Bjorn’s Corner: The challenges of hydrogen. Part 18. Airliner hydrogen tank

By Bjorn Fehrm

December 18, 2020, ©. Leeham News: After discussing the risk-reducing research programs we need to do before a program launch in 2027, we focus the next Corners on the hydrogen airliner’s biggest problem, the liquid hydrogen tank.

In this Corner, we start with the placement and discuss how it affects aircraft performance.

Figure 1. Airbus ZEROe turbofan airliner concept. Source: Airbus.

Tank placement and its effects

We have discussed the hydrogen tanks’ placement for an airliner in previous parts of the Hydrogen Corner series. Here we will focus on the placement proposed in the Airbus ZEROe concepts, presented on the 21st of September.

Figure 1 shows the ZEROe turbofan from the presentation. It’s 33 window positions suggest a capacity of ~200 seats. Note, the rear door is far from the tail. It’s because the tailcone houses two cylindrical LH2 tanks, Figure 2.

Figure 2. Airbus ZEROe turbofan with its two tailcone placed LH2 fuel tanks. Source: Airbus.

Figure 2 shows the tanks are not part of the fuselage structure. They are designed as two separate isolated tanks, filling the aircraft’s non-pressurized tailcone aft of the cabin pressure bulkhead to the trim jack for the horizontal tailplane.

Why two tanks? Probably for safety reasons, should there be a leak in one tank or its fuel system should develop a problem. We can assume the tanks have separate fuel systems, feeding both engines.

There are several advantages to this placement. The cabin is ahead of any leaking tank from a hard landing or accident. As hydrogen leaks and burns straight up, evacuating passengers are separated from the danger area.

Structurally the fuselage is kept close to round, which is good for fatigue reasons. The major fatigue problem of an airliner is the cabin pressure stretching the fuselage skins every flight. Then a round shape is ideal.

The same is valid for the hydrogen tanks. They have a slight overpressure to ambient air, and the thinner air at cruise flight levels cycles the skin stress level of the hydrogen tanks.

The drawback of the design is a varying center of gravity (CG) during flight. A typical narrowbody flight is around 800nm or two hours. It consumes about 4.2t of Jet fuel during such a flight. If we assume our hydrogen airliner has the same efficiency, it consumes 1.4t of LH2 on the trip. With reserves, we have about 2t LH2 in the tanks (today’s jet 6t Jet-A1).

Center of gravity travel

When the ZEROe turbofan lines up for takeoff, it has a weight somewhere around 65t. During the flight, 1.4t is consumed. We can assume Airbus designs the system so this consumption is focused on the forward tank, with the rear acting as a safety tank and holding the reserves.

The consumption of fuel closer to the center of gravity limits the effect of center-of-gravity travel. The loss of 1.4t of LH2 at the rear of the aircraft makes the ZEROe more nose-heavy than at the start of the flight.

An airliner uses the horizontal tailplane to balance the aircraft in pitch, Figure 3.

Figure 3. Forces on an airliner during flight. Source: Leeham News

At the beginning of the mission, we can assume the horizontal tailplane downforce to be on the aft side of the middle position of Mean Aerodynamic Chord (MAC). For the Airbus A320, the mid position is 28% MAC, Figure 4. So say around 33% if the ZEROe used a CG range diagram similar to the A320.

Figure 4. An A320 CG diagram with the acceptable cruise range in red and the CG travel of our flight. Source: Airbus.

As we approach landing, the CG would have moved forward. How much we discuss next Friday. The forward movement of the CG must be within the acceptable range for the aircraft. It’s about the horizontal tailplane’s authority in controlling the pitch during the landing and a possible go-around.

But the forward travel of the CG will also decrease the efficiency of the aircraft. A nose-heavy jet consumes more fuel as the trim drag of the horizontal tailplane increases, and the wing must carry a higher load.

How detrimental these effects will be for hydrogen airliners with the tanks in the rear we calculate in the next Corner.

