One way to increase the efficiency of a high-bypass jet engine is to turn the fan into an open rotor, a bit like returning to propellers, but using two rings of stubby counter-rotating blades.

ECONOMIST: A new way to build a greener jet engine

Both BA and KLM, after all, had direct experience of what flying through volcanic ash can do to a high-bypass turbofan jet engine.

ECONOMIST: The effect on business and leisure

However high-bypass engines are built in the future, at their core they will still have a basic jet engine.

ECONOMIST: A new way to build a greener jet engine