Bjorn’s Corner: The challenges of hydrogen. Part 19. Hydrogen airliner weight shift

By Bjorn Fehrm

January 8, 2021, ©. Leeham News: In our Corner before Christmas we discussed the hydrogen tank placement at the rear of the aircraft for Airbus’ ZEROe concept turbofan aircraft.

We now calculate how the weight transfer when emptying the tanks in the rear affects the ZEROe’s efficiency.

 

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

A rear tank’s influence on efficiency

We discussed the hydrogen tanks’ placement for the ZEROe turbofan aircraft (Figure 1). It has two LH2 tanks (liquid hydrogen tanks, see previous Corners why it uses cryogenically cooled liquid hydrogen, LH2) placed in the rear, Figure 2.

It’s two tanks for safety reasons. A tank can start leaking or lose its insulating vacuum, which means losing hydrogen as it boils off faster than expected. We also assumed the aircraft has two separate fuel systems that feed both engines, once again for safety reasons.

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

The drawback of the design is a varying Center of Gravity (CG) during flight, as the fuel consumed in the rear placed tanks gradually makes the aircraft more nose-heavy.

Loss of efficiency due to the center of gravity change.

Airbus has given data for the aircraft in Figure 1. It’s a six abreast single-aisle aircraft with around 160 seats in a single class high-density layout, just below the passenger capacity of the A320, and a maximum range of 2,000nm.

In Figure 3, I have sketched the aircraft to find the distance between the Center of Gravity (CG) and the consumed fuel. The placement of the tanks in the rear of the fuselage instead of in the wingboxes, as for today’s airliners, makes the design more rear heavy. On a statical level, this is fixed by placing the wing further back, so the distance between the center of gravity and the aircraft’s aerodynamic center is kept, thus keeping positive pitch stability.

The problem is the movement of the CG during flight. As the LH2 is consumed in the two tanks, the rear gets lighter and the aircraft gradually gets more nose-heavy.

Figure 3. A sketch of the Airbus ZEROe hydrogen turbofan concept. Source: Leeham Co.

We need to check to what extent such a design creates problems. We start with the influence of the CG shift on efficiency. To this, we need to calculate the increase in nose-down movement around the CG during flight.

A moment is force times distance. Our performance model gives us the weight change of LH2 in the tanks. Now we need the distances. The front tank is 8m from the CG, and the rear tank 13m. We then run the model to get the hydrogen consumption at max range (2,000nm) and the typical flight of 800nm.

The LH2 consumption at the max range is 3.3t with 0.8t in reserves. The change from 4.1t of hydrogen divided between the tanks down to 0.8t means the aircraft’s nose down moment increases by 340,000Nm. Airbus might schedule the forward to tank to take the main consumption so CG change is minimized, but there must always remain sufficient LH2  in the tank so the aircraft can land safely should the rear tank run into problems. We, therefore, assume an equal consumption in the two tanks to ease the calculation.

If we divide the increased nose-down moment with the distance between the CG and the horizontal tailplane’s aerodynamic center of 19m, we get an additional tail downforce at the end of the mission of 17,900N/4,000lbf (Figure 4 shows the main forces involved in an airliner).

Figure 4. Forces on an airliner during flight. Source: Leeham News

This increases the induced drag for the horizontal tailplane, and it also increases the induced drag of the wing as its lift force must increase with the same amount. Our performance model gives the efficiency loss from this change; it’s 1.4% for the aircraft’s max range flight.

For the typical 800nm flight, the hydrogen consumption is 1,500kg. This gives a nose-down moment increase of 154,500Nm, which increases the tail downforce by 8,100N/1,800lbf. Our performance model now shows an efficiency loss of 0.5%.

We can see that the effects on the efficiency of having a rear-mounted LH2 tank system in a domestic airliner with a maximum range of 2,000nm are marginal. The rear-mounted tank system works from an efficiency point of view for this type of aircraft. What other considerations must be made? This we discuss in next week’s Corner.

73 Comments on “Bjorn’s Corner: The challenges of hydrogen. Part 19. Hydrogen airliner weight shift

  1. A transition aircraft could be an hybridation of a current, regular liquid-fuel aircraft with a conical H2 tank in the tail empty space, and H2 injectors in the engines, to test the waters and enable a H2 infrastructure on ground.

    • A hybrid transition would be much easier and in parts even better.
      Only one bigger LH2 tank would be needed. For safety and reserves kerosin could be used and similar to the A330 pumped into the stabilzer to trim the plane.

