Chinese scientists say a cheap, lightweight foam could improve hypersonic weapons
- After testing carbon foam on a prototype vehicle in a wind tunnel, experts say it ‘has great application potential’ as a coating material
- Team finds it could reduce impact of shock waves by more than 20 per cent and dramatically improve aerodynamic stability
The team at the China Aerodynamics Research and Development Centre said their testing – carried out in a wind tunnel – suggested the material could reduce the impact of shock waves by more than 20 per cent.
And they said it could improve a weapon’s aerodynamic stability by up to two orders of magnitude at the hypersonic speed of Mach 6.
China’s existing hypersonic weapons can travel at five times the speed of sound, Mach 5, or faster. The smooth surface of their hulls – made from super-hard materials such as titanium alloy and carbon fibre – is designed to reduce drag. But the shock waves that can be generated at hypersonic speeds could jeopardise a flight.
Hypersonic weapons have sophisticated flight control mechanisms to deal with shock waves and maintain stability, but researchers are looking for simpler solutions.
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The team at the aerodynamics centre in Mianyang, Sichuan province, believe they may have found an answer.
Carbon foam “has great application potential” as a coating material for future hypersonic flight, said Tu Guohua, lead scientist of the study, in a paper published in the peer-reviewed Journal of Beijing University of Aeronautics and Astronautics on Tuesday.
Carbon foam can be made from many things including coal, natural gas and even wood. It is a lightweight, stiff material with a rough surface and irregular pores formed by a web of carbon atoms.
Those tiny, air-breathing holes could prevent or slow down harmful turbulence in a hypersonic air stream, according to Tu’s team.
They noted that coating a high-speed aircraft with a rough material is not a new idea, and was first proposed by Russian physicist Alexander Fedorov at the Moscow Institute of Physics and Technology more than a decade ago.
Researchers around the world have sought to put the idea to the test by creating holes or cracks in tough materials like metals, ceramics and carbon fibre, but they encountered challenges in real-life applications.
Carbon foam is widely used to insulate aircraft and rockets. Until now, no one had considered using this cheap material to coat a hypersonic weapon.
Tu’s team started by looking at how the foam absorbs sound waves, collecting a lot of raw data for a computer to predict its aerodynamic performance at various speeds.
Its rough surface was less efficient at lower speeds, but in hypervelocity most of the drag came from shock waves, according to the computer modelling. It suggested that carbon foam could have an advantage.
The scientists tested the theory on a prototype hypersonic vehicle at a wind tunnel in Wuhan, Hubei province. The foam was wrapped around an area where shock waves would have the biggest impact.
They found that its spongy surface significantly reduced the energy density of the shock waves by stretching them and increasing their contact with the hypersonic vehicle, according to the paper.
Recent studies have suggested that carbon foam has other advantages when used on a weapon. For example, its porous structure is said to be highly efficient at absorbing electromagnetic waves, meaning it could potentially be used to help evade radar detection and tracking.
It could also be strengthened and its other physical properties adjusted in the production process or by adding extra elements.
But there are concerns that carbon foam could burn when exposed to oxygen in high temperatures. Researchers at the China Aerospace Science and Technology Corporation last year said this problem could be avoided by using a super-strong carbon foam.
They claimed theirs could survive at temperatures up to 3,000 degrees Celsius (5,430 Fahrenheit) – similar to that reached on the surface of a hypersonic weapon.
