June 22, 2021

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Supersonic flights are about to return – here’s how they can succeed where Concorde failed

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(CNN) – United Airlines has announced that it will purchase up to 50 Boom Overture supersonic jets for commercial use by 2029, announcing the return of supersonic passenger flights nearly 20 years after Concorde’s decommissioning.

Supersonic planes cut the time it takes to fly from New York to London in half, from seven hours to 3.5 hours, but these airliners were abandoned after Concorde’s last flight in 2003. Concorde had become financially unworkable after a high profile accident in 2000, combined with excessive ticket prices, high fuel consumption and ever higher maintenance costs.

Whether Boom’s supersonic plane (pictured above) is successful will depend on overcoming these issues that derailed the Concorde. So can it be done?

Break the sound barrier

Supersonic flights are so called because they travel faster than the speed of sound. To do this, the aircraft must break through the sound barrier, which requires efficient aerodynamic design to reduce drag and considerable thrust from powerful engines to overcome the turbulence caused by the shock waves.

United Airlines will purchase 15 supersonic jets from Denver-based Boom Supersonic. With the proper safety, operational and sustainability requirements, United plans to make the jets fully operational by 2029.

Breaking the sound barrier also requires engines that consume a lot of jet fuel – a major drawback of the Concorde and something that has only become more controversial in recent years. You would then expect Boom, which is in the development stage of the Overture prototype, to focus its projects on increasing fuel efficiency.

The Colorado-based company is likely to choose between a turbojet engine and a turbofan. A turbojet produces all of its thrust from its exhaust gases when moving at higher speeds. A turbofan engine, meanwhile, derives most of its thrust from the amount of air it accelerates with its fan blades. The amount of this air defines the “bypass ratio” of the engine.

Turbofan engines with a higher bypass ratio are more fuel efficient than turbojet engines. Their slower exhaust rate makes them quieter, but they tend to be larger, resulting in more drag at supersonic speeds. This stamina penalty has exceeded the efficiency of turbofans for extended supersonic flight in the past.

A good compromise would be a low-bypass turbofan with an afterburner, which injects additional fuel to significantly increase available thrust and is commonly used on military jets. Such an engine was used in early production versions of another supersonic passenger aircraft, the Russian Tupelov Tu-144, but it was too inefficient because it needed to keep firing its afterburners to maintain supersonic cruise.

The Tu-144’s afterburner also helped create a very noisy cabin, humming loudly at 90 decibels – roughly the sound generated by a hair dryer – that exceeds regulatory safety limits. The Concorde’s turbojets, meanwhile, only needed afterburners on takeoff and to break through the sound barrier, improving fuel economy and lowering cabin noise during supercruising.

Supersonic jet noise

Due to the noise they generate, supersonic jets cannot fly over land. But these restrictions could be lifted with a refined aerodynamic design. For example, NASA’s research on its X-59 QueSST program hopes to produce optimized cell shapes that could significantly reduce ground-based sonic booms to a much quieter “thud” – coming in at 75 decibels instead of the 105 decibel boom. Concorde.

Getting the aerodynamics right could also open up the possibility of using modern, lightweight composites to allow for better thrust-to-weight ratios, perhaps eliminating the need for afterburners at takeoff.

Substantial developments in computational fluid dynamics software and other simulation programs since the 1970s will be crucial in evaluating these projects and certifying them under Boom’s tight production deadlines.

Sustainable aviation fuel

Boom is also promoting the green credentials of its aircraft. Part of the United agreement involves collaborative development to establish a reliable supply of sustainable aviation fuel. This will benefit the other aircraft in the United fleet and industry in general, which currently produces approximately 2.8% of all global CO₂ emissions from burning fossil fuels.

Sustainable aviation fuels include biofuels and synthetic kerosene which are manufactured using renewable and sustainable materials. An impressive 80% reduction in CO₂ emissions over the life cycle is often cited. The key word here, however, is “life cycle”; it does not necessarily mean less harmful emissions from the engine.

These sustainable fuels are compatible with conventional aviation fuel, which means that no changes to the airport refueling infrastructure or engine design will be required for their introduction, which is critical to their uptake. But these fuels are very expensive, because the raw materials needed to produce them are not available on a large scale. The total amount of sustainable aviation fuel currently in use amounts to only 0.1% of the total fuel spent in the air. Projections estimate that this must reach between 1.4% and 3.7% before such fuels become economically viable.

A return to supersonic flights?

Boom will be optimistic that he can overcome fuel efficiency challenges when his plane starts carrying paying passengers in 2029. These fares look set to be high, with Boom forecasting a price of £ 3,500 ($ 4,930) for place. In 1996, British Airways charged around £ 5,350 – £ 8,800 at today’s prices – for round-trip tickets from New York to London.

This means that, like the Concorde before it, the Boom Overture seems to be aimed at the luxury market, beyond the reach of even business class passengers. It is likely only frequented by those currently traveling by private jet, who may be lured by Boom’s claims of being a sustainable aircraft manufacturer.

So while supersonic passenger jets may return to our skies by the end of the decade, most of us will be closer to experiencing them when they unleash their signature sonic booms overhead.

Peter Thomas is Senior Lecturer in Aerospace Engineering at the University of Hertfordshire.

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