46 Comments on “Bjorn’s Corner: The challenges of hydrogen. Part 18. Airliner hydrogen tank

  1. If the engines can also burn jet fuel, one could imagine a forward fuel tank to balance the CG

  2. One window must not translate to one seat row and often doesn’t because airlines are using own cabin seat configurations.
    A320 doors are 0.81m wide and therefore three windows would translate to two seat rows.

    A320 doors are 1.85m high and are less than half in hight of the fuselage hight of the A320. But ZEROe doors are more than half in hight of the fuselage hight.

    This is not the A320 fuselage, it’s the A220 with 5 abreast seating.
    So using Bjorn’s assumption with 33 windows, it might have 22 seat rows with 110 seats.

    Beside other solutions Airbus is thinking about engine units with LH2 tanks with fuel cell and electic propulsion, which would reduce the unit costs and keep the fuselage space for seating.

    • Good observations Leon, thanks.

      I’ll dig deeper into it next week as I will do a platform sketch of the ZEROe to work out the CG effects. Let’s see if I come to the same conclusion when I’ve worked on it a bit. Could very well be the A220 cross-section.

  3. @Bjorn

    Airbus is clearly exploring a number of options.

    https://www.flightglobal.com/air-transport/airbus-explores-self-contained-fuel-cell-propulsors-for-zero-emission-aircraft/141635.article

    Would this approach be at all practical ? Could the pods contain enough LH2 even for a regional aircraft ?

    What are the implications for CG where the tanks are in pods on the wings as fuel is depleted ? Do the pod tanks replace wing tanks ?

    Did you ever use drop tanks on any of the aircraft you flew ?

      • Just to be clear, I was interested in Bjorn’s experience of drop tanks regarding any shift in CG as fuel is consumed. I.e. would Airbus’ podded fuel cell propulsor concept help or hinder the CG issue.

        I was not for one moment proposing that drop tanks are used as fuel storage, and then jettisoned.

        I’m actually still surprised that rocket companies are allowed to toss spent stages into the ocean, and are not required to clean up after themselves.

  4. One can imagne using 2ea LD45 container with LH2 cylinder tanks max in the fwd pos in the fwd cargo compartment as a trim tanks. This reduces available cargo but a A321neoLH2 with a shorter pax cabin don’t need equal amount of cargo space. Carrying all the fuel in the fuselage opens up design for an all new slender carbon wing and the decision who shall design it and where shall it be built post Brexit. Airbus most likely want to build them next door to the FAL’s but cannot yet due politics.

  5. Burning hydrogen in a turbine engine would virtually eliminate CO2 emissions. But what about the interaction between hydrogen and nitrogen, that makes up 78% of the atmosphere?
    Will hydrogen “burn” with nitrogen, as it burns with oxygen? A web search on this seems to show that hydrogen is able to burn with nitrogen. What are the combustion products, and can these be viewed as pollutants?

    • Reducing nitrogen with hydrogen creates ammonia and is an energy intensive process. Nitrogen gas is notoriously stable and the only way to reduce it for use in biomolecules are the Haber process or by nitrogenase in a particular group of bacteria. In both cases a metal catalyst (iron for the Haber process, molybdenum for nitrogenase) is required. If airplanes could produce ammonia it would basically be creating airborne fertilizer. I doubt the efficiency is anywhere above negligible compared to the amount of ammonia in the biosphere already.

    • The formulation of NOx in gas turbines is a real issue, however the latest engine designs are very good at keeping NOx levels down. The high tempererature generated burning hydrogen in air can cause NOx formation but the very high flame velocity of hydrogen oxygen combustion reduces the time at high temperatures as you quickly quench the steam with fresh air in the combustion chamber. Lots of modeling and testing (often just one to three burner sections) in enough to get good results is required but normal engineering work.

  6. Bjorn, if there’s value in your series (within the context of your erroneous climate catastrophic beliefs) you should be charging hydrogen airplane wannabes (and politicians maybe) for the advice.

      • Here I go, although I know people generally don’t want change their mind on this subject and would rather shoot the messenger. Yet, I did change my mind… I was a believer until 2007, when I started to dig below the hype and scaremongering.

