business Jets of the 1990s
The decade of the 1990’s brought a surge of companies working on supersonic business jets. The project scale and budgets of the previous decades influenced this new trend. Many started to look at an incremental approach to supersonic transport development as a viable path. While similar in mission, a business jet can be much smaller than a commercial supersonic transport. Five to ten seats is sufficient, and the small size of the passenger cabin is very attractive for an aircraft configuration layout that provides adequate space for other systems and minimizes drag through optimized shaping. The target market for a supersonic business jet is the ultra-wealthy, a customer that is not typically sensitive to aircraft purchase price and/or operating cost. Leading the charge in this category was Gulfstream. Their existing (and current) products are as close as you can currently come to an ultra-luxury, long-range, high-speed aircraft (which also comes with a very large price tag). Most of their development ideas and efforts have never been made public. But a glimpse of their concept can be seen in the proposed X-54 research aircraft. This aircraft never flew, but much effort went into its design, with a few details being published over the years. The aircraft was intended to be a “quiet boom” aircraft, with technology to attenuate the objectionable sonic boom that is produced when an aircraft flies overhead at supersonic speed. This boom follows the aircraft along its continuous flight path, and unfortunately high altitude does little to minimize its impact on the ground. Gulstream workedwith NASA to produce two technology demonstrations in support of the X-54 aircraft. These experiments (Quiet Spike and the Shaped Sonic Boom Demonstration) used fighter aircraft with altered external shapes to test boom mitigation ideas and get full-scale flight test data. Progress on the X-54 in recent years has faded with no further updates being issued by Gulfstream.
Another project started in the late 1990’s is the Supersonic Aerospace International (SAI) QSST. This company was started by the son of the Gulfstream founder, Allen Paulson . Any direct ties to Gulfstream were unclear. The QSST was also a business jet targeting the quiet boom market. Despite credible preliminary design effort and collaboration with the Lockheed Martin Skunk Works, a prototype never emerged.
A final 1990’s development project was the Sukhoi S-21 business jet. This project was originally a joint venture between Sukhoi and Gulfstream with the latter parting ways after funding proved to be elusive. Sukhoi continued the effort on their own. Few details have been published on this aircraft beyond the rough specification and a few models and renderings. The aircraft predated the quiet boom era and was intended for over-ocean supersonic travel.
A few other organizations also worked on supersonic business jet projects in this era including Tupolev, Dassault, and JAXA (the Japanese aerospace research agency). Designs are plentiful, prototypes are rare.
Aerion was formed in the early 2000’s as the next credible company in the supersonic business jet space. Aerion had a potentially winning formula of a marketable design specification, an impressive team of aerospace engineers/designers, and a wealthy founder willing to fully invest in the program.
The original wing design proposed by Aerion was quite unique and strived for low drag in several different ways. In a break from high-speed aircraft tradition, the wing had virtually no sweep. Most transonic and supersonic aircraft have considerable wing sweep, which accomplishes two things. First, the angle of the wing leading edge to the oncoming airflow reduces the strength of the shockwave created by the wing, and thus reduces drag from this source. Secondly, a swept wing has lower effective thickness relative to the airflow, which also reduces shockwave drag. Another advantage of a non-swept supersonic wing is a relatively short wing chord which minimizes skin friction drag. An unswept supersonic wing can work great, as long as the leading edge is relatively sharp, which prevents the formation of a large drag-inducing bow shockwave on the front of the wing. This type of wing was used with great success on the Lockheed F-104. That aircraft was a contemporary of the F-100 and F-102A. All three aircraft used a similar engine, yet the F-104 was roughly 50% faster at Mach 2.0. (To be fair, it was also smaller and lighter.)
A second wing innovation on the original Aerion was a laminar flow wing design. Laminar flow, contrasted to turbulent flow, is an elusive condition that is possible with the right wing shaping and attention to detail. All wings have a small amount of laminar flow at their leading edges. A laminar flow design extends this region to a majority of the wing surface and can reduce the drag produced by an incredible 25%-50%. Aerion did extensive research and wind tunnel testing of this idea in the early days. Unfortunately, very few aircraft have shown reliable capture of this effect in long-term service, mostly due to wing contamination and other real-world constraints.
The Aerion design went through several iterations over a time span of almost two decades. The final design showed a much more conventional swept wing planform, and references to laminar flow were dropped. Other changes included the number of engines, cruise Mach number, and sonic boom strength. In recent years, the company was advancing rapidly, including separate partnerships with Airbus, Lockheed Martin, and Boeing, and an engine deal announced with General Electric. Their executive leadership was also fortified with heavy hitters from the commercial aircraft world. Unfortunately the company shut down earlier in 2021 after failing to close a critical round of funding. It appeared they were on the cusp of building a production prototype, so this company closure was both a surprise and a blow to many in the world of supersonic development.
6. Boom Supersonic
Boom Supersonic launched their proposed product, Overture, in 2015. Overture is a unique development as it was the first project in several decades to solely target the commercial airliner market. Overture is a tri-jet design with a similar approach and configuration as Concorde. The company has gained a lot of traction since launch including raising several rounds of funding, signing an development agreement with Rolls-Royce engines and signing aircraft deals with several international airlines including Virgin Group, Japan Airlines and United Airlines.
The company has stated the desire to bring back the capabilities of Concorde, but with updated materials, engines, and aerodynamics. The goal is an aircraft that meets current takeoff noise standards and has operating economics comparable to business-class travel on existing subsonic airliners. Boom has decided to not pursue quiet boom technologies on their first product, and the company name is a (slightly provocative) nod to that decision. The aircraft is a little smaller and slower than Concorde, cruising at Mach 1.7 with a seating capacity of up to 88 seats.
