Supersonic Airliners on the Horizon

Seventeen years – that is how long commercial aviation has been stuck in the subsonic flight regime after British Airways ended Concorde flights in October 2003. In a world obsessed with speed and timesaving, that is remarkable. After all, the prospect of flying from London to New York in three and a quarter hours instead of the current seven hours or from San Francisco to Tokyo in five and a half hours instead of 11, would strongly appeal to most travellers.

That is why several companies are racing to get their supersonic jets to the market. These include Boom Supersonic’s Overture Mach 2.2 airliner, Aerion Supersonic’s AS2 Mach 1.4 Business Jet and Spike Aerospace whose S-512 Mach 1.6 aircraft has both Business Jet and commercial possibilities. And in August 2020, Virgin Galactic announced plans to collaborate with Rolls-Royce to build a Mach 3 airliner. In fact, although the aviation industry has been severely hit by the coronavirus pandemic, the nascent field of high-speed civil aviation is one area that is recording noteworthy progress. But can the supersonic hopefuls overcome the stiff challenges they face?

BAN THE BOOM

Concorde entered commercial service simultaneously with Air France and British Airways on January 21, 1976. However, it was soon slapped with stiff operational restrictions that forced it to stay subsonic overland. Many countries completely prohibit supersonic passenger overflights because of the booms they generate. But over the last decade or two, aerodynamic shaping technology has succeeded in making sonic booms more benign. The International Civil Aviation Organisation (ICAO) and the US Federal Aviation Administration (FAA) are reviewing the restrictions placed on civilian supersonic planes. In January this year, the FAA issued a “final rule” to facilitate the safe development of such aircraft. Once the review is completed, by 2025, some restrictions will probably be eased. The FAA’s decision will likely be based in part on NASA’s Mach 1.4 X-59 Quiet Supersonic Transport (QueSST) project. The Lockheed Martin X-59 aims to prove that it can operate generating only a 75 Perceived Level decibel (PLdB) “thump” on the ground – as loud as closing a car door. It has a long skinny fuselage so as to spread the shockwaves out and reduce their impact earthwards. It also has most of its external features above to make the shockwaves emanate upwards, not downwards. Further, an “aft deck” under the engine nozzle prevents shockwaves from the exhaust from travelling downwards. From 2022 onwards, the X-59 will undergo a series of supersonic flight tests to demonstrate low-boom technologies. Thereafter, community-response flight testing is scheduled over several US cities to evaluate the perceived noise generated by X-59 flights. Similar research is planned in Europe.

Environmentalists are already gearing up to fight supersonic airliners tooth and nail

Apart from meeting the latest noise standards, supersonic passenger jets will need to run entirely on Sustainable Alternative Fuel (SAF) if these are to have any hope of countering the objections raised by green activists. In fact, environmentalists are already gearing up to fight supersonic airliners tooth and nail, since such aircraft produce far more emissions than conventional aeroplanes. On their part, the manufacturers are planning to leverage offsets to claim that their offerings are 100 per cent sustainable.

AN OVERTURE IN MINIATURE

As yet, no full-scale supersonic flight prototype has been built. However, a significant step forward was taken by Boom Supersonic in October 2020 when it unveiled the prototype of a supersonic aircraft named XB-1. The XB-1 is a one-third scale model of Boom’s planned Overture supersonic passenger jet. Powered by three GE J85 engines, the 21m long demonstrator has a wingspan of 6.40m and will fly two test pilots at Mach 2.2 (about 2,700 kmph) over a distance of 1,900 km. Boom plans to conduct the world’s first 100 per cent carbon-neutral flight test programme on the aircraft. The XB-1 will undergo taxi tests in Colorado before being transported to Mojave, California, for flight tests later in 2021. The programme aims to quickly learn about cost-savings, safety, and efficiency of supersonic operations and also prove the aerodynamics of Boom’s Ogive delta wing and engine inlet design across the low-speed, transonic and high-speed regimes. It will also serve to validate production and commercial viability of the $200 million supersonic Overture.

