Bjorn’s Corner: The challenges of hydrogen. Part 20. Hydrogen airliner weight shift

By Bjorn Fehrm

January 15, 2021, ©. Leeham News: In last week’s Corner, we looked at how hydrogen consumed in the rear fuselage tanks of Airbus’ ZEROe concept affect the airliner’s efficiency.

Now we look at other aspects of the rear placement of the tanks.

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

How rear tanks influence a hydrogen airliner

We look at what to consider when a hydrogen airliner has its tanks placed like Airbus’ ZEROe turbofan concept in Figure 1. It has two tanks placed in the rear (two for safety reasons), Figure 2.

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

We could see as the Center of Gravity (CG) shifts during a flight when the Liquid Hydrogen (LH2) is consumed, it causes an efficiency degradation by 1.4% on the maximum range flight of 2,000nm and 0.5% for the typical flight of 800nm.

Now we look at other effects. How large is the CG shift from a flight? Will it run the aircraft into CG range problems?

An airliner’s CG range

An airliner has an acceptable range for the Center of Gravity. It’s determined in the design process and verified in flight tests. It’s about the authority of the horizontal tail at low-speed maneuvers like rotation, lift-off, and landing. The A320 range is how in Figure 3.

Figure 3. An A320 CG diagram with the calculated cruise range in red. Source: Airbus.

As a test, let’s assume this diagram applies to our airliner as well. We have an initial CG where the red line starts to the right and now calculate how much of the Mean Aerodynamic Chord (MAC) Center of Gravity range is eaten up by our LH2 consumption during the flight.

For the maximum range flight of 2,000nm, the increase in the nose-down moment was 340,000nm. A moment is force time distance. To get the CG travel of the aircraft, we divide the moment with the aircraft’s gravitational force which is 61.7t times the gravity constant, g = 605kN/136klbf.

The CG travels 0.56m forward for the 2,000nm flight. For the 800nm flight, the moment is 154,500Nm, and the CG travel is 0.25m. Is this change of CG acceptable? To understand it, we must understand how many % of Mean Aerodynamic Chrod CG travel the displacements represent.

The Mean Aerodynamic Chord of an A320 is 4.2m. Thus, the maximum range flight moves the CG 13% and the typical flight 6%.

A 13% move of the CG forward during flight is not ideal (it’s about the length of the red line in Figure 3), but it can be handled. But a hydrogen airliner with the tanks in the rear will have a narrower passenger and cargo loading CG range than today’s A320/737.

As you prepare for a maximum range flight you have to include a forward movement of the CG with 13% during the flight. Your allowed CG moves back 13% compared to an airliner that doesn’t shift the CG during flight. As the rear limits remain your CG range shrinks 13%.

The CG shift of 6% is less problematic, but once again, it shall be included in the pre-flight analysis.

A hydrogen airliner with a tank arrangement like Figure 2 can include means to compensate for the CG shift. One possibility is to shift masses around as Airbus does with the tailplane tanks for the A330. You can also move the Aerodynamic Center of the aircraft.

We look at some of these methods in the next Corner.

31 Comments on “Bjorn’s Corner: The challenges of hydrogen. Part 20. Hydrogen airliner weight shift

  1. Bjorn, isn’t the problem worse than this? Surely the preflight calculations must take into account the worst-case scenario which here would be the need to utilize the reserve fuel as well? After all, what is the point in carrying even the mandatory final reserve fuel if it can’t be used without putting the a/c outside its certified CG envelope?

    • Hi Andy,

      you are right, I should calculate the worst-case which is when the aircraft lands at the alternate with reserves close to 0kg. Then it’s 16.8% CG shift, another 3.8%. It starts to be problematic. We discuss what can be done next week.

      • Not sure I understand the A330 tank reference, if this is a Hydrogen powered air craft I am missing on how it applies to a Jet A tank.

        • You always need to trim somehow. Most planes do it with the stabilizer causing additional drag. The A330 does it with fuel weight in the stab without additional drag.
          For the A321 Airbus shows different payload-range curves with additional fuel tanks (ACT). It’s a trade off between weight of the fuel tank and pax. In general, planes with ACT have lower trip costs because the weight of the ACT is in the center. Without ACT more pax can be carried which mostly sit in the back and therefore more trim is needed which cause additional drag.
          For the A330 it’s the same payload-range curve which is only extended, there is no additional drag because it is trimmed with weight.

          For LH2 planes the same could be achieved with water. Fresh water in the front and waste water in the back.

          • You mean the kitchen in front and the toilets in the back. I have thought about it but the problem is that only gives a liter per passenger while fuel is 25kg per passenger iirc

          • It must be within the pressure cabin, otherwise the waste water would freeze.
            But still, if fresh water is in the nose it would help because moment is weight times distance, and the distance to the nose should be much bigger.

          • Perhaps it will be possible to carry a supplementary liquid fuel in the wings that can be pumped into the tail fins, tail cone or horizontal stabiliser. This ballast fuel will be part of the ICAO reserve and not generally consumed. The issue is how well can it be consumed by engines optimised for hydrogen. I would assume tailor made synthetic hydrocarbons might make this easier. Or one could have a dry reformer that converts hydrocarbon into CO and H2, such systems were developed for use in automobiles years ago.

      • With reserve fuel you could do things like moving the heavier passengers to the back and kids to the front.

  2. Todays A320neo’s with Airbus Cabin Flex and the Lufthansa Technik version to fit another row of pax in it moves the c.g. aft so much that LH had to block the last row from pax. Hence c.g. is important, the 737NG’s also has issues with c.g. and the need for a tail support rod on ground at certain loading distributions.