Boom’s first prototype is a 1⁄3-scale technology demonstrator called XB-1. Although the project does not exactly represent the full-scale shaping of Overture, the project will demonstrate the aircraft efficiency and engine installations in a piloted, large-scale flying prototype. The company plans to use the aircraft as a testbed to further evaluate design ideas for systems and aerodynamic features and gather flight test data in real-world conditions. The demonstrator was unveiled last fall and first flight is slated for later this year.
7. Late 2010’s Supersonic Explosion
In addition to Aerion and Boom, the late 2010’s saw an increase of other emergent supersonic companies similar to the 1990’s. These companies are focusing on a mix of the business jet and commercial airliner markets.
Spike Aerospace launched the S-512 project soon after Boom emerged. The aircraft will have a quiet boom and moderate cabin size. The project started as a business jet, but Spike has promoted a small airliner version in recent years as that market has started to gain momentum. Like many early-stage supersonic projects, Spike is trying to fund the project and cross the transition from early conceptual studies to preliminary development and prototyping. Spike released a press release in 2018 claiming to have flown a remotely-piloted subsonic demonstrator, but further details or photos were not released. An interesting design detail on the S-512 is the lack of passenger windows. Spike plans to install large-area digital displays on the cabin walls and ceiling to give the illusion of looking outside the aircraft. If accepted by passengers, this could significantly reduce the weight and cost of the fuselage, while increasing the high-altitude reliability. The passenger reception to this idea is still to be determined. Another recent entrant is Exosonic. Their aircraft specification is similar to Boom in size and speed. Very few other details have been released. They are advertising sustainability and quiet boom.
A final recent project is the Virgin supersonic project. The idea of billionaire Richard Branson, the aircraft is being developed at his Virgin Galactic company, which is also developing a suborbital spaceplane for space tourism. Richard took a flight on this space vehicle a few months ago and the company plans high-frequency tourist flights in the near future. Virgin’s combined experience of designing and building high-Mach spaceships and operating subsonic airlines gives them a lot of advantage in this field of commercial supersonic development. Their conceptual design is slated to fly at Mach 3, and they have signed a development agreement with Rolls-Royce for engines.
Hermeus Aerospace is a recent supersonic company with a rather impressive design concept. With a top speed in excess of Mach 5, the aircraft is technically “hypersonic” rather than supersonic. Very few aircraft have been designed to fly this fast. The primary challenges above and beyond supersonic are the extreme airframe heating experienced at hypersonic speeds (>1000°C) and the complexity of the propulsion system. The Hermeus conceptual design looks more like a space plane than a traditional supersonic transport, and it should! The aircraft will likely have no windows (they would melt) and the plane will fly at altitudes approaching 100,000 feet, which is two and a half times higher than a subsonic airliner. The aircraft will be painted black, similar to the famous SR-71 spy plane. At hypersonic speeds and temperatures, the plane can effectively dissipate heat through blackbody radiation.
Hermes is currently focussing on their biggest developmental challenge: the propulsion system. They also recently were awarded a research contract from the US Air Force to develop an autonomous or remotely piloted subscale hypersonic demonstrator nicknamed the “Quarterhorse”. The development goals of this demonstrator will equally benefit both the commercial and military interest in this realm of flight. If successful, the Hermeus aircraft would provide trans-Pacific flights in as little as a few hours, compared to twelve hours or more today.
A second demonstrator (in addition to the Boom XB-1) currently in the fabrication stage is the NASA X-59 QueSST aircraft. This is not meant to be a commercial product, but is a technology demonstrator and research aircraft intending to move forward the state of the art in this category of aircraft. The demonstrator is primarily focused on quiet boom technologies and will serve as a testbed to produce sonic booms of variable strength. This can be used to perform community flyover studies and gather more data on the acceptable sound pressure levels for establishment of a sonic boom standard.
The aircraft is being designed and built by Lockheed Martin and will be operated by NASA. The aircraft is mostly built and first flight is planned for 2022
10. You can just convert a bomber to carry passengers. Right?
The final entrant in this top-ten list of commercial supersonic aircraft projects has to be the crazy idea to convert supersonic bombers to carry passengers. There were a multitude of bombers for which this was proposed, using both American and Soviet bombers. None of these ever went beyond the conceptual stage. Generally these were proposed for military transport purposes, but occasionally there were thoughts given to possible commercial viabilty. Generally a military bomber would not make a good supersonic transport for reasons including cost efficiency, takeoff noise, and certification standards, to name a few. The various bombers would not carry a high number of people without major modification. Bombs tend to be more dense than human cargo, and thus bomb bays are not conducive to a satisfactory passenger experience. So the craziness of this idea is grounded in a lack of practicality. It is fun to sketch though!
And don’t think this is just a crazy idea from the 1960s. Russia was proposing to convert old Tu-160s for passenger service as recently as 2020.
Joe Wilding has 25 years of aircraft development experience including business jets, military transports, small and large UAVs, light sport aircraft and sailplanes. Joe has BS and MS degrees in Aerospace Engineering from Wichita State University. He has worked on several programs from initial concept through regulatory certification. His responsibilities have included composites structural development, aircraft loads, flutter analysis, conceptual aircraft design, flight test engineering, and programme management. Joe has been a co-founder in four startup companies and he is currently splitting his time between engineering consulting, technical mentoring, and communications coaching for engineers.