Boom’s new supersonic jets want to succeed where the Concorde failed

(CNN) – United Airlines announced that it will purchase up to 50 supersonic Boom Overture aircraft for commercial use in 2029, heralding the return of supersonic passenger flights nearly 20 years after the Concorde was withdrawn from service.

Supersonic aircraft cut the time it takes to fly from New York to London in half, from seven hours to 3.5 hours, but these types of aircraft were abandoned after the last Concorde flight in 2003. The Concorde became unviable financially after a notorious accident in 2000, combined with excessive ticket prices, high fuel consumption and increasingly high maintenance costs.

In order for Boom’s supersonic plane to succeed, it must overcome these problems that derailed the Concorde. It’s possible?

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

Breaking the sound barrier also requires fuel-efficient engines, one of the Concorde’s main drawbacks and a fact that has become more contentious in recent years. So it is hoped that Boom, which is in the prototype phase of the Overture, will focus its designs on increasing fuel efficiency.

The problem of engines

The Colorado-based company will likely have a choice between a turbojet engine or a turbofan. A turbojet produces all its thrust from its exhaust gases when it moves at a higher speed. A turbofan engine, meanwhile, gets most of its thrust from the amount of air it accelerates with its 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 turbojets. Their slower exhaust velocity makes them quieter, but they tend to be larger, which means greater resistance at supersonic speeds. In the past, this drag penalty has outpaced the efficiency of turbofans for long supersonic flights.

A middle ground could be to opt for a low “bypass ratio” turbofan with afterburning, a system that injects additional fuel to significantly increase available thrust, and is commonly used in military aircraft. This type of engine was used in early production versions of another supersonic passenger jet, the Russian Tupelov Tu-144, but it was too inefficient because it needed to keep firing its afterburners to keep the supersonic cruiser going.

The Tu-144’s afterburning also contributed to the cabin being very noisy, with a 90-decibel hum – about the sound generated by a hair dryer – that exceeds regulatory safety limits. The Concorde turbojets, meanwhile, only needed afterburning at takeoff and to overcome the sound barrier, improving fuel economy and reducing cabin noise at supersonic cruising speed.

An old problem: the noise of supersonic planes

Due to the noise they generate, supersonic jets are not allowed to fly over the ground. But these restrictions could be removed with a more refined aerodynamic design. For example, NASA research in its X-59 QueSST program hopes to develop optimized airframe formats that could significantly reduce “sonic booms” on the ground to a much quieter “thud”, with 75 decibels instead of 105. produced by the Concorde.

If proper aerodynamics are achieved, modern, lightweight composites could also be used to improve thrust-to-weight ratio, which could eliminate the need for afterburners on take-off.

Substantial advances in fluid dynamics computing and simulation programs since the 1970s will be critical in evaluating these designs and getting them certified for Boom’s tight production schedules.

Sustainable aviation fuel

Boom also promotes the green credentials of its aircraft. Part of the agreement with United involves collaborative development to establish a reliable supply of sustainable aviation fuel. This will ultimately benefit other aircraft in United’s fleet and the industry as a whole, which currently produces around 2.8% of all global carbon dioxide emissions from fossil fuel combustion.

Sustainable aviation fuels include biofuels and synthetic kerosene that are made from renewable and sustainable materials. An impressive 80% reduction in life cycle carbon dioxide emissions is often cited. However, the key word is “life cycle”; it does not necessarily mean that there are fewer harmful emissions from the engine.

These sustainable fuels are compatible with conventional jet fuel, which means that there will be no need to change airport fueling infrastructure or engine design to introduce them, a critical factor for their acceptance. But these fuels are very expensive because the raw materials needed to make them are not available on a large scale. The total amount of sustainable aviation fuel used today amounts to only 0.1% of the total fuel consumed in the air. Projections estimate that it must reach between 1.4% and 3.7% before these fuels are economically viable.

The return of supersonic flights?

Boom is confident that it can overcome fuel efficiency issues by the time its plane begins carrying passengers in 2029. Fares will be steep, as Boom expects a price tag of 3,500 pounds (US $ 4,930) per seat. In 1996, British Airways charged about 5,350 pounds (8,800 pounds in current prices) for round-trip tickets from New York to London.

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

So even if supersonic airliners return to our skies by the end of the decade, the closest we’ll get to experiencing them will be when they unleash their signature “sonic blasts” overhead.

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

Republished under a Creative Commons license from The Conversation.