How does a turbojet start

More efficient and more climate-friendly

The engines of the new supersonic aircraft have also made decisive progress: Instead of the Concorde's single-jet turbo jets, which were loud and required an afterburner for their top performance, there are now quieter and more efficient bypass engines. In these engines, also known as turbofans, a large part of the air flow is accelerated by a turbine, the "fan", and directed past the combustion chamber. At the rear end of the engine, this bypass flow emerges with the core flow and provides the thrust.

Quieter and more economical thanks to the turbofan

The advantage here: The sheath flow is less compressed and accelerated than the hot core flow from the combustion chamber and generates less turbulence when it exits the nozzle. That lowers the noise pollution. Because the efficiency of the turbofan engines is higher, these engines also save fuel and thus emissions. In the typical engines of modern passenger planes, the bypass flow therefore accounts for up to 90 percent of the total air flow.

However, to accelerate an aircraft up to supersonic speeds, the thrust of the common turbofan engines is not enough. They must therefore be optimized with additional turbines, compressors and more powerful combustion trains so that they cause less noise and emissions despite the higher proportion of core electricity.

The aircraft manufacturer Aerion Supersonic wants to use a special engine from General Electric for its supersonic aircraft. According to the manufacturer, its core should be based on a tried and tested jet plan, but thanks to adapted compressors and sheath flow areas, it should meet the strictest noise protection level 5 at startup and still achieve a speed of Mach 1.4 to 1.6. Competitor Boom Supersonic relies on Rolls-Royce engines, which are said to be particularly low-emission, for its "Overture" supersonic machine.

Fuel from air and sun

Both manufacturers are also striving to use more climate-friendly synthetic fuels instead of kerosene. Such hydrocarbon mixtures can be obtained by chemical processes from biomass, methanol or even gaseous hydrogen and carbon monoxide, the so-called syngas. Initial pilot tests are also running with sun-to-liquid technologies, in which sunlight and a catalyst are used to convert CO2 and water absorbed from the air into syngas.

For the fuels of its machines, Aerion cooperates with Carbon Engineering, a company that specializes in air-to-fuel technologies - the production of fuels from the carbon dioxide in the air. “We wanted an aircraft that does not depend on fossil fuels, but can run 100 percent on synthetic fuels from day one,” emphasizes Aerion CEO Tom Vice.

For the production of these fuels, Carbon Engineering wants to first filter and compress CO2 from the air by means of air capture. At the same time, electrolysis systems use electricity from the sun or wind to generate hydrogen by splitting water. In the third step, CO2 and hydrogen are then chemically combined to form high-energy organic compounds. “The fundamental benefit of fuels made from atmospheric CO2 is that they create an emissions cycle,” said Steve Oldham, CEO of Carbon Engineering. Because the CO2 emitted by the aircraft originally came from the air, the net emissions are close to zero.

Air-to-fuel technology is being tested in a Canadian pilot plant. © Carbin Engineering

Carbon fibers instead of aluminum

The particularly light structures of the supersonic aircraft also help to reduce emissions. While the Concorde mainly consisted of aluminum partially reinforced with titanium, new types of carbon composite materials are now available that make aircraft significantly lighter. Boom Supersonic wants to use a special 3D printing process to make titanium-reinforced components of its supersonic aircraft lighter through tiny ducts and at the same time to cool them with air flow.

However, all of this has its price. Despite optimized designs and better materials and engines, the Concorde successors will still be significantly more expensive to build and operate than classic traffic machines. Whether there will really be a renaissance in supersonic travel therefore also depends on whether enough people are willing to pay more for the luxury of fast flying ...

May 7, 2021

- Nadja Podbregar