Developing hydrogen propulsion
One alternative fuel that has drawn significant attention is hydrogen, which produces no carbon emissions when it is burned.
Aircraft manufacturers are laying out plans for hydrogen-fueled aircraft to enter service in the next 10 to 20 years, and Raytheon Technologies is participating in U.S. Department of Energy research and development projects to test hydrogen as an effective zero-carbon option.
In one of those projects, the company will validate the use of hydrogen and hydrogen blends to operate Mitsubishi Power Aero’s FT4000, a gas turbine unit that is a land-based variant of Pratt & Whitney’s PW4000 turbofan aircraft engine.
That fuel test will complement a Pratt & Whitney-led Department of Energy project known as the Hydrogen Steam Injected Intercooled Turbine Engine, or HySIITE. That engine is expected to achieve zero in-flight CO2 emissions while reducing nitrogen oxide emissions by up to 80% and fuel consumption by up to 35%, mostly thorough its use of cryogenic hydrogen combustion and the recovery of water vapor.
The project's use of water vapor is particularly novel in aviation, Webb said. The engine would capture vapor and run it through a heat exchanger, using it to do things like create steam, generate electrical power, control the hydrogen flame and reduce its nitrogen oxide emissions. The technique itself "has been known for a number of years, but our people have figured out how to make a flying version of it," Webb said.
"This is our brainchild," he said. "We have a group of highly talented people who do nothing but think of how to do things more efficiently."
Separately, the company will work with the University of Connecticut School of Engineering on another Department of Energy project that focuses on the use of ammonia – which is composed mostly of hydrogen – as a zero-carbon fuel for power-generating turbines.
History shows hydrogen can be viable as fuel – Pratt & Whitney, for example, built an engine that ran on hydrogen in the 1950s. But there are obstacles to overcome, and they start with airframe design. Hydrogen's low energy density means it is commonly compressed for use as fuel and stored in large, heavy tanks – meaning it probably can't be stored in the wings, as jet fuel is.
While that’s a challenge, it wouldn’t be the first time a new type of engine drove design changes, Webb said. And with projects such as HySIITE, the efficiencies could offset any design penalties associated with hydrogen-powered engines and make the fuel more economically enticing for operators.
"We’ve always been a company where we first think: 'What's the right engine for the market?' and airplane makers will build their airframe around that," he said.
Making hydrogen work as aircraft fuel will require collaboration between airframers and propulsion providers, and, much like SAF, it will require new infrastructure efforts by energy companies and governments around the world.