        Climate change is real. But how much of it is man-made? The climate has always changed, and will continue to change. The last Ice Age ended only 11 000 years ago, and this was certainly not because of human-generated CO².

        Climate change is caused by natural factors, the most significant being Milankovitch Cycles: Changes in orbit and axis of rotation of the Earth. There are some very well-done videos about Milankovitch Cycles on YouTube, such as these:

        https://www.youtube.com/watch?v=82p-DYgGFjI&ab_channel=AryanNavabi

        https://www.youtube.com/watch?v=PFfwIOzVlh8&ab_channel=PhilosophicalInvestigations

        Human activity has little, if any, effect on global climate, although it can produce local effects, such as the urban heat effect.

        Man-made Global Warming / Climate Change has unfortunately become a dogma, used to explain almost every natural disaster. There is no reason to be worried about the 0,45 C° natural cyclical warming over the last 40 years.

        There is much more to this dogma than science. This 2007 TV documentary is still relevant:

        https://www.youtube.com/watch?v=YrsUQ5jw_B4&ab_channel=JesseWells

        Billions are spent to fight this imaginary bogeyman that would be better spent to solve actual environmental problems and adapt to unavoidable climate change.

        • Baranrd:

          Good you brought it up though I don’t say we have no impact. How much depends on models and those are only as good as the assumptions written into them.

          Reading Maeve Leaky on her work, 7 million years ago we began to see a major warming event. Likely related to our devlopment as Humans.

          I have seen a number of observation going back to Magellan (deep ice on the tip of South America 700 years before he went through first ).

          We had one glacier type who was probing a glacier to get time samples. At 250,000 years, no more ice.

          As she was in a crater, ice had to have melted all the way down then started filling back up.

          $64 issue is are we adding to it? It seems like we are, but it was ramping up a long time ago.

          On the other hand, the less we pollute the better off we are, so I vote for clean air and water.

          Hydrogen as an answer for aircraft I am not at all convinced, but overall I am 100% on board with cleaning up our crud.

        • @ BernardP
          I see merit in some of your points, and disagree with others.

          You certainly are correct is saying that there is no “consensus” on this subject; the concept of “consensus” rarely exists in emerging areas of research. But the debate has been hijacked by particular groups that like to proclaim “consensus” so as to drown debate. Climate modelling is horrendously complex, and we can expect that self-proclaimed “experts” will have to make many adjustments to previously held dogmas as time goes on.

          A contra-example: in astronomy/cosmology, physicists have been debating for decades on the nature of dark energy / dark matter, all sorts of theories are floated, various theories fall in and out of favor, old assumptions are regularly re-examined…and, after all that, there’s still no “consensus” in the field, and people are always open to new input. But, then again, this field has not been hijacked by activists.

          What is peculiar is that the Paris Climate Agreement and other such constructs actually make zero effort to address the current atmospheric CO2 problem. By attempting to reduce CO2 emissions, all they’re doing is attempting to reduce the magnitude of future increments to today’s problem…nothing at all is being done to reduce the CO2 levels already present in the atmosphere. Idealistic greenies think that trees can solve this problem, but they rarely do any serious research into the gargantuan problems/shortcomings associated with such a “solution”.
          Also peculiar: nobody is discussing curbing birth rates, even though this is the most efficient solution to the CO2 problem, and multiple other problems. In fact, amusingly, greenies are often front of the line when it comes to having kids.

          I do agree with TW that we should make every effort to reduce and clean up our “crud” while the “debate” is ongoing…to the extent that debate is possible with entrenched activists.
          I don’t agree with demeaning any effort to get alternative energy sources up and running — although there should always be full and open debate as to the true merits and disadvantages of such novel sources. Where that’s concerned, I expect an avalanche of knee-jerk protest from entrenched activists as nuclear fusion projects get closer to going online.

    • I need aerospace, I need the climate,
      climate doesn’t like aerospace.
      Something must be unfair, otherwise I’ve a problem.

  7. Acknowledging the desire to make as few changes to existing practices as possible, a question or two…

    Passengers are not supposed to be able to get into the cockpit any more anyway, so why not put one tank forward of the wing, aft of the cockpit separating it from the passenger compartment?