      But Airbus chose a complete new design.
      Figure 1 is the same plane Airbus introduced Sep 21.
      Now, planes with pods including LH2 tank and fuel cell make more sense to get LH2 going.

      • You want to have a jet engine that can burn kerosene and H2 efficient? You also have to deal with the ice-cold H2 plumping. Hybridizing a A340/A380 is good for testing but i doubt that its is smart to build more than single digits

      • A question is how serious Airbus is.

        My impression is that in recent years they have a number of projects that pander to climate catastrophists and other environmental activists.

        Perhaps they are doodling in the back room as Boeing used to do, but getting politically correct PR from them.

        • I tend to agree. Although I’m glad to see that at least OEM seems to be taking the issue of future propulsion technology seriously, I also question to what extent this Airbus program is just “window-dressing”. It could be for PR purposes, or it could be because of all sorts of political pressure in the EU; for example, governments have to increasingly show that investments/participations have a “green element”, and more and more investment analysts/funds are also devoting increasing attention to the “sustainability” of corporations. Having a program like this might be a way for Airbus to tick off some sort of “sustainability list”. I hope not, but it certainly is a possibility.

  2. If you need to go-around, you have less controllability due to the nose-heavy configuration. You also have higher loads on the nose landing gear.

    What also speaks against a rear-fuse tank is that you consume the LH2 at the wings, but the tank is in the rear. So you end up with longer (cold?) fuel lines than necessary. These long lines also cost you performance (weight and “boil-off” volume).

    • Pax have more weight than LH2. 4.1t LH2 translate to 7 seat rows, so it makes sense to have LH2 tanks in the back if the tanks nearly have the fuselage diameter.

      Inside the cold fuel lines is H2, not LH2.

  3. A fwd canard and further aft mounting of the wing can help, as H2 is consumed the canard gives more lift keeping the center of lift at desired position.

    • CE is spot on.

      Amazing to see only two readers identifying so far the obvious solution to the CG travel issue caused by aft fuselage LH2 tanks: canards.

      In fact, it is also amazing that we still accept ‘elevators’ that do exactly the opposite: fly downwards. They may ‘elevate’ the nose by flying even more downwards. But that doesn’t take away the insane nonsense of elevators working against the main wing.

      We should thank LH2 for bringing about a much delayed and needed change to airliner design: upward flying canards instead of downward flying ‘elevators’.

      • “”it is also amazing that we still accept elevators””

        Elevators are used to control the plane, not to trim. Here we talk about trim.
        When elevators are not needed they are hiding behind the stab, canards always have additional drag.
        It’s also a reason why Boeing chose MCAS instead of strakes which would produce drag.
        Better to trim is the A330 stabilizer with included fuel tank, it can be trimmed with fuel weight instead of stab degrees with additional drag.

        • What I am trying to say is that we should not accept any control AND trim surfaces that fly downwards.

          Every control and trim surface on the airplane should fly upwards.

          With modern electronic flight controls, there is no reason to have something onboard, rowing backwards.

          • Yes, the Piaggio P.180 was able to reduce the area of its main wing for lower drag by having the tail surfaces for ‘lifting’ and a small fore wing. – also lifting.
            The integrated design also allowed the wing well back behind the cabin and the spars through the fuselage , a better aerodynamic location than under the fuselage for all other corporate aircraft ( except the forward swept wing on the HFB.320).

    • A loaded canard has other advantages too, like stall performance. Or put one tank just behind the cockpit.

  4. Could the engines be made dual-fuel users? Then the second fuel could be liquid, carried as the emergency reserve in the wings and pumped back into a tank in the rear of the plane as the hydrogen is burned off

    • That is pretty easy to do. You had water injection on early widebody engines like the P&W JT9D-7AW and today with 3D printing it should be even easier to manufacture. Still I think Airbus can solve it aero wise maybe having a long strake on the new slender carbo wing to body joint fitted with one tank each that work as center tank/trim tank up to the fwd cargo door. We will see what they choose maybe at a virtual Paris Air Show and vaccinated guests only at Le Bourget.

      • “You had water injection on early widebody engines like the P&W JT9D-7AW”

        Uhh… water injection cooled engine blades and added mass to exhaust flow. Tip: water is not flammable.

        Injecting fuel is more like afterburners, which are crude things that guzzle fuel.

        For your scheme you’d want dual systems to deliver two very different fuels into the combustion chambers, they and turbine blades would have to cope with any differences in combustion of each.

