Leeham News and Analysis
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Leeham News and Analysis
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Bjorn’s Corner: The challenges of Hydrogen. Part 10. Airbus’ Hydrogen ZEROe concepts
September 25, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners, we look at the three hydrogen-based concept aircraft Airbus presented this week.
They are called ZEROe and are concepts and not products, but their design tells us a lot about where Airbus is with its studies and how the hydrogen demonstrator aircraft might look like come 2026-2028.
How to understand Airbus’ Hydrogen concepts
Are the pictures of Airbus’ hydrogen-based ZEROe aircraft from Monday’s presentation also a hint of how future products will look like? Airbus EVP development Jean-Brice Dumont pointed out that concepts are not products.
The concepts are created to give Airbus and suppliers concrete example aircraft to work around when they look at configurations, aerodynamics, system solutions, and needed avionics to build efficient hydrogen-fueled airliners.
If we discard the Blended Wing-Body (BWB) variant, which I judge is there to spawn ideas and excitement more than show a realistic concept for the next move, the concepts still say a lot about what might come. Let’s look at what we can conclude from the Airbus presentation.
Hints for the 2027 demonstrator and 2035 first product
A demonstrator aircraft is scheduled for 2026-2028, but the program has a main supporter in the French government and the Paris Air Show is 2027. So let’s use June 2027 as the likely presentation date for a hydrogen-fueled demonstrator.
Will this be singular or plural? Airbus owns 50% of ATR and the turboprop concept looks very much like a more exciting ATR72, Figure 1.
Figure 1. Airbus hydrogen turboprop concept. Source: Airbus.
At the same token, the Turbofan concept has several similarities with the Airbus A220, Figure 2. It would not be difficult to put the chain saw in the rear fuselages of former prototypes of an A220 and an ATR72, install the hydrogen tanks in modified rear tail cones as shown, and put the aircraft together again, now a bit shorter in the back (we come to why).
Figure 2. A hydrogen turbofan for the heart of the market. Source: Airbus.
As we have seen, the challenge with hydrogen aircraft is the hydrogen tanks. The change of the rest of the fuel system components and the conversion of the Jet fuel-based engines to hydrogen is less of a challenge.
Airbus confirmed that all three concepts are hydrogen fueled gas turbine aircraft (also the BWB, I wasn’t sure directly after the presentation). It has come to the same conclusion as I, the step to a fuel cell-based hybrid with electric propulsion is risking a lot with no gains at the moment.
At the announcement Grazia Vittadini, the Airbus CTO said the gas turbine-based engines will have electric motors built-in. This is the electric starter generators I talked about in this Corner.
By moving the air starter and generators from the engine auxiliary gearbox to one of the shafts of the core, the acceleration of the engine can be faster and the core can be optimized with smaller margins to compressor stall. This leads to a more efficient engine compared with today’s engines with air starters and generators on the auxiliary gearbox.
I could talk to Vittadini in connection with the presentation and she confirmed that changing the APU to a hydrogen fuel cell that produces electrical energy is attractive, now that the main fuel is LH2. There was no confirmation or denying that this would find its way to the demonstrators or ultimately a product. We just have to wait and see.
Converting the APU of an A220 based demonstrator to hydrogen is straight forward, should the APU remain to reduce the risk budget. The ATR72 doesn’t have an APU so no problem there (but a hydrogen variant might, the concept turboprop has an APU).
How much do the concepts say about a 2035 Airbus product?
Probably a few things:
- It will have hydrogen-fueled gas turbine Turbofan propulsion.
- The likely seating will be at the heart of the single-aisle market. I would guess around 180 seats.
- The general configuration of the aircraft will likely be like the Turbofan concept, with the tank in the rear of the aircraft. The heavy LH2 tank placed in the tail cone behind the cabin will move the wing back to balance the aircraft.
- The wing can be made more slender as there is no volume requirement on the wing’s wingbox to house fuel. Many would say thinner but the profile of the wing and by it the thickness is dictated by super-critical transonic considerations and wing thickness is important for low wing weight. The wing cord (the width) will be shorter however to gain a high aspect ratio when paired with a large span. Folding wingtips might be used to get into the 36m gates of narrowbodies.
