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

Its Radmax engine uses two wave-shaped cams that rotate above and below a stationary rotor.

FORBES: How to get 51mpg from an engine the size of a large turkey.

It was three o'clock in the morning when Graham fired up the engine, and as the frost glistened on the young fruit, the giant rotor began to turn.

BBC: Graham Hewitt and his wind machine

It derives its name from the two rotating systems that are the basis of the design: the main engine at its base, which will put it into orbit, and the rotor on top, which will enable it to touch down gently.

ECONOMIST: On a rotor-blade and a prayer | The

There is another way to build a greener jet engine: by bypassing the turbine to an even greater extent, with an open rotor.

ECONOMIST: Monitor