Pratt & Whitney engineers and scientists are leveraging technology developed in the 1950s to help them achieve modern hydrogen-fueled aircraft.

Known as Project Suntan, the secret programme carried out by the US Air Force in 1956-1958 saw tens of millions of dollars spent to determine the feasibility of an aircraft designed to be fuelled with liquid hydrogen.

The Pratt & Whitney division of United Aircraft Corporation was one of the engine designers chosen to explore the technology.

Fast forward to 2022 and the company is once again making strides to achieve hydrogen-fuelled flight.

Seventy years on, similar challenges

Francis Preli, vice president, propulsion and materials technologies at Pratt & Whitney, said some of the same challenges faced by those early pioneers still presented barriers today. But that has not stopped his team from making vital progress.

“Pratt & Whitney successfully built and tested hydrogen-fueled engines in the 1950s,” Preli told FINN.

“Back then, if you wanted to run a hydrogen engine, you actually had to build your own hydrogen production plant because there was no hydrogen available. The reason the programme didn’t go forward was because they couldn’t figure out how to get enough hydrogen on an aircraft. And that’s still an issue.”

Pratt & Whitney’s latest venture is the Hydrogen Steam Injected, Inter‐Cooled Turbine Engine (HySIITE) project.

This aims to use liquid hydrogen combustion and water vapour recovery to achieve zero in-flight CO2 emissions, while reducing nitrogen-oxide (NOx) emissions by up to 80 percent and reducing fuel consumption by up to 35 percent for next generation single-aisle aircraft.

US government backing

In February of this year, Pratt & Whitney was selected by the US Department of Energy (DoE) to work on the project as part of the Advanced Research Projects Agency-Energy (ARPA-E) OPEN 2021 programme.

“Our HySIITE programme is really leveraging the learning from way back in the 1950s to come up with a whole new architecture for a hydrogen engine,” Preli noted.

“The reason we think this is so important is, as you move to hydrogen, if you move to hydrogen, people will be really looking at the infrastructure costs, the aircraft costs, the engine costs, and to be able to take 35 per cent off of that total cost we think is a big deal.

“Especially if there’s a lot of SAF [sustainable aviation fuel], people could say, we’ll make more SAF, but if there’s another alternative that says, here’s something that’s cleaner, 35 per cent better, that might be more interesting.”

Making hydrogen work

Explaining the process, Preli said: “You have cryogenic liquid hydrogen, and there is a tremendous amount of energy that’s expended by the phase change. When that liquid turns into a gas you have a tremendous availability of high pressure hydrogen, and it’s cold. So what HySIITE does is that phase change but actually in the back end of the engine, which is what the 1950s Project Suntan did.

“What you can do is you can condense out a lot of the water … and then you can use that to cool and inject into the combustor, which improves thermodynamic efficiency. Because it’s cold, we’re going to operate the engine at a lower temperature, which means lower NOx formation; we’re predicting NOx formation in that engine is 80 per cent lower than today’s engines.”

Carbon benefits undone?

Challenges remain, including tackling contrail formation which Preli acknowledged could undo the benefits of reduced CO2 emissions. Other issues include how to make and transport the hydrogen, and ultimately how to fuel an aircraft with it.

Preli concluded: “We need advances all along the way. With cryogenic hydrogen, there is no containment system for it that’s perfect, so there’s always some bleed out. Are we really going to have a couple of 100 aeroplanes all bleeding hydrogen at the gates? How are you going to deal with that?

“And when you make the hydrogen and use the hydrogen, they need to be closely coupled in time, otherwise you’re going to lose all your hydrogen by just storing it. So there are a lot of challenges, but Pratt & Whitney are looking at the engine technologies.”
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