        • I indended to show that the injection system of 2 different liquids is not uncommon for gas turbines. Many stationary gas turbines have “wet” systems some of which is direct water injection into the combustor housing, these are for lowering the compressor outlet air temperature and increase massflow thru the turbines. In this case the issue is can one design a combustor with fuel injectors that handle both fuels either in one nozzle by mixing in JET-A1 into the LH2 flow or by using 2 nozzles like the old primary and secondary nozzles in some jet Engine fuel injectors. GE stationary turbines designed to burn a mix of H2 with fuel does it already since many years, https://www.ge.com/power/gas/fuel-capability/hydrogen-fueled-gas-turbines

    • They should be powered by coal fired boiler feeding steam turbines, more in keeping with their age.

  5. A design using a fuel-cell driven electric turbofan would generate liquid water that could be diverted into shaped ballast tanks at the rear of the fuselage, to counteract the weight loss due to hydrogen consumption. It could also generate water for consumption on board by passengers…including enough for showers 😉
    Not sure to what extent (partial) water capture could be effected in the case of a combustion-driven turbofan — it strikes me as being a hell of an engineering challenge, in view of the exhaust speeds and pressures, but who knows?

    • Zeppelins tested water condensation from exhaust gases to stay in equilibrium. fuel out, water in.

      • With hydrogen as fuel, you would keep only a fraction of the condensed water, since liquid water is 18 times heavier than liquid hydrogen. Otherwise the aircraft would get heavier as hydrogen is consumed.

        Reason being that you are accumulating oxygen in the water after combustion, rather than consuming it as with liquid oxygen rocket propellant. Conservation of mass applies regardless of the source of oxygen.

        • Bryce, that wasn’t meant as criticism, if anything it makes your idea more viable, if only 1/18 of the water vapor needs to be retained.

          That might be done with a turbine exhaust air bleed, which is routed through an internal tube along the leading edge of the wing, to act as condenser. This would also heat the wing to provide some deicing. Since the exhaust is clean (no carbon products), it shouldn’t become fouled or pose a cleaning problem.

          The condensate could then easily be pumped to a 1/16 sized trim tank (allowing for greater density of liquid water) near the LH2 tank, to result in no change in weight or CG during flight. Or if the trim tank is located farther aft, the volume of water could be even less, to reduce weight during flight and still maintain CG.

          Bjorn would have to run his aircraft model to see if retaining weight for the entire flight would kill the efficiency gain due to constant CG. But it’s a good idea nonetheless.

    • “counteract the weight loss due to hydrogen consumption.”

      Existing hydrocarbon powered jet liners lose weight from fuel burn as well. Its not the change in weight per se thats an issue ( its good for reduction drag and flying at higher altitudes), but the weight distribution. The numbers Bjorn gives show the loss of aerodynamic efficiency is minor.

      Bring back the Sonic cruiser with its forward ‘canards/lifting surfaces’ and rear wing surfaces…maybe not as fast as the original design

    • Water kunnen genereren voor consumptie?
      Dat water zou deminwater zijn, dat kun je beter niet drinken.

      Google translation
      Generate water for consumption?
      That water would be demineralised water, you better not drink it.

      • Up here in the rainy land of Keith and Scott, drinking water does not have much mineral content compared to dry areas using wells. I don’t think it is needed.

        (Seattle, Vancouver WA, and Victoria BC have protected watersheds to collect water, and reservoirs to store it in.
        Filtering and treatment is done to eliminate bacteria and parasites that come from animals in the watershed.

        Trivia: more chlorine taste in some of the Victoria BC area this month as the water system rebuilds one of its disinfection methods. Combinations of methods are used, in public swimming pools UV is often used in a section of pipe.

    • Fuel cells are by now well-known technology for hydrogen fuel.

      AFAIK a bust for liquid fuel, much money spent on reformers to create a gaseous fuel for the cells, without much success.

      Check PR of Ballard in Vancouver BC, though IIRC he’s moved on to something else as he ages.

    • if you compensate weight lost by adding water, you do not take advantage of fuel savings over the trip due fuel consumption.
      In the article it said that you only lost a 0.5% of efficiency with this method, you would lose a lot more if you keep your induced drag constant over the whole trip.
      But still, I am not a fan of Airbus ideas for Hydrogen Airliners, looks like a PR, trying to show “that they are working on it” without really doing it.
      I really think than an Hydrogen airplane could achieve much higher performance than a conventional, but they really need to break an leg and commit to a paradigm change, new designs, everything.

