Engine makers are quick to note that their newest turbofans are significantly more efficient than those they replace.
Perhaps so, but the old adage “What have you done for me lately?” still rings true, evidenced by government and public demand for next generation airliners to deliver a revolutionary efficiency bump.
For an industry that historically achieves perhaps 15% better efficiency with each aircraft generation, “revolutionary” does not mean the next narrowbody jets, expected in the 2030s, will be buzzing around on battery power.
But the aerospace industry does have ambitious goals: for those aircraft to be 20-30% more efficient than today’s Airbus A320neos and Boeing 737 Max.
“I think we are on track to deliver more than 20% [additional] fuel efficiency. That is just the propulsion system. The aircraft would add on that,” says GE Aerospace general manager of advanced technologies Arjan Hegeman. “The remainder of this decade is going to be filled with demonstrators to ensure the maturity of all those technologies.”
NASA is backing industry efforts through various projects, including its Sustainable Flight National Partnership, under which it and Boeing are developing a truss-braced-wing demonstrator.
But to get close to a 30% CO2 reduction, engine makers like GE Aerospace and Pratt & Whitney (P&W) must deliver.
Hegeman says GE Aerospace and affiliate CFM International are pursuing a three-prong approach toward the 20% target: developing an open-fan engine, working to shrink engine cores and advancing the state of electric aircraft systems.
“There are 400 to 500… tests in our plan,” Hegeman says. “On any given day we have 20 or 30 of them running.”
Many of those tests involve the open-fan engine that CFM (jointly owned by GE Aerospace and Safran Aircraft Engines) is developing under its Revolutionary Innovation for Sustainable Engines (RISE) programme, launched in 2021.
Considering technological change comes slow as molasses in the aviation industry, open-fan engines are fairly radical. They are similar to traditional turbofans except in one glaring way: their fans are not enclosed in nacelles, but rather spin freely in the outside air.
Ditching nacelles means less weight and drag, providing an immediate efficiency bump, says Hegeman. But also, without bulky and restrictive nacelles, engineers can give open-fan engines wider fans (and hence greater bypass ratios), which improves efficiency – all without increasing overall width.
Such designs are not new. GE Aerospace test flew such an engine in the 1980s but shelved the effort as fuel prices declined. Open fans have also been stymied by their propensity to be screaming loud and due to the risk that, because they lack containment rings, engine failures could fire components into passenger cabins.
But Hegeman says GE Aerospace and its partners have solutions.
“Our computing capability has massively increased [to] where we can now be so exact and precise in our analytic predictions,” he says. “We have been able to optimise those airfoil shapes to get sound levels below today’s narrowbody engines”.
He says armoured aircraft fuselages can mitigate failure risks and that composite fan blades are exceptionally durable.
CFM aims this decade to begin flight testing its open-rotor demonstrator with support from Airbus, using an A380.
GE Aerospace and P&W, a Raytheon Technologies subsidiary, are also each working to squeeze more power from smaller turbofan cores. They both won contracts under NASA’s Hybrid Thermally Efficient Core (HyTEC) effort to develop a smaller engine with a 15:1 bypass ratio that burns 5-10% less fuel that today’s turbofans. It also seeks to extract 10-20% of the engine’s power as electricity – enabling expansion of electric systems.
NASA hopes to begin HyTEC ground demonstrations around 2026 and to have such technologies ready for prime time in the 2030s.
The companies are also creating hybrid-electric propulsion systems with megawatt-class electric motors, which could potentially be fitted to regional turboprop aircraft or mated to turbofans on larger jets.
Raytheon subsidiary Collins Aerospace has been developing a megawatt-class hybrid-electric system for testing in partnership with Pratt & Whitney Canada and De Havilland Aircraft.
That effort involves fitting the system on a Dash 8 turboprop. P&W is also working under a European Union project to develop a hybrid-electric modification of its geared turbofan.
GE Aerospace, meanwhile, is conducting its hybrid-electric work partly through NASA’s Electric Powertrain Flight Demonstration (EPFD) programme, which the agency hopes will culminate in new electric systems entering service no later than 2035.
Progress is being made, with GE Aerospace having tested its megawatt-class hybrid-electric system at simulated altitudes up to 45,000ft. The company expects in the mid-2020s to begin ground and flight tests using a Saab 340B turboprop.
Additionally, GE Aerospace in May disclosed plans to invest $20 million to expand its hybrid-electric testing capabilities.
Hegeman stresses that technologies under development by his team would be capable with engines burning either hydrogen or sustainable aviation fuel (SAF) – both of which have garnered recent hype as potential solutions to aviation’s emissions problems.
“To top it all off, all this is agnostic to the energy source, whether hydrogen or SAF,” he says.
The aviation industry has been leaning heaviest on SAF, which is typically a biofuel, as its saviour. That, despite SAF remaining prohibitively expensive, largely unavailable due to meagre production volumes, and having debatable environmental benefits.
The viability of hydrogen also remains unclear. Though Airbus has pitched development of hydrogen airliners entering service in the 2030s, Boeing has held back.
Hegeman notes that hydrogen propulsion is less advanced than other efficiency reducing technologies.
“We are not that far along on [the hydrogen] journey… Hydrogen is the least mature and has the longest way to go,” he says.
Story updated on 19 June to specify in the second-to-last paragraph that Hegeman thinks hydrogen propulsion less advanced than other technologies.