    Getting a bit more far-fetched (?) hasn’t there been research with pilots flying by video? Does the cockpit have to remain the most forward section of the aircraft? One could put a forward tank where the cockpit is today and have the cockpit remain contiguous with the passenger compartment.

    • Its all trade offs. Pilots need a loo.

      Also a tank in front of Pax and a fuselage that is pressurized , un-presurized, pressurized , un-pressurized.

      Pilots can no longer be the cabin Captain (tank up front)

      No vision and more regulatory aspect not certified for.

      How many changes can you afford when you are making a big leap?

      How about Drop Tanks?

      • Well, Bjorn has already said the design center is for something like a four hour flight maximum, so tell the pilots to hold it?-)

        Is there much difference between a tank in front of Pax and a tank behind Pax when it comes to pressurization cycles?

        Fair point on certification of alternative vision mechanisms.

        Drop tanks?? It is one thing when a fighter is dropping tanks over a combat zone, it is quite another for a commercial airliner to be doing so. Blue ice writ large as it were.

        • Agreed on the drop tanks but lets think outside the air-frame.

          Establish a drop corridor!

          And whats the risk?

          Tens of thousands of them reined down in WWII and I never heard of anyone getting hit with one!

          Tank in front of PAx means a Hydro Sandwich.

  8. How do you keep the hydrogen cold enough to remain liquid
    A fatal flaw I think !!!!!
    Come on, get real, it’s not possible

    • It is possible with the correct tank design, if only needed for a 4 hour flight. Any LH2 boil-off is consumed by the engines or APU.

      The tank design extremes are rockets, were there is minimal tank insulation as the fuel is to be consumed in about 15 minutes, with continuous replenishment on the launchpad. And ground storage tanks which are heavily insulated vacuum flasks, that can store LH2 for months.

      The aircraft tank design will need to be in between these extremes. Much lighter than ground storage but retaining vacuum insulation for probably 8 to 12 hours.

      The more significant issues are LH2 loading/offloading cycles required for every flight. Also a loss of vacuum event in flight (why there are two tanks and a venting stack). And of course crashworthiness, as the LH2 represents multiple hazards to the passengers (freeze burns, heat burns, asphyxiation) All these things will need to be worked out. It will be challenging.

  9. sHello Bjørn. Very interesting series of articles on the hydrogen plane. This latest one looks into CG and the placement of the tanks.
    You do not bring in the BWB concept which Airbus has studied extensively.
    The width of the BWB allows to get the tanks inside the fuselage without conflicting with the cockpit/passenger area.
    A fragmentation of the tanks would handle the CG issue.

    • The BWB concept is considered in the LH2 studies as a possible replacement for larger widebody long-range aircraft. Tube & wing doesn’t have enough storage potential for that application. BWB has its own design and certification challenges.

      An assumption for the smaller short-range aircraft is that they can remain viable while retaining the traditional tube & wing format. That eases the design, certification and operational burden.

      • TU Delft has a novel design in KLM colors with 2 A320 size fuselages in a V-formation in a blended wing-body design. One could add a center fuselage that is the tank that is separate from the passanger cabines. It adds up to lots of surface area but it might have its merits for certian routes.

  10. “How detrimental these effects will be for hydrogen airliners with the tanks in the rear we calculate in the next Corner.”

    With fuel in the payload area TOW will be nearer MZFW than (design) MTOW of the airframe Hydrogen propulsion is grafted on.

    For a start you could pump ballast water around :-()

    • Or just move people around.

      Water = Weight. Weight is not your choice for efficient when you are compromised on range and all new systems to get clean already.

      • You aim for manholes in the LH2 Tanks and moving of obnoxious passengers there when empty? 🙂

  11. Yes the TFL is optimised, but a new problem occurs on the dash extremity. I forsee nodules with XWZ articulates extending the HDL. A further problem occurs at altitudes where X=Y over T. This may be overcome with dash nodules arching on wide split planes. Comments to profairplanes@jollygoodfun.com

  12. The P-80 found the wing tanks lent themselves to better performance though they were put on to gain range (those early jets really did suck fuel)

    Perhaps we could have a combo wing tank and winglet?

