Fanjets
Fanjets are similar in concept to a turboprop. Instead of powering a conventional propeller, they have a high-chord, low-span fan that is designed to operate at jet velocities. They promise much higher fuel efficiency than a turbofan, but high development cost, unit cost, noise problems, and relatively inexpensive fuel have kept the fanjet from entering production. If it did, it would fill a middle range in performance between low-speed, high-efficiency turboprops and high-speed, lower-efficiency turbofans.
Several concepts for propfans employ advanced technologies. One is the free power turbine, in which the turbines counter-rotate, acting as each other's nozzles. These in turn power counter-rotating props. Such a system is elegant on paper and should increase efficiency dramatically, but is difficult to implement.
T406 : Advanced