- There will be Fly-By-Wire and aerodynamic adaptations to cater to the change in the Center of Gravity (CG) for the aircraft when the 1.5t hydrogen sitting behind the CG gets consumed during a typical 800nm to 1,000nm sector. We can view the large swept-back winglets as such an adaptation. We will probably see more innovations to cater to this CG shift.
- These concepts have a hydrogen-based APU. Will the 2035 product? This will depend on the developments for the fuel cells. Will these be small, light, and reliable enough by 2035 to take over this role?
- Materials for the LH2 tank and all fuel related components will be intensively researched and tested. A major task for the demonstrators will be to test materials under realistic operational conditions over a longer time.
- Major aspects that are not shown in the concept pictures are the ground handling and airport infrastructure needed for hydrogen airliners. Once again, an important task for a demonstrator aircraft to test out.
As can be seen, we can deduce a number of hints from these concepts. We shall just avoid thinking these concepts define a 2035 product. Dumont said, one of the reasons is the market for a hydrogen airliner is not clear.
The demonstrator will be vital for airlines to understand all the aspects of operating a hydrogen airliner compared with today’s carbon-based types. When this becomes clearer, including the cost picture of hydrogen contra carbon-based airliners and state subsidies/taxes, products can be defined.
Will there be a hydrogen turboprop demonstrator and then a product from ATR? Probably. The French €15bn program announced in June contained program components for the regional industry, and in France this means ATR. We can expect a Turboprop demonstrator, based on a converted ATR72 and this can be followed by a product should the market reaction be positive.
Wouldn’t LH2 tanks aft of the rear pressure bulkhead be too exposed to the risk of tailstrike? How about the crashworthiness of LH2 tanks in general? Wouldn’t passengers and LH2 tanks have to be separated, inherently leading to external tanks or twin boom/twin fuselage configurations?
The behavior of a crashed or cracked LH2 tank is in many ways less troublesome than for Jet fuel tanks. LH2 expands to gaseous H2 and mixes with air. As such it’s harmless. If ignited it burns straight up from the leak which is better than carbon fuels that spread and generate fires over larger areas (ref Superjet that made a hard landing in Moscow).
You want to avoid leaks into a confined space that can have a mix of H2 and air that can be ignited but this is valid for Jet fuel as well. This speaks to integral tanks (the tank walls are also part of the tail cone outer skin), to minimize the number of close spaces around the tanks.
There are precautions with hydrogen tanks and fuel systems, as there are for carbon fuel tanks/systems. But these are not more difficult than for Jet fuel it seems, just a bit different.
It’s true that Brewer stated that the integral tank is superior compared to the non-integral one in terms of overall aircraft weight and more suitable accessibility of the components,
e.g., for inspections. However, this is true only for designs having non-integral cryogenic tanks located within a conventional semi-monocoque fuselage shell.
Interestingly, on the Skylon spaceplane the LH2 tanks (i.e. with the LH2 tanks having a common bulkhead with the LOX tanks) would only be carrying the thermal loads of cooling and warming, the pressurisation (which normally is the biggest of all loads) and acceleration loads on the contents, all other loads are carried by the truss and the skin.
In fact, the structure design of the Skylon spaceplane is essentially a fuselage truss structure. Hence, the main structure of the spaceplane is more akin to that of an airship than a conventional aircraft. An internal lattice like structure (constructed of CFRP struts fitting into titanium alloy end fittings, known as nodes) provides the main structural component with the aeroshell mounted on the exterior and the LH2 propellant tanks internally supported.
http://www.shorturl.at/dtCNW
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As with the the space shuttle orbiter, the unpressurised compartments in the fuselage and wings of the Skylon spaceplance would be vented during ascent and re-entry.
https://spaceflight.nasa.gov/shuttle/reference/shutref/purge/index2.html
There is also the cryo-hazard to consider. The energy withdrawn from LH2 in liquefaction is very large. That energy will be reabsorbed by H2 to drive vaporization, and it will be sucked at a large rate from anything it touches in a spill. Along with the energy extraction, there is a large temperature drop, which radically alters the properties of materials.