Overture, for which Rolls-Royce is the chosen propulsion partner, has a planned first flight in 2026. It is 51.8m long and has a wingspan of 18.2m. It will carry between 65 and 85 passengers at Mach 2.2 when it enters service around 2030. Boom already has pre-orders for 30 jets from Japan Airlines and Virgin. In another sign of confidence, Boom has signed a contract with the US Air Force to design a future supersonic “Air Force One” – the jet that will fly the US President.

AERION’S AMBITIONS

Aerion Supersonic has long been the national leader in the civil supersonic sweepstakes. But its AS2 Business Jet is years behind schedule. The re-designed AS2 finally wrapped up low and high-speed wind tunnel testing in November 2020. Its first test flight is now planned for 2025 so as to reach the market by 2027. One hurdle that supersonic aspirants face is that no current engine can meet noise restrictions, emission limits and operational reliability over thousands of hours of high-speed flight. Aerion’s key advantage, therefore, is General Electric’s promise of the Affinity turbofan that should do just that. Affinity is a new engine with 20,000 pounds thrust and special acoustic linings to limit takeoff noise. It will help the AS2 meet Stage 5 Airplane Noise Standards, currently the most stringent landing and takeoff noise regulations. It will also make the AS2 the first supersonic aircraft to use only synthetic fuel.

The technological problems of supersonic airliners can probably be resolved fairly soon

Cruising at Mach 1.4 at 57,000 feet without fuel-guzzling afterburners, the AS2 will fly New York to London in just four hours. Aerion believes that its “boomless cruise” technology will make the sonic boom refract off a denser, lower layer of air, never reaching the ground. For this, real-time weather information will play a crucial role. However, Aerion still plans to fly the AS2 subsonic overland and supersonic over water at least initially, thus freeing it from dependence on lifting the current ban. It estimates a market for 300 supersonic Business Jets and already has a $3.5 billion order backlog under its belt. Long-term, Aerion sees the AS2 as a stepping stone to a larger AS3 airliner.

SPIKE AND VIRGIN

Spike Supersonic aims to tap both the business and commercial market segments. Its 18-seat Mach 1.6 S-512 jet will feature a low sonic boom over land and could be developed either as a Business Jet or commercially or both. Spike is building the S-512 with composite materials lighter than aluminium and capable of tolerating higher temperatures. It also plans to modify an existing engine to save time and costs. Whether or not it succeeds, remains to be seen. In a surprise announcement in August 2020, Virgin Galactic said that it will team up with Rolls-Royce to create a supersonic delta-wing commercial aircraft that will carry between nine and 19 passengers and reach speeds of Mach 3 above 60,000 feet. The project is still in the initial planning stage.

THE WINNER TAKES ALL

The technological problems of supersonic airliners can probably be resolved fairly soon. More worryingly, the aerospace industry is in dire straits due to the pandemic, with numerous airliner orders either delayed or cancelled. Yet, the supersonic aspirants feel that since time is a precious and irreplaceable commodity, there will always be takers for their jets. They believe the market will return to near-normal around 2025 and demand for supersonics will surge.

Other experts are not so sanguine. For instance, Richard Aboulafia, an aerospace analyst with the Teal Group, is quoted as saying, “Supersonic Business Jets remain an intriguing idea. Supersonic airliners remain unlikely.” He perhaps has based his opinion on the fact that supersonic flights would need to offer only first-class fares to be financially viable.

Although optimists identify up to 500 possible routes for supersonic flights (many limited to subsonic overland), the number of city pairs with sufficient travellers willing to pay first-class fares, is limited. There is also the problem of funding the huge development costs of supersonic airliners. It is by no means clear that there will be enough investors to get all these wannabes off the ground and into the market. More likely, there will be an early shakeout leaving only one or two strong players in the field. And everyone knows that whichever supersonic jet reaches the market first, will have a clear advantage over its competitors. The race has begun!