      • I don’t think that Hydrogen Airliners are a PR stunt of Airbus.
        Working on an LNG project a few years ago, I found a worldwide patent deposited by Airbus regarding the storage of Liquid Hydrogen somewhere by the end of 2014, that investment would be far to expensive as to just use it only for PR.
        Looking at the initial image figure 3, the LH2 tanks are placed inside the passengers ‘space’. In case of a serious calamity, despite the presence of hoses to evacuate the hydrogen in a controlled way, this would corrupt immediately the living space. It would be a serious infringement of ‘good practices’: never put fuel storage in the same zone as passengers (as in vessels there will be no fuel in the passengers area).
        The construction as shown in figure 3 would obstruct any release of certification because of these restrictions. The crash of flight 295 made it all to clear: no fuel (or burning material) together with passengers.
        Given the fact that an airplane has an active balance control system, it looks to me wiser to put LH2 tanks (about 15-20 tanks) with a smaller diameter along the central top section of the plane, separating it physically from the passengers ‘living bubble’, the outside of the tanks into the air so during the normal live of the plane, the tanks are exposed to -60°C, another fuel efficiency. Their should also be no problem to put these type of tanks in the wing area, making it part of the structural strength of the wing like an engine in a F1 car.
        Likewise, in case of calamity (a heavy tailstrike), there will be no risk for passengers and the tanks could immediately be evacuated in the atmosphere from the top of the airplane in case of emergency.
        Likewise the front and back tanks could be consumed first, gradually switching more towards the center tanks.
        A hybrid airplane should be avoided, it would create just a very heavy airplane with no advantages.
        There is also no sense of accumulating water, it would only increase the weight of the airplane for no purpose besides flushing the toilets with deminwater.

        • To Chris Van Oevelen:
          The cost for an international patent request is not more than 4000 usd, which is nothing for a company like Airbus, this type of corporations usually buy all kind of patents related to their business just to slow down the pace of innovation of other companies, also patents does not need to prove that something works or not, so it does not requiere any investment or research on the matter.
          This mean that these corporations can keep their business strategy without relay on risky investment to compete and denied at the same time the same kind of innovation that may come from other companies, without allowing the use of their patents.
          Going back to the tank placement, I remind you that most airplanes also had fuel tanks under the seats, I would bet that those tanks are not even close to the structure integrity of a liquid hydrogen tank.
          About increasing the numbers of tanks, the problem with that is that your tank surface=heat lost, increase a lot for the same total volume.
          I guess hydrogen irrational fear is being taken to the extreme for absolutely no reason.

          • A worldwide patent costing not more than $4.000???
            This is what we call ‘fake news’.

            May I kindly ask you not to write anymore such totally wrong information? Please inform yourself (maybe at Jacobacci) before writing wrong information.

            Depositing a patent is 1 cost, paying the yearly cost is another.
            Depositing a worldwide patent at the EPO will set you back about €150.000, the annual cost would be around €100.000, not to mention the cost of the Patent bureau and studies regarding existing patents. You also have to add all translation costs.

            Of course patents do not exist to slow down innovation but they exist so you would incourage you to innovate and consequently, protect own technology and cover development cost.

            About multiplying the tanks, may I remind you about the topic we are writing in: Airliner weight swift.
            Dividing the total quantity in different/separate tanks along the fuselage will solve that problem 100%, no other solution will get you to this 100%, compensating the efficiency loss of storage weigth shifting.

            The very small increase of surface is largely compensate by the shift of 1) exposing the tanks to temperatures inside the fuselage (25°C) or 2) exposing the tanks to ambient flying temperature (-60°C). Calculating a simple heatbalance will proof you that.

            The positioning of the tanks as shown in figure 3 will expose them to damage just by a heavy tailstrike.

            By difference to kerosene, there will always be a pressure in the LH2, ruptering will cause an immediate and full termination of the passengers area, not only by asphyxiation but by the certenty of an explosion in the fuselage or, in the worst case, a BLEVE.

        • The LH2 tanks are outside the pressure cabin.

          Hybrid shouldn’t be bad. A1-fuel only for reserve, so only one LH2 tank would be needed. A1 could be pumped to the back during flight to improve CG.

