The four largest manufacturers of turbine aero engines for commercial aircraft all have launched significant projects to research and develop hybrid-electric propulsion systems able to power regional, single-aisle, and eventually even widebody airliners.
At the same time—and in some cases within the same research program—CFM International, GE Aviation, Pratt & Whitney/Pratt & Whitney Canada, and Rolls-Royce are working to improve turbine-engine efficiency significantly, substantially reducing CO2 and other emissions.
CFM International on the RISE
In its Revolutionary Innovation for Sustainable Engines (RISE) R&D program announced in June, CFM partners GE Aviation and Safran Aircraft Engines (SAE) are jointly developing technologies that would reduce fuel consumption and CO2 emissions by more than 20 percent compared with today’s engines. With RISE, CFM intends to mature a range of new technologies for engines that could enter service in the mid-2030s.
Central to its RISE research is the development of a new Open Fan architecture that builds upon, first, the original open-rotor research GE Aviation performed and flight-tested in 1988 in its Unducted Fan (UDF) project and the considerably more advanced—and vastly more environmentally quiet, compared to the UDF—Counter-Rotating Open Rotor (CROR) project SAE conducted successfully in 2017.
Travis Harper, GE Aviation’s program manager for RISE, said that—unlike SAE’s 2017 two-rotor-stage design—CFM’s Open Fan design contemplates only a single fan stage. Today, CFM envisions 12 fan blades for the Open Fan stage, aerodynamically three-dimensional in design and made of woven carbon-fiber composite material—a technology developed and perfected by SAE.
The blades will cover a much greater diameter than those in CFM’s latest Leap production turbofan engines. But because no fan case will contain the blades, the overall fan diameter could match the overall outer diameter of the Leap nacelle, according to Harper. With such large blades—and also making use of a smaller core than typically found in today’s engines—CFM expects its Open Fan R&D design to produce an eye-opening bypass ratio of 75:1, nearly an order of magnitude greater than the 11:1 achieved by the Leap-1A and other latest-generation turbofans.
The Open Fan will produce almost all its thrust as cold air accelerated to a relatively low speed close to the overall airspeed at which the aircraft itself is traveling, in order for the engine to achieve the highest-possible propulsive efficiency. Its improved propulsive efficiency will account for more than half of the overall fuel-efficiency improvement achieved by the design, according to Harper.
Because of their large diameter, the fan blades of the Open Fan development design will rotate relatively slowly so their tips don’t reach supersonic speeds. As a result, a reduction gearbox between the low-pressure spool and the fan drive shaft will reduce the drive shaft’s rotation rate greatly compared with that of the low-pressure turbine stages. Harper said the reduction ratio will far exceed that in Pratt & Whitney’s Geared Turbofan engines of today.
He says CFM used “the world’s most sophisticated super-computers” to achieve significant improvements in the Open Fan design’s acoustics and aerodynamics compared with today’s turbofans, without compromising function or weight. In addition to its increased propulsive efficiency, the Open Fan design will also feature greater thermal efficiencies than today’s engines.
CFM will achieve those efficiencies in part by using improved ceramic matrix composites (CMCs) and other advanced materials, such as new thermal barrier coatings. Additionally, however, “the Open Fan system does require a step-change in the amount of controls” for the engine, said Harper.
Software-driven, the controls ensure the Open Fan design operates at its optimal thermal efficiency during each phase of flight—takeoff, climb, cruise, etc.—and that engine performance always runs optimally to the design of the aircraft and its systems. The controls will also monitor engine health in real-time, allowing proactive resolution of issues and improving the engine’s utilization rate. Extensive use of software controls will also mean that, for the first time, CFM will be able to control the development design’s thrust using variables other than fan speed, which remains the only means of thrust control in today’s turbofans, according to Harper.
From the outset, CFM’s RISE program includes research into integrating the Open Fan design into a hybrid-electric system to optimize engine efficiency and to enable electrification of aircraft systems, as well as for the design to use 100 percent SAF and, potentially, hydrogen.
CFM has planned more than 300 different component and module tests across 150 test vehicles for its Open Fan R&D program. Rig tests will take place at sites throughout the world, including some module tests at SAE. Wind-tunnel testing of the fan and engine-level tests will happen at GE’s Peebles, Ohio, facility, and flight testing using GE’s Boeing 747-400 testbed will take place from Victorville in California. CFM expects to achieve ground- and flight-testing of the design in the mid-2020s.
GE Aviation Teams with NASA
Separately from its CFM RISE work, GE Aviation continues work under NASA’s Electric Powertrain Flight Demonstration (EPFD) program to develop, ground-test and flight-test by the mid-2020s a megawatt-class hybrid-electric propulsion system capable of being scaled to power regional, single-aisle, and possibly even larger airliners.
Without much fanfare, GE has worked with NASA on the EPFD program for at least the past five years, according to Christine Andrews, GE Aviation’s hybrid-electric systems manager. The partnership came about following more than five years of work GE did on its own accord to research and develop hybrid-electric propulsion technologies, “particularly at the component level,” she said. “I think probably what is most surprising to the industry is that GE has been working on this for as long as it has.”
GE’s previous EPFD work culminated in NASA awarding the bulk of a $260 million contract (program partner MagniX USA, which develops electric propulsion systems for aircraft, won a $74.3 million award) on September 30 to advance the EPFD program to flight-test within five years. NASA ultimately aims to introduce electrified aircraft propulsion (EAP) technologies to U.S. airline fleets by 2035.
Under its latest EPFD contract, GE will use a modified Saab 340 testbed to flight-test the combination of a GE CT7-9B turboshaft engine, battery power, motor generator, inverter, and converter, forming a system that “can support future architectures in conjunction with an airframer,” said Andrews. GE chose the CT7 as the ideal size for testing a system that designers can scale and modify to “support a variety of platforms.”
