When the engine’s operation stabilised during the recent test flight “it produced a faintly blue, transparent flame”, said lead author Wang Chao, in a paper published in the peer-reviewed Chinese-language Journal of Experiments in Fluid Mechanics.
Computer modelling in previous studies has shown that a rotating detonation engine can achieve fuel efficiency 50 per cent higher than that of a jet engine, where fuel burns under full control but much more slowly.
Many prototype detonation engines have been built in recent decades – mostly by the military – but for most the operation has been unstable.
The ones that have worked all used hydrogen as fuel. But to store liquid hydrogen, a large fuel tank with a complex cooling facility is needed, meaning it is not suitable for most military purposes.
Hydrocarbon fuel is cheaper and safer to handle, but its vapour is more difficult to produce and ignite. The challenge increases with speed, as the incoming air becomes so hot it can cause an early explosion that could choke the engine. For that reason, it is generally believed that a rotating detonation engine cannot operate at hypersonic velocity.
The team in Sichuan is being led by Le Jialing, lead scientist in China’s hypersonic weapons programme. Their new engine was fired up in a ground experiment that simulated a low-altitude flight at Mach 5.
The initial shock waves produced fast spins and triggered more explosions, some of which moved in the opposite direction to incoming air, according to the paper.
If they had reached the air inlet, the engine would not have been able to “breathe”, so the team added a device between the inlet and detonation chamber to try to stop this from happening.
They also found that the engine needed far more fresh air than for normal combustion to maintain a stable operation – with less air, it backfired and shot out a purple flame from the exhaust that was several metres long.