    More thinking outside the wing box.

  13. Why stick to wing mounted engines? With fuselage rear mounted engines the center of gravity would be closer to the fuel tanks and there would be no need to route hydrogen fuel lines past the rear cabin pressure bulkhead.

    • Fuel line routing is the least of the issues.

      Rear mounted engines would probably help the CG issue.

      But they also add to structural issues.

      But then again this is not about efficiency so much as driven by one aspect to the detriment of many others.

      We probably would be vastly ahead to plant more trees than spend all that money on hydrogen for aircraft.

      Its PR driven at best as opposed to best bang for the buck.

  14. 1 The point of this series is not to debate the advisability of quitting fossil fuel but rather, assuming a requirement to do so, what might the LH2 alternative aircraft look like.

    2 The obvious issue with the proposed configuration is the large CG movement with fuel burn. It is stated the fuel burn is 1.4 t, with takeoff weight of 65 t. Guessing from the illustration, the distance from the center of lift to the center of gravity of the fuel might be about 3 to 4 mean chord lengths. That implies an aircraft CG movement of about 4 x 1.4 / 65 or about 9% of the mean chord length.

    This is large but not an impossible shift. However this was with range limited to about 2000 nm. This limited range might be unacceptable to the users. Also stretching the fuselage would drive up the CG movement. Not much growth potential for this particular configuration, I think.

  15. Could water ballast help? Would moving the center of effort, the position of the wings and adding water or a liquid that could be pumped for ballast help. The extra weight will reduce range. If it was hydraulic oil it could add an extra reserve for control systems but it might add another safety hazard because of having a flammable at a location that is not advantageous. If the pumps fail would the center of gravity be uncontrolled?

  16. I came back to check on a comment I made about ballast tanks. I don’t see it so I guess it was a waste of time. I enjoyed the comment about moving obnoxious passengers to the empty fuel tanks.

  17. More of a question than a comment – would be interested to get Bjorn’s thoughts.

    Would a 3 lifting surface configuration similar to the Avanti offer any advantages to cope with the CG shift?

    • The Avanti is really a distributed wing design. It allowed the wing box to be shifted outside the cabin area, but without enlarging the main wing area as is typically needed. That in turn allowed it to retain high speed performance. It also allows the CG to shift forward with fuel consumption, similar to what is proposed for the LH2 aircraft.

      Control of the aircraft is more complicated with multiple wings. Also there are structural ramifications of distributing lift. Also if the main wing moves far enough back, you end up with a T-tail. It would be interesting to learn why these designs are not pursued for commercial airliners, or what the trade-offs would be. I’m sure they have been considered before.

      • “… allowed the wing box to be shifted outside the cabin …”

        HFB 320 Hansa Jet ( Germany, 60ties business jet using forward swept wings)

  18. the effect of burning away 2tons of fuel at say 15m behind CG could be compensated at 50% by 500L/half ton of sanitary water moved from nose tip to tail tip trim tanks located 30m away of each other. So considering most flights will keep a lot of hydrogen in reserves, control surface would not have much extra compensation to do.
    It seems 500L is just 2-3 times what a plane like this would carry.
    Not a big weight penalty.
    Special fare for swing passengers for seating at a front row then move mid flight to a special last row for them.

  19. The design has to cope with a leaky front tank that is empty and a fully depleted rear tank after a diversion and a go around, so must cope with a 4 tonne change to the CoG, at maximum moment when the rear tank is used. Tanks forward of or under the passenger compartment won’t pass the safety sniff test. What about permanent aerodynamically shaped under-wing tanks (not drop tanks)? While the drag would be permanent it would reduce the drag from the rear stabilizer as the CoG moved forward, and the range of adjustment required by the tailplane.

  20. How about putting fresh water in the front of the plane and routing waste water to the back, close to the CG ??
    This would compensate CG shifts and water usage should be proportional to fuel usage.
    In case of emergency, leftover fresh water could be dumped to move CG further to the back.

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