There’s no disguising the fact that business aviation is among the most challenging industry sector when it comes to sustainability.
Yet, in an ironic twist, two of the leading business jet engines—GE Aerospace’s Passport and Rolls-Royce’s Pearl—are in the forefront of researching the use of hydrogen as a potential zero emissions fuel for the civil aviation industry.
GE’s Passport is being modified to run on direct combustion of hydrogen for flight tests on an Airbus A380 as part of the airframe manufacturer’s ZeroE program. Rolls-Royce meanwhile plans to run ground tests of a Pearl business jet engine with gaseous and liquid hydrogen following tests of a modified annular combustor from the Pearl 700 engine variant with 100% hydrogen at conditions representative of maximum takeoff thrust.
Yet, despite these pioneering test roles, the use of hydrogen as a fuel in business aviation is a long way off. Instead, both the Passport and Pearl are being used as testbeds because of their modern design, compact architecture and high performance.
“When you look at the way the Passport engine is mounted to a Bombardier Global 7500, it was an ideal candidate for the pylon they want to put it on the A380,” says Melvyn Heard, president of the GE Honda Aero Engines business and Passport Engine Programs at GE. “So, combining that from an integration on the aircraft standpoint and having a next generation type of turbomachinery engine was a great candidate for them [Airbus] to use this as the platform to go test out the hydrogen program.”
In preparations for the A380 test campaign, researchers from the German Aerospace Center (DLR) and GE Aerospace Advanced Technology have begun ground rig tests of 100% hydrogen combustion under flight representative conditions. The tests are being conducted at the DLR Institute of Propulsion Technology in Cologne, Germany, as part of the European Clean Aviation’s three-year, €116 million ($123 million) HYDEA (hydrogen demonstrator for aviation) project led by Italy’s GE Avio Aero.
HYDEA aims to develop a zero-emission direct hydrogen combustion engine by 2026 and to demonstrate its feasibility in a series of integrated ground tests. GE will ground test the modified hydrogen-burning Passport at its Peebles, Ohio, site prior to its installation on the A380 in the 2027–2028-time frame. The ZEROe flight demonstrator is intended to pave the way for development of the world’s first all-new hydrogen-powered commercial aircraft by 2035.
Testing on the Passport does not mean, however, that GE is “pushing for an engine that will burn hydrogen on a Global 7500,” Heard says. “But we are making sure that we have the right technology in place that we could have an engine out there in the 2035 time frame that could actually burn sustainable hydrogen fuel. It is all getting us ready for the next decade for what technologies may be available, especially when we combine what we’re doing with the ZEROe demonstrator with a suite of technologies that’s going into the RISE engine,” he adds, referring to the CFM open fan engine in development with CFM partner Safran.
The Rolls-Royce Pearl is also the focus of on-going hydrogen combustor tests currently underway at DLR’s Cologne facility, where specially developed spray nozzles where recently evaluated for the first time at high-pressure, high-temperature full-thrust conditions. Developed for hydrogen use by the UK National Center for Combustion and Aerothermal Technology (NCCAT) at Loughborough University, the individual nozzles were initially tested at intermediate pressure at Loughborough’s recently upgraded test facilities and at DLR before the final full-pressure combustor tests took place at Cologne.
‘Art Of The Possible’
The tests form part of Rolls-Royce’s broader test focus on hydrogen fuel for future use either for direct combustion in modified zero carbon turbofans or as part of various hybrid-electric propulsion efforts including fuel cell-augmented gas turbines. However, because hydrogen burns far hotter and more rapidly than kerosene its use in direct combustion engines is particularly challenging. Rolls says the nozzles tested at DLR were able to control the flame position using a new system that progressively mixes air with hydrogen to manage the fuel’s reactivity.
“We are trying to tease what is the art of the possible,” says Dirk Geisinger, Director Business Aviation, Rolls-Royce. “It’s a multi-fold test process—one is you are burning hydrogen in a gas turbine, and we started doing that already. Then we are moving on to burn liquid hydrogen. It’s a bit more aggressive because you must take care to control the flame because it burns at a very high temperature and requires lots of alterations to the nozzle. So, we did that, and it was very successful,” he says.
The build up to the full annular rig tests at takeoff conditions follows a long series of atmospheric, intermediate pressure and sub-atmospheric pressure testing at NCCAT and continued testing at facilities at Rolls-Royce Germany and DLR. Results from the rig tests will feed several key new UK national and European fundamental research programs including three initiatives awarded by the UK Aerospace Technology Institute (ATI)—HYEST, LH2GT and Rachel, as well as the Clean Aviation Cavendish program.
HYEST (Hydrogen Engine System Technologies) is a £14.8 million ($18.1 million) Rolls-Royce led project to develop technologies and the sub-system architecture for the combustor element of a liquid hydrogen gas turbine. RACHEL (Robustly Achievable Combustion of Hydrogen Engine Layout), also led by Rolls-Royce, is a £36.6 million project aimed at advanced hydrogen-related technology for the nacelle, engine externals and the powerplant itself, and looks at the fully integrated propulsion system as well as hydrogen storage. The third ATI effort, LH2GT (Liquid Hydrogen Gas Turbine) is a £31.4 million project to develop technologies for the delivery of a liquid hydrogen fuel system for a hydrogen gas turbine.
Under Europe’s Clean Aviation aeronautics research program, Rolls-Royce Germany is leading the €29.2 million Cavendish project targeted at integrating lean-burn hydrogen combustion into a Pearl 15 donor engine for ground testing on liquid hydrogen starting in late 2024. A second objective of Cavendish is to work on aircraft integration and formulate a route to a flight demonstration with Dassault in the second phase of Clean Aviation.
But overall, the prospects for hydrogen-fueled business aircraft remain slim—at least in the short- to medium-term for longer range missions Geisinger says. “If you look at business aviation obviously, we are not seeing hydrogen as a real possibility for long range. The engine is not the issue, it’s the aircraft infrastructure and how to deal with the high pressure, low temperature and volume of the big tanks. But on short-haul missions, absolutely, there’s an opportunity in the regional market and so on. So, we obviously want to be ahead of this, and if something develops, we want to understand what’s going on,” he adds.
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