Kerry Lynch stepped into the role of editor of the&nbsp;Aviation International News&nbsp;monthly magazine in May 2022 after serving with&nbsp;AIN&nbsp;for eight years. She joined&nbsp;AIN&nbsp;in December 2014 as a senior editor covering Washington and business aviation news. She has also served on Convention News teams, including as editor of&nbsp;HAI Convention News. Lynch previously spent 25 years with Aviation Week. During her time there, she reported for and later steered&nbsp;The Weekly of Business Aviation, oversaw business aviation content on the&nbsp;Aviation Week Intelligence Network, served on the&nbsp;Show News&nbsp;staff, and held roles as the Washington columnist for&nbsp;Business &amp; Commercial Aviation&nbsp;and&nbsp;Overhaul &amp; Maintenance&nbsp;monthly magazines. In 2021 she was honored with the National Business Aviation Association Platinum Wing Award for lifetime achievement in journalism and was the 2004 recipient of the National Air Transportation Association’s Aviation Journalism Excellence Award. She also has been a recipient of an Aerospace Media Award for business aviation (2022) and NBAA Gold Wing Award (2003, 2024). Lynch graduated from The Ohio State University with a bachelor of arts in journalism.

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Kerry Lynch

Pratt &amp; Whitney's research into its Hydrogen, Steam Injected, Inter-cooled Turbine Engine (HySIITE) architecture is demonstrating not only the viability of the technology but up to a 35% improvement in energy efficiency and 99.3% reduction in nitrogen oxides (NOx), company executives said. These results give promise of the feasibility of bringing liquid-hydrogen power to the commercial aviation market through this technology, the executives added.&nbsp;
While this new&nbsp;<a href="https://www.ainonline.com/aviation-news/futureflight/2025-01-01/fueling-future-engine-makers-bet-hydrogen-power">hydrogen propulsion technology</a> may not be ready to commercialize until about 2050, proving its feasibility is critical given the infrastructure investments that would be required to switch to liquid hydrogen, said Neil Terwilliger, technical fellow—advanced concepts for Pratt &amp; Whitney.
“We need to be clear-eyed about the risks of hydrogen, the challenges of hydrogen,” he told a briefing for journalists. “There are real timelines for infrastructure, technology, safety, and certification. We can't make this engine right now, and even if it were at the airports today, there is no hydrogen at the airports.”
All of this requires incentives to make those investments and the technology must prove its potential value, Terwilliger maintained. &nbsp;In addition, that value is important because of the weight penalty associated with carrying liquid hydrogen.
While its hydrogen research is not complete and technical challenges remain, the engine maker recently wrapped up a two and one-half year nearly project supported by the U.S. Department of Energy Advanced Research Projects Agency (ARPA-e) that explored the use of hydrogen in a Brayton (thermodynamic) cycle engine with steam injection to reduce NOx.
In a presentation for the Towards Sustainable Aviation Summit 2025 being held this week in Toulouse, Pratt &amp; Whitney executives said rig tests conducted at the RTX Technology Research Center in East Hartford, Connecticut either met or exceeded expectations for the program.&nbsp; A single-nozzle combustor right test with hydrogen fuel reduced NOx by 99.3%, which Pratt &amp; Whitney chief scientist Michael Winter noted essentially eliminates it. &nbsp;
Also in accordance with design requirements, the condenser demonstrated the ability to capture a gallon of water every three seconds, and the evaporator functioned in an exhaust gas path.&nbsp;
Ultimately, the project indicated a hydrogen-enabled engine with water recovery and steam injection could triple net energy savings when compared with more conventional sustainable aviation fuel produced using power-to-liquid technology, the company maintained.
<h2>Hydrogen Shows Plenty of Promise</h2>
The hydrogen project is part of a multi-faceted approach for Pratt &amp; Whitney toward a more sustainable future for air transport. The company has worked to produce efficiencies through advancements such as the GTF geared turbofan, sustainable aviation fuel, and extensive hybrid-electric propulsion work, explained Winter, but he added: “Hydrogen remains a very significant possibility, and we're really excited about hydrogen.”
However, challenges accompany hydrogen such as the fact that it takes up four times the volume so it needs to be stored as a cryogenic liquid at -253 °C. Further, hydrogen produces nitrogen oxides, which contribute to global warming, and it also releases more water vapor that can help form contrails, he conceded.
The HySIITE technology is designed to tackle those challenges, Winter explained. “We took all of these challenges, and we combined it into an opportunity.”
HySITTE uses the cold of hydrogen, condenses excess water, and injects it into the engine thermodynamics. “By doing that, we have demonstrated that we can then control the oxides of nitrogen, essentially driving them to almost zero.”
