Published by Todd Bush on February 3, 2025
Air enters a small reverse-flow core at the rear of the engine, powering the fan and then exiting via an evaporator, condenser and water separator.
Hydrogen fuel may offer attractive pathways toward the goal of zero carbon emissions, but turning that vision into a practical propulsion system is another matter.
Now Pratt & Whitney thinks it may have taken the first steps along that path with the Hydrogen Steam-Injected, Intercooled Turbine Engine (HySIITE), a novel hybrid engine configuration that combines the advantages of the fuel’s cryogenic properties with the thermodynamic benefits of steam injection.Pratt & Whitney has unveiled details of the concept, which has been studied under a two-year $3.8 million U.S. Energy Department Advanced Research Projects Agency–Energy (ARPA-E) effort. While Pratt acknowledges the cycle is complex and requires more study, it is encouraged by the results, which show potential for as much as 35% lower energy use compared with current state-of-the-art engines.
Steam injection is key to increased performance
The concept could cut NOx emissions more than 99%
The work could pave the way for a range of new powerplants, Pratt says, including radically more efficient commercial geared turbofans with zero carbon emissions. The studies also indicate the concept could minimize emissions of nitrogen oxides (NOx), a greenhouse gas produced as a byproduct of hydrogen’s higher combustion temperatures.
HySIITE, which ended in December, was focused on notional component and system design along with feasibility tests of some critical high-risk elements. These included burning hydrogen in a heavy steam-air mix, evaluations of a practical evaporator design, and tests to see how much water could be produced by a condenser.
“There’s certainly far more technical challenges yet to go, but those were the ones [through which] we could get quick answers on key components to find out if it’s worth continuing,” said Neil Terwilliger, technical fellow for advanced concepts at Pratt & Whitney.
“HySIITE is about us imagining if there were going to be hydrogen and that it was a viable decarbonization pathway,” Terwilliger told Aviation Week on the sidelines at the American Institute of Aeronautics and Astronautics SciTech Forum in Orlando, Florida, on Jan. 9. “What kind of engine would take the best advantage of it? Should it look like engines do today, or should we do something different?”
“We broke down that question to ask, ‘What are the elements of hydrogen that we might take advantage of?’” Terwilliger continued. “We’ve got heat recovery. We’ve got superconducting power electronics with the cryogenic temperatures, and we’ve got the fact that it produces a lot more water vapor when it burns, which is a somewhat obscure factoid, but we can use that. It ends up being the thing that really enables us to catch water from the back of the engine.”
HySIITE single-nozzle combustor rig tests were carried out at the RTX Technology Research Center. Credit: Pratt & Whitney
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On the outside, the HySIITE concept notionally resembles a conventional engine, but the inside is completely different from a turbofan in most respects. Although most airflow is accelerated by the fan through a bypass duct, as in a turbofan, there is no conventional multispool core or forward-facing compressor.
Instead, some of the incoming air is ingested into a small core at the rear of the engine and makes a U-turn to be flowed forward through a reverse compressor, similar to the configuration used in Pratt & Whitney Canada’s PT6 turboprop. The small core is possible because of the greater power capacity of steam, Terwilliger explained.
As the incoming air passes through the compressor, it mixes with steam, which is injected between compression stages upstream of the combustor. There, gaseous hydrogen is ignited with the compressed air before the combusted products are expelled through a power turbine that is connected via a shaft to the gear-driven fan.
The hot exhaust gas, rather than exiting the engine rearward through a conventional exhaust nozzle to generate additional thrust, is passed through an evaporator—a type of heat exchanger. The exhaust is then routed to a series of condensers that are integrated into the bypass duct to take advantage of the structure’s large surface area.
Some heat from the condenser is used to recover some of the energy required to liquefy the hydrogen by turning the cryogenic fuel into a gas ready for combustion.
Meanwhile, the condensers are cooled with a stream of bypass bleed air split off from the fan, and they in turn chill the flow to cool the exhaust gas, turning it into water.
