Gold Hydrogen Program coalition launches program for subsurface biomanufacturing of hydrogen
Polestar announces first partnerships for the Polestar 0 Project to develop a climate-neutral car

Pratt & Whitney awarded $3.8M by ARPA-E to develop novel hydrogen-fueled propulsion for aviation: HySIITE

Pratt & Whitney was selected by the US Department of Energy (DoE) to develop novel, high-efficiency hydrogen-fueled propulsion technology for commercial aviation, as part of DoE’s Advanced Research Projects Agency-Energy (ARPA-E) OPEN 2021 program (earlier post).

The Hydrogen Steam-Injected, Inter‐Cooled Turbine Engine (HySIITE) project will use liquid hydrogen (LH2) combustion and water vapor recovery to achieve zero in-flight CO2 emissions, while reducing nitrogen-oxide (NOx) emissions by up to 80% and reducing fuel consumption by up to 35% for next-generation single-aisle aircraft.

The HySIITE engine will burn hydrogen in a thermodynamic engine cycle that incorporates steam injection to reduce NOx emissions. The semi-closed system architecture planned for HySIITE will achieve thermal efficiency greater than fuel cells and reduce total operating costs when compared to using “drop in” sustainable aviation fuels.

This truly is an exciting opportunity to start developing the key technologies that could bring the industry’s first hydrogen steam injected, inter-cooled engine from concept to reality. For nearly 100 years, Pratt & Whitney has been at the forefront of innovating cutting-edge technologies to continually advance the efficiency of aircraft engines, and we are thrilled to be selected to work on what could be the next breakthrough technology for aviation.

—Geoff Hunt, senior vice president, Engineering and Technology, at Pratt & Whitney

This is the first direct collaboration between Pratt & Whitney and ARPA-E.

Separately, Airbus announced a collaboration with CFM to develop a direct combustion aviation engine also fueled by liquid hydrogen. (Earlier post.)

Interest in using liquid hydrogen as an alternative to conventional hydrocarbon aviation fuels reaches back at least to the 1950s, when the impetus was primarily improving military aircraft performance. In the 1970s, the interest switched to concern over fossil fuel depletion (as perceived at the time), the oil crises, and air pollution. Now the concern is greenhouse gases and climate change.



I am not sure what they intend to do with the water vapor that they recover. They are burning H2 with a molecular weight of 2 and generating water with a molecular weight of 18. So for every kilogram of hydrogen consumed they replace it with 9 kilograms of water. After a bit, the plane would no longer be able to fly. Maybe, they are planing to dump the water as a liquid or, at altitude, they can make ice cubes and dump it as a solid. Anyway, I do not expect hydrogen fueled planes any time soon.

I am not sure what they intend to do with the water vapor that they recover.

They're going to make steam to inject to dilute the fuel to reduce the combustion temperature.  This is for NOx control.

The water will eventually go out the tailpipe, as vapor.  Gonna be some pretty healthy contrails from those things.


I have not done the maths for aeroplanes, but some time ago we ran through that for water emitted from fuel cell cars.

The water was about the same, although the far lower temperature in the fuel cells compared to an ICE meant that it was more likely to pool instead of rapidly rising.

So as a first guess it seems probable to me that the quantities of tailpipe vapour should be similar to those of conventional planes, although of course an engine combusting hydrogen is not the same as a fuel cell operating in a car.


If they achieve these goals, it could be a game changer: "The semi-closed system architecture planned for HySIITE will achieve thermal efficiency greater than fuel cells and reduce total operating costs when compared to using “drop in” sustainable aviation fuels."
I will believe it when I see it.


Thanks, E-P. When I first read this, I thought that they were going to try to cut down on the contrails. Maybe they could collect some of the water vapor and make course crushed ice which would at least fall into the lower atmosphere:)

@Davemart, You should end up with more water vapor as some of the energy from burning jet fuel comes from the carbon forming CO2 unless the gain in efficiency is sufficient to offset this.

@GdB, I do not believe that I will live long enough to see it:) I think that in the short to medium term (up to at least 2040), short to medium haul aircraft will become battery electric and the long haul will use some type of bio-fuel. This is primarily based on economics plus the inherent problems of dealing with liquid hydrogen. Also, as we convert more of our ground based transportation to battery electric, the ethanol that in now used E10, E15, and E85 gasoline can be used to make aviation fuel.

Roger Pham

@sd and GdB,
This is a stoichiometric-combustion gas turbine / stean turbine hybrid engine that consumes all the O2 in the intake air while diluting that mixture with steam generated using wasted exhaust heat in order for the steam to generate extra thrust, thus a built-in exhaust heat recuperator. That is how this can obtain a 35%-gain in fuel efficiency vs a conventional gas turbine. Thus, we will have a very small compressor section that consume much less input power, that is followed by a steam injector right behind it for charge dilution and an H2 injector within the same chamber, to produce a mixture of air, steam and H2 in order to greatly reduce NOx during combustion, feeding this combustion product into a much larger turbine section.
Then, some of the exhaust gas will be fed to a condensor section to recover water to be injected into an exhaust heat recuperator coil section to produce steam again. This hybrid gas/steam turbine engine will thus be heavier and a bit more complicated than a comparable gas turbine, ...BUT...the much lighter LH2 fuel load will more than compensate for the increase engine weight that will be much more efficient than conventional gas turbines thus will help reduce LH2 fuel mass and increase the economic prospect of LH2 as aviation fuel.

This is a really exciting development for H2-burning gas turbines that will make LH2 aviation fuel to become cost-competitive with Jet A fuel much sooner, due to the gain in efficiency inherent in it, while achieving truly low emission...truly exciting development.


This “ Steam-Injected, Inter‐Cooled Turbine Engine” is similar to the work that MTU is doing with it’s water-enhanced turbofan (WET) a Geared Turbofan with which uses “wet” combustion to cut NOx emissions and reduce contrails. MTU is a partner of Pratt&Whitney and the Geared Turbofan is probably a PW1100G.
Read about it here:
Watch this video on “Potentials of the Water-Enhanced Turbofan (WET)” - - that shows the aircraft mounted condenser and water recovery system.


combustion of LH2 produces no aerosol particles.
LH2 ice crystals, which instead form on background
aerosol, are consequently 1–2 orders of magnitude fewer than in a kerosene

The comments to this entry are closed.