    • I have seen that on 747F and the MD-11 and 777 F have nose tethers but not on a Pax aircraft.

    • Need to block pax seats may depend on loading calculation method, computers do that with assigned seating, otherwise the calculation is approximate based on assumptions and probability, I forget how much margin was used.

      (Special case is a group of heavy pax like a football team sitting together, F/As have to watch for that. And perhaps a group of tourists from SE Asia, lighter people than most here.)

      The 727-100C had an optional forward CofG limit to help with cargo in the forward cabin, that required use of Flaps 30 not Flaps 40. Was manageable at CYRB with its 6000 feet of gravel at sea level, but PW had an early simple HUD to monitor the final approach and I presume the runway had an ILS then as it does today.

      Real-time CofG measurement was tried circa 1970s-80s. Measuring strut pressure did not work well because of friction of seals. Strain gage affair inside axles was better but with challenges. Strain gage bolted to lugs forged into truck beam likewise. I don’t know if anyone solved the challenge.

  3. Hydrogen is not the solution to pollution. Hydrogen combustion produces water. Water at altitude produces water vapour and crypto clouds.
    In quantity and at altitude these vapour trails produce persistent overcasting. Given any future widespread use of hydrogen we can look forward to permanent overcasts. So goodbye blue skies.

    • Current airplanes also produce condensation…look at all those contrails in the sky! But it’s still blue.

      • Bryce:

        It is not Blue where the contrail is and I have seen them spread across an entire area (we are a crossroads of international travel so a lot of flight per day over us)

        We need all the sunshine we can get, note our dark of winter is just past thank you!

          • We pay our dues come winter!

            Clouds also trap heat, if it does any good it has to reflect it.

    • water vapor is in equilibrium. partial pressure hangs on temperature.
      hotter atmosphere, more water vapor.

      • So hot deserts are very humid then ?

        More water vapour needs a source, in the humid areas the heating of the day just produces more thunderstorms as they work to cool the atmosphere.
        Ive lived in different places in the same country where a coastal area with
        seas all around has high humidity both summer and winter, another location inland at higher altitude but with multiple lakes would be lower humidity but hotter during day and colder at night.

        Even contrails at altitude vary with the background humidity

        Depending on a plane’s altitude, and the temperature and humidity of the atmosphere, contrails may vary in their thickness, extent and duration. The nature and persistence of jet contrails can be used to predict the weather. A thin, short-lived contrail indicates low-humidity air at high altitude, a sign of fair weather, whereas a thick, long-lasting contrail reflects humid air at high altitudes and can be an early indicator of a storm.

    • I asked this in a past article and the response was that the amount of water wouldn’t significantly alter things especially since it is a byproduct of today’s combustion. TBH I trust what I hear from LNA on this more than other sources so based on that it shouldn’t be an issue vs today’s aircraft

    • “The combustion of 1 kg of kerosene uses 3.4 kg of aerial oxygen and produces 3.15 kg of carbon dioxide. (CO2), 1.25 kg of water vapor (H2O) plus several other. compared to the energy content of 1 kg of kerosene, the combustion of an energy-equivalent amount of hydrogen generates only 3.24 kg of water vapor,…”

      this is from

      So yes, hydrogen produces twice as much water vapor as kerosene, but as you may have observed, even when lots and lots of kerosene propelled aircraft fly their exhaust is dissolved in the relatively dry high altitude regions of the troposphere rather quickly.

    • There is a discussion on contrails and if you don’t have combustion soot particles to attach to what will the water vapour do as it condensates?

      Should not be that hard to test and calculate with todays multi-physics software packages like Ansys, Comsol if not solved already. The US Navy needs good knowledge on weather at sea to predict sensor, communication and weapon performance during changing metrological conditions. Just having sensors seeing thru high speed boat prop massive water spray plume can be a challange.

    • There will be a solution. Hydrogen production by fuel cell or some engine cycles may be so efficient the amount of water is minimal a turbo-compounded solid oxide fuel cell for instance could be 85% efficient. The output could be condensed into water and held over till at lower altitude (maybe a flow low for part of the flight) or converted into ice (small hail stones) and released. I personally think PtL Power to Liquids is a better solution.

  4. Having the fuel close to the engines probably, as in the Airbus patent, solves part of the CG issues and also risks during crashes. Also less isolated tubing and a more practical usage of the fuselage.

    Significant hydrogen tanks in the wings seems challenging because of the shape, requirements, but behind the engines seems feasible space. It would mean a highly loaded wing though, e.g. during hard landings.

    There must be a reason Airbus projects 6 engines iso 4.. Maybe redundancy? rapid fuel dumping during emergency situations? Wing loading? Thrust / reliability of hydrogen fueled engines?

    • Those are fuel cells. Only at take off more thrust is needed, during cruise two engines could be switched off.
      The LH2 tanks on these pods don’t seem to be big, that also restricts range.

      • Dont know about highly loaded wing, the vol hydrocarbon fuel is greater than that used LH2. Previous jet versions have 4T of fuel that a LH2 version needs 2T.
        The tuboprop is even more efficient for its flight profile. Multiple pods and engines distribute the load.
        Tupulev added pods to his wings for aerodynamic reasons -and storing undercarriage. Also on Convair 990.

  5. Maybe 2 extra engines, if you needs the payload-range / redundancy. Done before (A330/340).

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