GE’s EPFD R&D work is not officially linked to its research within the CFM partnership to develop and mature new fuel-efficient, low-emissions technologies for future airliner engines under the RISE program. But Travis Harper, GE Aviation’s RISE program manager, said the EPFD contract work will provide “foundational technologies” that will “directly benefit the systems in the Open Fan” CFM is designing within RISE.
Pratt & Whitney Canada’s Hybrid Bid
In July, P&WC announced that, following a C$163 million investment by the governments of Canada and Québec, it would advance research with program partners Collins Aerospace and De Havilland Canada to develop a hybrid-electric propulsion system to flight demonstration. Over a 250 nm sector distance, the project aims to demonstrate a 30 percent reduction in fuel burn and CO2 emissions compared with a modern regional turboprop airliner such as the DHC Dash 8-100—one of which the partners will use as the flight-test demonstrator.
P&WC had already been working on the project—previously known as Project 804—with Collins since 2019. “Building on advances in conceptual and component design made during Project 804, we are continuing our collaboration with Collins, and, realizing the need to work closely with an airframe OEM to achieve a flight demonstrator, are working with De Havilland Canada to drive the technology towards its next phase of development and eventual demonstration,” said Jean Thomassin, executive director, new product and service introduction for P&WC.
Collins Aerospace will provide the 1-megawatt electric motor and motor controller. P&WC leads the development of the battery systems and is working with a number of collaborators on potential configurations. De Havilland Canada will support the integration of the new hybrid-electric propulsion technology and batteries within the Dash 8-100 airframe, including designing a modified nacelle to house the hybrid-electric system. It will also provide the cockpit interfaces needed to monitor and control the hybrid-electric technology.
The project will replace one of the Dash 8-100’s installed PW121A engines with a less-powerful turboshaft engine provided by P&WC, in combination with the 1-megawatt electric motor provided by Collins. Together the two motors will provide 2 megawatts of power, which is similar to the 2,000 shp total power output of the PW121. They will drive the propeller already installed on the Dash 8-100 demo aircraft. During takeoff and climb, the electric motor will provide a power boost, allowing a smaller fuel-burning engine to be optimally sized for the cruise phase, thereby creating fuel-efficiency benefits.
Meanwhile, P&WC parent Pratt & Whitney is working on two programs, one under a new NASA contract and the other in partnership with the FAA, to develop new fuel-efficient, emissions-reducing technologies for commercial aircraft.
As part of NASA’s Sustainable Flight National Partnership, NASA has awarded P&W a contract to develop advanced high-pressure turbine technologies for next-generation single-aisle aircraft through the Hybrid Thermally Efficient Core (HyTEC) project. HyTEC seeks to develop next-generation CMC materials that will operate at higher temperatures than current CMCs, environmental barrier coatings, and advanced cooling and aerodynamic approaches to enable new component designs and efficiencies.
P&W and the FAA have committed to investing a total of $50 million in R&D to develop an ultra-quiet engine fan and advanced combustion technology designed to reduce noise, emissions, and fuel consumption as part of the third phase of the FAA’s Continuous Lower Energy, Emissions and Noise (CLEEN III) initiative. Pratt & Whitney has served as an FAA partner since CLEEN began in 2010.
Rolls-Royce UltraFan Advancing
Rolls-Royce is now building the first demonstrator engine in Derby for its UltraFan advanced turbofan program and the demonstrator will run entirely by SAF for the first time next year. According to R-R, the UltraFan will offer a 25 percent fuel burn improvement over the first Trent; a 40 percent reduction in NOx; and a reduction of up to 35 percent in noise. It will produce at least 50 percent less non-volatile particulate matter at airports and almost zero nvPM at cruise.
The OEM has designed the UltraFan to produce from 25,000 to 100,000 lb of thrust. It features a new engine core architecture, advanced ceramic matrix composites, and a geared design. Earlier this year, the demonstrator’s power gearbox set a new world record at 64 megawatts while on test at the company’s facility in Dahlewitz, Germany.
“The success that we’ve seen with our power gearbox and other technologies that are going into UltraFan, give us great confidence going forward,” said Andy Geer, chief engineer for the UltraFan. “Our first run of the engine demonstrator will be a great moment—a brand-new engine design, running on 100 percent SAF from the outset.”
Rolls-Royce participates in electrified-propulsion work for projects such as small propeller aircraft, air taxis, commuter, and regional aircraft. Its planned electrical propulsion systems range from kilowatt power to megawatt power, the OEM often working with start-up companies to establish a strong presence in a rapidly evolving sector.
One example—Accel—seeks to set an airspeed record for an electrically powered aircraft. R-R developed a battery system with UK start-up Electroflight to serve as a potential power and propulsion system for pure and hybrid-electric aircraft designers.
The engine manufacturer is also involved in a wide range of inter-city and intra-city programs in the kilowatt class and hybrid-electric programs incorporating gas turbines. They include the 2.5-megawatt Power Generation System 1, now undergoing testing in Bristol. Capable of powering future hybrid-electrical regional aircraft, the PGS 1 is being run with a Rolls-Royce AE2100 turboprop engine—the engine type that powers the Lockheed C-130J. According to Rolls-Royce, the PGS 1 also would also power a more electric larger aircraft.
R-R also continues research on hydrogen-based propulsion for small-to-regional aircraft. It contends that, although hydrogen does not readily lend itself to application as a simple drop-in fuel, its viability more likely lies with its power density as a fuel cell instead of a battery or as a fuel combusted in a gas turbine—in which case the only emission would be water vapor.