So this means directly fatal to humans, as well as secondary effects of the collapse of structure & materials around them. Freeze burns are no less damaging than heat burns, the transfer of energy is just reversed in direction, but not reduced in magnitude.
If the tanks are in the fuselage, there will need to be protective containment measures implemented in the bulkheads, to prevent distribution of the LH2 in the event of tank rupture. The containment will need to be sufficient to absorb the forward momentum of the LH2 that may occur in a crash, and also to allow for safe vaporization (materials resistant to cryogenic temperatures).
These will be some of the challenges of using hydrogen. They are not insurmountable problems, but will require significant research and testing to meet safety standards.
One visible change is the front window designed for a single pilot. Looking at the space industry LH2 tanks in the US are welded Al-Li structures and making similar for an airline with insualtion and a thin outer non structural skin is doable. To minimize fuel consumption and increase TO performance makes you look at counter rotating UDF’s. Safran has one EU demon unit build around a M88 Engine that is only ground tested but can be hard to certify and produce for a profit without the help from GE.
Using an A220-300 to make a demo can be difficult with its PWA Engines and its wing as you need to strethch the fwd fuselage to get c.g. where you want it with a heavy tail. Still with German money to MTU converting the geared fan and a limited volyme aft tank it is doable. Using an A321neo with a small fwd stretch “A322neo” and convert the aft fueselage to an Al-Li LH2 tank looks easier keeping the LEAP-1A Engines converted to LH2. I think Airbus with time will make it look more like the Boeing Sugar with high wings and UDF’s.
Instead of prolonging the forward fuselage, you can shorten the rear fuselage when you have it apart (we are talking the demonstrator here). You need to increase the size of the tail surfaces to keep the directional stability (increased tail volumes). The high vertical tail with an H2 chimney in the Turbofan concept serves this purpose.
I agree on the chimney and it will be interesting what material, welding method (friction stir welding as NASA choose for the Orion?) and system pressure they choose for the tank with pressurisation method (Helium as SpaceX and other space rockets or let the evaporation build up pressure until safety valves open in the Cold environment). Airbus needs a new wing for the A320neo successor and this program might devlop one probably with UK Money and similar with SAFRAN develop a new LH2 fuel system for the LEAP. At some Paris Airshow in the future we will see and hear what they choose and what they got funding for (not Always the same). Canada, Ireland and Germany might fund a big chunk of the A220LH2 demo?
Both look like A220, both must have a 5-abreast cabin. The pictured turbofan might have the size for 90 pax. I doubt Airbus will come up with a 180 seater, that would be complete defeat for Boeing. I know these are only photo models, and even when the ATR is old, it might be easier to make an ATR demonstrator. In few years robots might make the A220 fuselage in one piece, then it could make sense to replace the ATR with an turboprop A220.
Interesting is to reduce the power on same sized engines and of course the turbofan wings. How much energy would they save. How much takeoff field length would such a turbofan need.
@Leon
The turbofan powered variant has a larger A320-type freight door sized for LD3-45 containers.
Thanks OV,
turbofan and turboprop have the same fuselage.
R1-door is much too big for an A320-fuselage.
A320 cargo door has same width as R1-door height.
Turboprop has the same cargo door.
To me they are A220.
@Leon
You’re wrong. I’d suggest that you do take a closer look at the video provided by Airbus:
https://www.youtube.com/watch?v=5Fi65k2K3Mw
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Clearly, the fuselage lower lobe on the LH2 turbofan powered aircraft is larger than the fuselage lower lobe on the turboprop powered aircraft.
So – again, the single cargo compartment door on the LH2 turbofan powered aircraft is the same size as the forward and aft cargo compartment doors on the A320.
