A typical flight from Brussels to Tokyo takes over 11 hours – but imagine if that time was shaved to just two and a quarter.
That is the sort of possibility offered by hypersonic jets, which travel at many times the speed of sound – and which researchers in Europe are trying to make a reality.
‘Getting in a couple of hours to the other side of the world is quite impressive and nearly unimaginable,’ said aerospace engineer Dr Johan Steelant of the European Space Agency in the Netherlands. ‘I’m still amazed that classical aeroplanes weighing 500 tonnes are able to hang in the air travelling at 800 to 900 kilometres per hour – but just imagine if we could crank this speed up to seven to eight times faster.’
The speed of sound – 1 200 kilometres per hour – has been broken by civilian aeroplanes before, albeit only by two models: the Anglo-French Concorde and the Soviet Union’s Tupolev Tu-144, both of which flew at about twice the speed of sound. Both are now retired.
But even those supersonic aircraft would be left well behind by the prototype being developed by Dr Steelant and colleagues. Known as HEXAFLY, it is expected to travel at seven or eight times the speed of sound.
Such speeds would not easily be reached. One problem will be generating enough thrust to overcome air beating past at over 8 000 kilometres per hour, and even then there are issues of stability and managing the thousand-degree temperatures generated by aerodynamic friction.
Fortunately Dr Steelant has had prior success: HEXAFLY’s precursor project, LAPCAT-II, saw the researchers test a 1.2-metre physical model in a wind tunnel at 7.4 times the speed of sound. In HEXAFLY, he and his colleagues want to test a 3-metre prototype at similar speeds in the open air.
The basic design of the aircraft is nearly complete, and they are now optimising its mass before the proposed launch in 2018 or 2019. This particular prototype will not generate its own thrust and will be launched from a rocket in order to test flight stability and other factors.
‘This test will demonstrate that we have mastered the different aspects of the design and the related technologies,’ said Dr Steelant.
One of those related technologies has been developed as part of a project that Dr Steelant also coordinated. Known as ATLLAS-II, it sought to create materials that could withstand the heat generated at hypersonic speeds.
The result of that project was a range of composites called ceramic matrixes. They are similar to the composite materials already used in aircraft, but are treated during manufacture so that the only materials left over are those such as carbon or aluminium oxides which can survive high temperatures, while still being sufficiently light to minimise fuel consumption.
‘Just imagine if we could crank speeds up to seven to eight times faster.’
Dr Johan Steelant, European Space Agency
Fuel itself is a consideration for hypersonic flight, as the kerosene used by conventional airliners is not only heavy but a potent source of greenhouse gas emissions. Liquid hydrogen, liquid methane and even liquid oxygen are prime alternatives, but have to be stored at cryogenic temperatures of -200 to -250 degrees Celsius.
Dr Martin Sippel, an aerospace engineer at the German Aerospace Centre (DLR) in Cologne, coordinated a project called CHATT to investigate fuel management on hypersonic aircraft. The project is now finished, but the researchers believe they got some way towards identifying the best approach.
One of the problems with cryogenic fuels is that they take up a large volume, yet tend to slosh around in large tanks which affects the stability of the plane. For this reason Dr Sippel and colleagues have performed a lot of computer modelling to work out how to reduce sloshing.
The researchers have also built several different demonstrator tanks to see which was best able to withstand cryogenic temperatures. A particularly promising one was made from a carbon-fibre thin-ply material, which, unlike other carbon-fibre designs, did not need a protective internal liner to prevent the propellants seeping through or reacting with the tank’s walls.
However, Dr Sippel says there is a still a lot left to do. ‘(We will only know that) all problems are solved when a hypersonic vehicle is in safe and reliable routine operation,’ he said.
The next step is to develop a cryogenic tank demonstrating thermal protection and ‘health monitoring’ – that is, monitoring of the technical system’s condition over many cycles of filling and depletion.
That would be yet another step towards civilians travelling at eight times the speed of sound. Dr Steelant believes commercial flights could become economically viable towards the middle of the century.
But eight times the speed of sound may just be the start. Some of the concepts investigated in CHATT would be suitable for planes such as DLR’s SpaceLiner, a rocket-propelled craft that is designed to travel at 20 times the speed of sound, making Brussels to Japan in about an hour.
‘I think it's about the dream of creating something new and making a difference to today’s conventional subsonic airliners,’ said Dr Sippel.
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