The U.S. Department of Energy project conducted over the past few years explored the “highest risk” items of the technology, he said.
Terwilliger stressed the DOE project was not just to see if they could burn hydrogen. “Engines are pretty agnostic; they'll burn most fuels—put in a specialized fuel nozzle, and it'll work.” But the project was designed to explore how to take advantage of hydrogen and how to design an engine differently if hydrogen was the fuel.
“We had this contract to demonstrate some of the key risk items, and we did that successfully,” Terwilliger said. “We don't just want to say there's a cool engine out there. We want to show if the engine can be this much better, what’s included in that number, what’s separate from the airplane penalty, and how should people think about a hydrogen-enabled engine as they think about whether or not it makes sense to convert aviation to hydrogen.”
<img alt="HySIITE technology diagram" data-entity-type="file" data-entity-uuid="a6f4c53d-c7dc-4de1-ad2a-dfde6c4e6af4" src="/sites/default/files/inline-images/HySIITE%20diagram%202.jpg" width="2082" height="735" loading="lazy">Terwilliger provided a glimpse behind the HySIITE technology:&nbsp; Cryogenic hydrogen will readily accept heat because it is so cold, so Pratt &amp; Whitney designed an engine that would inject heat from different sources into the hydrogen. These sources are heat that would otherwise be wasted and difficult to reject.&nbsp; In addition, the system is designed to recover water produced from hydrogen.
From a technology standpoint, HySIITE was designed to fit within a traditional nacelle and is a turbofan engine, at least up front, Terwilliger explained. It begins with a typical thrust-producing turbofan that is powered by an engine core and a power turbine—“just like normal,” he said. “That part is pretty traditional.”
Where the less traditional technology enters is in the exhaust of the power turbine.&nbsp; “It's really what happens in the exhaust of the power turbine that makes HySIITE unique.” &nbsp;HySIITE incorporates an evaporator, condenser, and water separator that uses the hot exhaust from the engine to create steam that is cycled back into the thermodynamic process, improving efficiency and reducing waste.
Engines typically have a hot exhaust that flows out of the back and goes into the atmosphere. “But there’s a lot of useful energy in there at a very high temperature,” he said. The evaporator uses that heat to boil water to create a steam-air mixture from the burning hydrogen. The condenser condenses the water from the steam, and the water separator removes the liquid water from the steam, which then goes back to the evaporator. The system provides for other cycle effects such as intercooling, he added.
Terwilliger noted one of the key issues with burning hydrogen is NOx. Hydrogen has a high flame temperature. But with HySIITE, Terwilliger said, “that high flame temperature is actually what lets us burn so much steam, burn air, and hydrogen.” The steam stems the NOx production.
<h2>Weakness Leads to Opportunity</h2>
“This is a rare situation in system design where some of the weaknesses end up enabling a new opportunity, and in the end, the NOx is gone,” Terwilliger said. “Also, because we're capturing water from the exhaust, we have the opportunity to reduce how much water actually leaves the engine.” That reduces the potential of contrails.
“Then you put it all together, and you get a very significant efficiency benefit compared to the current state of the art.”
Under the DOE ARPA-E program, researchers demonstrated the function of the combustor, evaporator, and condenser under simulated conditions.
While experienced with combustors, Pratt &amp; Whitney does not typically work with combustion with a mixture of equal parts air and water.&nbsp; “We wanted to prove that we could do hydrogen combustion in the presence of so much water with controlling the flame, measuring how much Nox we actually make, making sure it's stable. And we did that.”
As for the evaporator, “this is a heat exchanger with small tubes and that has to have high-pressure water and superheated steam on one side and sit in the exhaust of a jet engine on the other side,” Terwilliger said. “So as you can imagine, that's a pretty challenging environment. We wanted to prove that it lasts in that environment, that it can successfully evaporate the water and not fall apart.”
The condenser, meanwhile, is a heat changer cooling exhaust. “We wanted to show that the amount of pressure loss it takes to cool the exhaust is what we think it is, because if it's much more then the performance of the whole system may fall apart,” he said.
Encouraged by the results, Pratt &amp; Whitney will continue to look for opportunities to advance the technologies as with remains active in several other programs.
However, a key lesson is emerging from HySIITE: “Hydrogen engines, even without steam injection, can be more efficient than staff engines. But if you are willing to add steam injection, I think this is a unique opportunity,” Terwilliger said, because 35% more efficiency over the baseline—Pratt &amp; Whitney used its own GTF as the measuring stick—is a significant enough change to capture the required interest and potential investment.

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