The water is centrifuged to the walls of an air-water separator as it exits the condenser. From there, the dried air is ejected into the mixed exhaust flow at the back of the nacelle while the water is fed back to the evaporator, where it is converted into steam. This convective recuperation process further recovers the waste heat from the exhaust as part of a steam bottoming cycle.
Although the ARPA-E contract did not include demonstrations of deriving available energy, or exergy, from the liquid-to-gaseous hydrogen conversion process, Pratt’s analysis indicates the entire cycle is viable because of the temperature difference available using the cryogenic fuel. “We could not get it to trade without hydrogen,” Terwilliger said.
The steam completes the semi-closed-loop system by being injected as a high-pressure spray into the compressor and combustor, increasing mass flow and improving efficiency while simultaneously cooling the flow and—Pratt says—dramatically lowering NOx emissions.
“Our first pass at this, we blew away the metrics,” says Michael Winter, chief scientist of Pratt & Whitney parent company RTX. “We got a 99.3% reduction of oxides of nitrogen compared with a certified engine today and produced 1 gal. of water every 3 sec. That’s an astonishing amount of water.”
A crucial part of HySIITE testing focused on the combustibility of hydrogen with a high steam-air ratio of up to 0.8. “So it’s almost as much steam as it is air when we’re directly burning it,” Terwilliger said. Whereas air is the working fluid of a turbine engine, steam is the working fluid for HySIITE, he added.
“The compressor is just an oxygen pump where we’re trying to put oxygen into our steam so that we can directly burn [the hydrogen],” Terwilliger explained. “We can get it up to very high temperatures, much higher than you can do in a steam powerplant, because it has to go through a heat exchanger there.”
Injecting steam directly into the compressor also helps raise overall pressure ratio (OPR), a function of inlet pressure ratio and compressor pressure ratio in a modern turbofan that is used as a measure of engine thermal efficiency. But higher OPR is traditionally limited by the material temperature capabilities at the exit of the high-pressure compressor. The use of steam injection as a coolant provides a way around this, Terwilliger said, as well as a new solution for intercooling, which is done conventionally by inserting a heat exchanger into the flowpath.
“If you have water, you can spray it in the compressor, and you cool the air in between compression stages,” he said. “You do less compressor work, you take much less volume and pressure loss than you would if it was a heat exchanger intercooler. The fact that we can do that intercooling and have that higher OPR, I would say is almost equally important to performance.”
Despite unavoidable pressure losses in the core flow and intercooling, Terwilliger said the overall benefits more than compensate, particularly with overall efficiency.
“One of the things we’re finding out is on a heat recovery cycle—especially one that’s intercooling and one that has a very low core flow—is that those core flow pressure losses just don’t matter as much,” he explained. “You have a much smaller amount of flow that has a pressure loss on it, and it’s no longer robbing from OPR. It used to be if you lost 5% in your compressor, that’s 5% lower OPR, because you still have the same [compressor exit temperature] limit. But we just intercool a little more, compress a little more and get back to where we were.”
With water being such a critical aspect of the concept, part of HySIITE’s condenser technology testing was focused on “what was coming out of the back of this thing,” Terwilliger said. “Even if the thermodynamics say you’ve condensed 1 gal. every 3 sec., what form does that water have? Is it a fog? Are you going to have to centrifuge it for a year to actually have water, or do you just hold a bucket behind it and you’ve got your water?” The answer, to Pratt’s relief, was the production of water “just like a kitchen faucet.”
For takeoff, particularly on hot days, the engine would be supplied with a small water reservoir. Pratt says additional water will accumulate during descent, meaning the aircraft will arrive with a full water tank for the next flight. “You could have added more condensers, but it trades more favorably just to have a little water tank,” Winter adds.
Pratt is building on HySIITE hydrogen propulsion studies through several initiatives, including the company’s recent award under the NASA Advanced Aircraft Concepts for Environmental Sustainability (AACES) 2050 project. Pratt also is working with the Netherlands’ Delft University of Technology on novel thermal energy recovery engine configuration studies, while sister company Pratt & Whitney Canada is working on a modified hydrogen-burning PW127XT turboprop under the Canadian-backed Hydrogen Advanced Design Engine Study (HyADES) project.
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