Furthermore, a five abreast 180 seat aircraft with a hydrogen tank located aft of the wing would be more than 50 m long. Hence, it would not be a sensible thing to do for an OEM to design a base version with 180 seats and a 5 abreast configuration, when using LH2 as a fuel. A stretch version would be at least as long as the 757-300.
Bjorn came up with the 180 pax number, not Airbus.
Watch how high the fuselage is under 1R-door on A320, there would be nearly space for another 1R-door. But on ZEROe turboprop there isn’t space under the door.
Also the A320 nose looks completely different.
Bjorn,
I understand Reaction Engines is looking at the possibilities of using Ammonia as a replacement for jet fuel.
Ammonia is heated, a cracking reactor uses a catalyst to break the Ammonia into Hydrogen, and Nitrogen, a modified jet engine burns the fuel mix. Waste products are Nitrogen, water vapour, and potentially some Nitrogen oxides which can be reduced using … Ammonia.
I’m wondering if this is a serious option for shorter flights ?
If I remember the energy density of Ammonia is roughly a third that of Diesel.
Perhaps the subject of a future corner ?
https://www.ammoniaenergy.org/articles/zero-emission-aircraft-ammonia-for-aviation/
“”On Thursday, sustainable aviation start-up Zeroavia operated the first commercial-grade flight fully powered by hydrogen. The six-seater retrofitted Piper Malibu aircraft flew to 1,000 feet above Bedfordshire in the UK, leaving nothing but water and heat in its wake.””
https://simpleflying.com/first-hydrogen-flight/
A Piper Malibu isnt really ‘commercial grade’
“was fuelled with 4lb 6oz of hydrogen gas. ” …Yep that will make all the difference.
Zeroavia is deliberately aiming at the low-end turboprop market initially (19 seats or below), that’s why they go for gaseous H2 to have the same filling apparatus as the hydrogen car. They don’t have the leverage of Airbus so they must adapt to what works for a first entry infrastructure-wise.
By it, the whole cryogenic problem for a small operator is also avoided. It’s one step too far unless you are a big player and a market maker like Airbus.
The same for Eremenko’s startup Universal Hydrogen. These concepts lose efficiency compared with carbon fueled alternatives (because of the large, heavy tanks), they can only be operationally sustainable in jurisdictions with tax benefits or subsidies (which is fully possible, States that want this change will chip in to get the ball rolling IMO).
Zero Avia is a startup that operates on a thin budget. We California FCEV drivers have seen them around the H2 stations in California with their Ranchero with a H2 tank strapped to the back. It’s Silicon Valley Start Up Gangster Style! Very cool, imho! Zero Avia’s goal is to get the thing in the air with 5,000psi gaseous and then later switch to LH2. But for now they need to keep it simple. Better to accomplish something with 5,000psi H2 than going broke trying to implement LH2.
Interesting analysis. But there is a major challenge regarding LH2 that was not mentioned. I remember reading an article about a powerplant in Austria using Hydrogen to power a gas turbine. The biggest challenge faced by the engineers was the sealings. The gas was leaking at many joints. They could not figure out how to solve it. I guess it will be a great challenge when implementing such a concept on an airplane.
Over a certain size, hydrogen cooled generators have long been the norm. Hydrogen dissappates so rapidly in the air that the accidentally trapping the hydrogen somehow is all that needs to be usually needs to be avoided to address small leaks.
“CIGRE (International Council on Large Electric Systems) estimated that there may be more than 40,000 hydrogen-cooled generators in service around the world. Despite the large number of systems that use pressurized hydrogen to cool generators, for the most part, few incidents or problems occur.”
There seems to be inherent problems with hydrogen as a fuel, caused by physical properties of this element. This article from 2003 discusses these properties and the intractable problems they cause.
https://www.researchgate.net/publication/232983331_The_Future_of_the_Hydrogen_Economy_Bright_or_Bleak
I’m working at a research instutute and we’re dealing – mostly – with production processes and digital monitoring of hydrogen and cell technologies.
The article is heavily outdated and biased. However, challanges exist from the diffusion capacity of hydrogen beeing the smallest of all elements.