Propfan

A propfan, also called an open rotor engine, open fan engine^[1][2] is an aircraft engine combining features of turbofans and turboprops. It uses advanced, curved propeller blades without a duct. Propfans aim to combine the speed capability of turbofans with the fuel efficiency of turboprops, especially at high subsonic speeds. It is sometimes called a "ultra-high-bypass (UHB) turbofan".