Aviation

Concorde’s grandchildren

On 21 May 2003 Concorde made its final flight from New York to Paris in under four hours, at a speed of almost 2 500 km/h (1 553 mph). During its 27 years in the skies, this beautiful Franco-British bird failed to nest permanently in the long-distance aviation hypermarket, which has stayed resolutely subsonic. Will times change? The Lapcat project brings together the elite of Europe’s aviation engine specialists to design hydrogen jet engines which could propel regular commercial aircraft at speeds of over 5 000 km/h (3 107 mph).

Prototype of the A2 supersonic civil aircraft that could be in the skies by 2023. ©Reaction engine Prototype of the A2 supersonic civil aircraft that could be in the skies by 2023.
©Reaction engine
Two very different profiles: the Airbus A2 (140 metres) and the A380 (73 metres nose to tail). ©Reaction engine Two very different profiles: the Airbus A2 (140 metres) and the A380 (73 metres nose to tail).
©Reaction engine
Prototype drawing of the Scimitar propulsion unit. ©Reaction engine Prototype drawing of the Scimitar propulsion unit.
©Reaction engine

Concorde was born at a time when breaking the sound barrier – 1 224 km/h (761 mph) or Mach 1 – was a powerful symbol of modernity. Was Concorde simply too far ahead of its time? The plane was revolutionary for its day, but depended on the technologies available at the time. It was heavy, loud and drank kerosene like it was going out of fashion. The oil crises in the 1970s put paid to its ambitions. Airlines concluded that supersonic flight was unprofitable and aircraft manufacturers abandoned it. Subsonic Airbuses and Boeings (both flying at under 1 000 km/h / 621 mph) became the workhorses of the world’s skies. For the past thirty years, supersonic flight has been a strictly military affair. Fighter planes have climbed the “Mach ladder” from supersonic to hypersonic, the term used for speeds above Mach 5. NASA’s unmanned X-43A prototype set the absolute record in November 2004, hitting Mach 9.6 or 11 250 km/h (6 990 mph).

Hypersonic for all

Breaking the hypersonic barrier (6 000 km/h / 3 728 mph) for commercial air transport is now a very serious research field for European aircraft manufacturers . It may all still sound very science-fiction, but engineers believe it is time to start planning “Concorde’s grandchildren”. These aircraft will be designed to carry 300 passengers and a total load of 400 tonnes. Unlike rockets, they will take off horizontally with their human and other cargo and all the fuel needed for an “Antipodal express” flight, reaching cruising speed at an altitude of 20-30 km. Lapcat (Long Term Advanced Propulsion Concepts and Technologies) is the codename for this quantum leap. The project is coordinated by ESTEC, the engineering branch of the European Space Agency (ESA).

The aircraft materials and structures necessary to withstand the heat generated by air friction when flying at these speeds pose the first major restraint. Major technological progress has been made here, borrowing technologies developed for bringing spacecraft back into the earth’s atmosphere. But the main challenge that Lapcat’s 14 partners are trying to meet is the radical revolution in engine design which is needed to make commercial hypersonic flight feasible.

Brussels-Sydney in four hours

“We are working on liquid hydrogen engines that are totally different from the traditional kerosene turbo-jets,” project coordinator Johan Steeland (ESA) is keen to point out. “We want, for example, to get from Brussels to Sydney in four hours – with fuel savings that make the transport costs sufficiently attractive. In today’s increasingly globalised environment, potential demand for such hyperspeed intercontinental flight is spurring on aircraft manufacturers and commercial airlines. Many of them are involved in Lapcat.” Traditional jet engines are air-breathing. The thrust that propels the plane forward is produced at the rear end by expelling the gas created by combusting kerosene on contact with oxygen in the compressed air sucked into the engine through blade turbines.

These turbofans, which are fitted on almost all present-day subsonic commercial aircraft, can take a heavy airplane well beyond Mach 1 (as in the case of Concorde); but 3 000 km/h (1 864 mph) is the limit. Engine experts have therefore turned to the ramjet concept, a futuristic air-breathing propulsion system dreamed up by physicist-engineers as far back as 1912. The ramjet went through a number of experimental stages during the 20th century, but never reached production. Combustion takes place in a chamber with no moving parts. The aerodynamic shape of the air inlet itself establishes a stable compression level allowing the oxygen in the air to burn the fuel.

Ramjet and scramjet

Ramjet design is simple in principle. The amount of thrust it can deliver explains why, over the past two decades, research mechanics have focused increasingly on this technology, which offers the best performance for lifting an aircraft right into the hypersonic zone. One major operating handicap remains, however: it is only when the plane has reached several hundred km/h that the ramjet ignition can produce a real thrust. The new plane therefore needs dual power sources: turbo-jet (for take-off and initial flight), and then ramjet.

Once the ramjet engine reaches 6 000 km/h (3 728 mph) (Mach 5), another problem arises. At this speed, the pressure created by the incoming air produces heat levels that are incompatible with combustion stability and traditional materials. This has led to the more complex scramjet concept, in which specially developed cryogenic systems cool the air entering the combustion chamber. The US X-43A prototype uses this system and it is inspiring the Lapcat teams as well.

All hydrogen

Reaching hypersonic speeds means flying at very high altitudes of the order of at least 20 000 metres (65 617 feet), in strata where the reactor can still find enough oxygen to combust. At these heights, the fuel of choice for both the aerospace sector and aviation researchers is liquid hydrogen. It is by far the most energy-efficient, the lightest and, with no carbon emissions, causes the least damage to a fragile stratospheric environment. It is also the cryogenic source for cooling the reactor. The rocket-launching and space engine industry has long been developing very advanced high performance technologies for this ideal fuel. Transferring its use to commercial aircraft engines poses, however, major and as yet unresolved problems, one of the greatest being its high flammability Right now, a British company, Reaction Engines, one of the project partners, is developing the research prototype under the name of Scimitar. This engine combines turbo-jet functions for take-off and landing and for subsonic flight over inhabited areas with ramjet, once past the sound barrier.

The A2 – shape and concept

A powerful, reliable engine alone does not give you a good racing car. Project engineers are therefore also looking closely at the aerodynamics of this generation of hypersonic planes, bearing the modest title “A2”.

The aircraft itself would consist of a long, thin fuselage, almost 140 metres long (compared to the 73 metres of the Airbus A380) and 7.5 metres in diameter, with delta wings at the centre, each carrying two engines. The passenger cabin, above the wings, would be 32 metres long, with almost all the remaining fuselage taken up by the hydrogen tanks.

Brussels-Sydney in four hours? This feat of engineering prowess could be reality by 2023, at the cost of a present-day business class ticket.

Matthieu Lethé

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