An unducted fan or propfan's a modified turbofan engine, with the fan placed outside of the engine nacelle on the same axis as the compressor blades. Propfans are also known as ultra-high bypass (UHB) engines and, most recently, open rotor jet engines. The design's intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. The problem Turboprops have a fairly strict sweet spot at speeds below about 450 mph (700 km/h). The reason's that all propellers lose e… (More on Propfan)