By Bjorn Fehrm
February 21, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why the development has been slow.
Last week, we reviewed the present fallout of lower emission projects that have not reached their goals and where investors, therefore, have decided not to invest further.
There is a well-known project failing every month at the present pace. Some recent ones: Universal Hydrogen’s ATR conversions, Volocopter and Lilium’s bankruptcies, Airbus freezing the CityAirbus eVTOL (Figure 1) and pushing out the ZEROe hydrogen airliner, hibernation of the Alice battery aircraft, etc. There will probably be more in the coming months.
Airbus’ way forward
Some of these projects stopped because batteries did not evolve as fast as expected. Still, most did so because their assumptions about the use and cost of present batteries and what was needed in terms of energy for operational flights were just naive.
There is no point in taking an airline flight of 100-150nm unless you live in an archipelago. More is not possible with present batteries when all factors are considered (bad weather, headwind, safety reserves beyond the legal ones, etc.).
The one technology that can enable longer flights is the exchange of the kerosene fuel for hydrogen, as the hydrogen has three times the energy content per weight unit (and aircraft are hypersensitive to weight). We looked at the Fuel Cell option in Part 8 and concluded that while it has true zero emissions (producing water), it has limitations in engine size and power.
I was at the Airbus 2024 results conference earlier in the week, and there, Airbus CEO Guillaume Faury said that it had chosen the Fuel Cell path instead of Hydrogen burn for its ZEROe airliner project. At the same time, he said that the present technology (including Airbus’s own fuel cell project) would not produce a commercially viable aircraft with present technology. With that, he meant that the air transport performance would be too low in relation to its operating costs.
The technological development inside Airbus around fuel cells for aircraft applications will now continue for another 10 years, with a plan to produce a viable aircraft using fuel cells to drive electric motors and propellers on the far side of 2040.
Hydrogen Burn
Airbus has concluded that the hydrogen burn alternative, where a turbofan’s core is converted to burn hydrogen instead of Jet-A1, is an alternative that Airbus will not pursue further. Why was not given.
It could be because hydrogen burning in a gas turbine core produces NOx emissions, though at a lower rate than for a Jet-A1 burning core.
It could also be that Airbus realizes it’s prudent to start a hydrogen-fueled airliner project outside the hot single-aisle segment, where about 2,000 aircraft per year will be delivered in the 2040s. A fuel cell machine will come in at about a 100 seater Turboprop in performance, well different from the A320 series and its successor in size and speed.
Introducing an alternative propulsion architecture in the segment that pays the bills of the industry, can lead to major disruption and confusion. Such an aircraft can’t be ramped to scale anytime soon after its introduction, but it can upset programs running at 50 to 75 units/month (i.e., two to three aircraft produced per day), and that is not what OEMs want to happen.
We will probably not find out anytime soon what the real deciding factor behind Airbus’s decision to go for fuel cells is. But it’s a bit of an irony that as Airbus decides on fuel cells, the hydrogen burn alternative announces its most interesting development in 10 years: the DARPA-sponsored RTX and Pratt & Whitney HySIITE project.
HySIITE is a long-term project that changes how hydrogen is burned in a gas turbine. Such a process produces a lot of water (more than the Jet-A1 burn process), which has, to date, been expelled with the gas turbine’s exhaust.
HySIITE changes this. It uses the water to improve the hydrogen burn process in a major way. How we will cover in nexts weeks Corner.
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