What is very exciting about the open rotor fan is that it has had a long history and is still under development. The “IATA Aircraft Technology Roadmap to 2050” rates this technology as being about the TRL level 3 (demonstration and analysis of critical functions). While the open rotor idea has been about for some time, its development has been slow. This technology offers a considerable fuel efficiency benefit – in the order of 20%! This significant fuel efficiency benefit from the starting gate keeps the open rotor fan development going, despite its relatively lower TRL level. The consequence of all this is that this technology will be considered for Entry Into Service (EIS) by 2030[1].
Airbus and Boeing recently presented a joint vision for developing the open-rotor fan. These engines will be designed to be capable of using sustainable biofuels or even hydrogen. Both airframe manufacturers can envision how this engine will join their lineups in about a decade.
Figure 1: Safran counter-rotating Open Rotor Developed in Clean Sky
Figure 1 shows the unique design and implications of the open-rotor fan. The open rotor is a hybrid between a propeller and a turbofan engine, with the composition characterized by its two counter-rotating, unshrouded fans. Of course, a large exterior fan means more noise and less airspeed. On short flights, this would not be a concern. Current engine development is focused on resolving noise challenges.
The predecessor to the open rotor was the propfan concept outlined in 1975 and was a response to the 1973 oil crisis and consequent petroleum price spikes. Like many aerospace ideas, NASA supported this novel research.
Research into noise issues was considered over the subsequent decades. General methods for reducing noise involved lowering tip speeds and decreasing blade loading (the amount of thrust per unit of blade surface area). Since contra-rotating fans can be louder than turboprop or single-rotating propfans, other sound management methods were introduced[2]. One of these methods included increasing the gap between the propellers. Another considered keeping the rear propeller blade lengths shorter than the front propeller. In this way, the back propeller blades avoid cutting through the blade tip vortices of the front propeller (i.e. blade-vortex interaction). Another method was using different numbers of blades on the two propellers to avoid acoustic reinforcement. Lastly, one design method turned the front and rear propellers at different speeds to also prevent acoustic reinforcement.
Many of these ideas will need to be dusted off and recalibrated for the current designs of the open-rotor engine.
Figure 2: The CFM open-rotor jet engine – a front-facing design
Airbus and Boeing, in June 2022, proposed a joint vision for their approach to the open rotor fan. In this case, their engine developer is CFM, which is a joint venture between GE and Safran. Figure 2 shows the CFM concept for the open-rotor engine. The CFM open rotors are front-facing rather than rear-facing, as in the Clean Sky version.
Consistent with the aviation technology flow of ideas of the day, the open-rotor engine will be compatible with sustainable fuels and hydrogen. But as suggested earlier, this engine will not be ready for about a decade.
The Aviation sector is particularly challenged in addressing carbon emissions. While this sector only accounts for 2.5%[3] of global CO2 emissions, aviation travel will grow annually by over 4%[4], which is much larger than anticipated global economic growth. Through ideas such as the open-rotor engine, the aviation industry is demonstrating that it focuses on the big picture and the need to cut overall carbon emissions.
[1] https://www.airliners.net/forum/viewtopic.php?t=1461795
[2] Hager, Roy V.; Vrabel, Deborah (1988). Advanced turboprop project. NASA SP-495. Lewis Research Center, Cleveland, Ohio: National Aeronautics and Space Administration (NASA) Scientific and Technical Information Division. hdl:2060/19890003194. OCLC 17508419.
[3] https://ourworldindata.org/co2-emissions-from-aviation
[4] https://www.forbes.com/sites/oliverwyman/2022/02/24/why-aviation-will-grow-for-a-decade-but-there-are-headwinds-besides-covid-19/?sh=5a15dd5e3d10