          • The LH2 tanks as shown in figure 3 are inside the fuselage, that would be against ‘good and common pratice’, even if they are placed outside the pressure cabin.
            Even if they were placed outside the pressure cabin, they should not be placed at the lower part of the airplane for obvious safety reasons.
            An hybrid airplane would carry the weight of a double system: double storage, double feeding system, double space necessary, engines to run from 0%-100% and 100%-0%, etc…
            That would reduce your payload in unacceptable way, not even mentioning how to find the space to put all double systems.

        • Wrong information? I give you the exact value that it takes to fill an international patent request for 150 countries for 30 month.
          https://english.rvo.nl/topics/innovation/patents-other-ip-rights-topic/apply-patent/worldwide/costs
          All the other cost that you mention are mostly optional, all independent inventors make their own research and work to complete their patents registration, after that it may require some extra cost which many I ignore, so I sorry if I mistake, but I guess most of your numbers is by hiring companies that do everything for you.
          Companies like Airbus has their own group of attorneys, for patent registration, buying other patents and suing those who use them by error.
          Even if it cost them 10.000 usd by year to maintain a patent (which is a big “IF”) for big companies like Airbus is nothing.

          About that patents does not slow down innovation:
          https://www.washingtonpost.com/news/innovations/wp/2015/03/11/heres-why-patents-are-innovations-worst-enemy/
          https://business.inquirer.net/84818/how-patents-slow-down-the-pace-of-technological-innovation
          There are also books and scientific papers about that.

          Going back to the tank discussion:
          First.. Why you dont acknowledge that increasing the numbers of tanks, you are also increasing by a lot the heat flow, you are also increasing by a lot the total tank mass which also depends on surface.

          If you just want 2 tanks, then, that is not a problem, but when you want to add 4, 6 or 10 different tanks.. then you have a problem!
          So for a efficiency lost of 0.5%, you prefer the use of pods that would increase the dry weight and heat lost by a lot.
          And extra aerodynamic loses.

          You can also have a tank that would work as fuselage divided in as many parts as you want, this way, it does not increase heat flow or mass, because the middle walls does not need to be insulated or resist any pressure, beyond the mass of the LH2 which is 70 kg by m3.

          About taking advantage of the -60C that you have at high altitude, it does not matter, because you have the same advantage with your tail tank, even if it is inside or as part of the fuselage.
          Second.. -60C only reduce your deltaT from 277C to 210C.
          I would count the extra heat lost that you can have due convection (a cooler on a 900kmh wind), of course it does not matter either because the max heat flow is given by the insulation, that is the bottleneck that matters.

          A heavy tailstrike seem harder to achieve than hitting the wing tips with the runway or something else.

          I doubt even in that case that lives would be lost.
          There are videos of similar airplanes losing the tail, and the whole section is split flat, from the passagers to the tail, even with an explossion, it would be rare to cause several damages to the passagers.
          Hydrogen seem dangerous, but in most cases is less dangerous than normal fuels.

          Even with a tank rupture over pressure, the hydrogen would not mix as fast as you think with air to produce a detonation (wave blast).
          LH2 first needs to evaporate to combust, and the little amount of H2 inside the tank has around 4 times less energy by volume than natural gas.

          The kinetic energy of H2 escaping is also low, it weights 10 times less than air.
          Hydrogen diffuse super fast on air, it rise at 20 m by second, it burn going up, not at floor base as any other fuel.
          So passagers are more safe from burning and asphyxiation than with other fuels, which combustion also produce co2.

          • Mr. Panelli, please go back to your own article of reference and do not stop reading the article just after reading the title.

            The $4.000 in the title you are refering to, is the annual cost ONLY to maintain the patent in ONLY 1 country!
            Not the cost of a either depositing or maintaining a patent in the world.

            Read further down below in your own article: “The costs for the whole PCT-procedure (application, European and national phase and costs for a patent attorney) vary from € 50.000 to € 100.000.”

            This is the cost only for a Eu patent, depositing a worldwide patent will set you back a multiple of this mentioned €100.000, not to mention the maintaining annual cost.

            Moderator, can you please remove this insistingly wrong and misleading information about a worldwide patent costing $4.000.

          • @Chris: You have refuted the error with facts. The post you reply to doesn’t violate Reader Comment rules, so I will allow it to remain.

            Hamilton

        • to Chris Van Oevelen:
          I would edit my own comment if I could if such “misleading” value bothers you so much, but you are really taking all the discussion out of context.
          My first comment was that apply for a “patent request” cost 4000 usd.
          Which is not wrong.. patent request is not equal to have a patent approval, your patent idea over those 30 month will be defended in case you finish your patent applications and fees on all the countries.
          Now.. in the second comment I even said “sorry if I ignoring some of the cost”, acknowledging that I ignore much of the process (although I was interested 5 years back in apply for a patent on my own).

          But all this patent discussion started when you assumed that if Airbus had applied for a LH2 storage patent in 2014, it means that they really want to research and develope this technology, which may not be the case!

          Airbus has 37000 patents (from their official webpage), Then according to this site:
          https://simpleflying.com/folding-airline-seats-airbus/
          Airbus apply over 700 patents by year, when only moves forward with a handful of them.
          So.. only from those numbers (ignoring how much they pay to register each patent) is clear that patent application is not always equal to product development.
          Now if we consider the strategic advantage to block most competitors to use the most obvious tech pieces to develope a new technology, it means that even without risking investment on their own, they have certain security that others would not develope a disruptive technology that can compromise their current business plan.

          If they are doing that or not, I can not prove it.. I just suggest that “it seem” like a PR stunt from my perspective, because their original idea and values seem way far from the possibilities, and because now they are taking into account the hydrogen pod approach, which all logic points that it would perform much worst, unless they want to develope a small propeller plane.
          So all that makes me the idea that is a small group of people making brainstorming, not a whole company committed to develop an hydrogen airline.

          And if you are so committed to the truth that you need to call for a moderator.. Why you did not acknowledge all the corrections that I made to you on my last reply?
          About the surface-volume ratio issue, your -60 degrees that you claim as advantage for the pod approach, hydrogen properties, etc. (it was a long post).

          Have a nice day.

      • Forget about this first ZEROe version in figure 1. This was their first thought, an A220 fuselage but they say with 6-abreast seating.

        The pods with included LH2 tank and fuel cell make much more sense. Maybe not for an 160 seater, but for everything below it.

        • To Leon:
          This is another proof that Airbus did not even wasted much time on research on their previous Hydrogen initial plans if they are already jumping to the hydrogen pods idea, which is not even their idea!
          The first time I saw this was 3 years back in the Horizon hydrogen page:
          https://www.hes.sg/element-one
          But as you said, this only has relative applications for small airplanes, because you only need to certificate the individual pods, then airplane designers just need to include as many of them as require.
          The refill process can also be easier, replacing pods instead filling a tank.
          So if Airbus now are looking into this, I can only imagine that the hydrogen team is just 3 or 4 guys taking ideas from internet and making some power point presentations to give the idea that they “are working on it”.

  6. Another solution to the weight/balance problem is asymmetric tank design. For, example, 2 opposing “D” shaped tanks over the wing and in the traditional cabin space would solve the balance problem and still allow a walk thru from forward cabin to aft cabin. Further development of modern materials, such as carbon graphite, could make this feasible. We are thinking into the future here.

  7. How much could the wing design without intergated fuel tanks be improved?

    • Not a lot I’m afraid. We will see. The modern transonic profiles allowed quite fat profiles that hold a lot of fuel in the wingbox so I don’t think wings like the 787/777X and A350 had a lot of compromise in them. We will see an increased aspect ratio (more span and shorter cords) but that is just normal evolution, the present fuel in the wingbox didn’t impair that.

      • Though I think we could see new wing architectures that enable simpler and lower cost construction. Multi-spar configurations that have very few ribs. Also weight savings from deleting access panels every few feet between each rib. Also weight savings of deleting fuel sealant in the wing tanks. It’s probably not an insignificant savings even if the wing doesn’t get much thinner. Oh and rear spar placement is less constrained (not competing with fuel volume) so your control surface actuators have more room to thin down and minimize fairings, reduce drag.

        • The DC-8 was one of the last airliners to use 3 spar wings. It didnt compete with Boeings lighter ‘more flexible’ 2 spar approach. It did mean the DC-8 especially in cargo versions had a longer service life.
          Without a fuel load in the wings and in flight variable load alleviation I cant see the need for more spars in stiffer wings like they do for fast military jets ( including sinusoidal spars)

      • I think you will see much different wing designs inboard and outboard of the engines. You still need wing beams to hold the powerplant and make room for systems routed to/from it. Outboard of engines you can have superlight and efficient wings like Fig 1. (they do the transition further out), hence center/trim tanks can be fitted into the extended wingbox if required. Still more elegant with small canards front of the fwd cargo door. It will have its issues like require deicing and failsafe actuation ( