Open rotor

Open rotor jet engine designs could significantly increase jet engine efficiency (by as much as 28% on aircraft of size class 2-3). The concept has been under development for decades but major barriers remain, including noise generation (however the technology would still comply with noise standards) and safety. Moreover, this technology has an optimum speed of 0.75 Mach, which is lower than the common 0.85 Mach. This implies that the technology is impractical for size class 4-5 with long-distance operations (for instance, a previously 10-hour flight would take over 12 hours, preventing airlines operating one return flight per day. Additionally, the large diameter of open rotor engines makes them incompatible with current aircraft design (the engine could not be mounted on the wing, but would have to be installed at the rear of the aircraft). The efficiency improvement potential of open rotors is similar to ultra-high bypass ratio engines, yet the latter seems to be more compatible with current aircraft design.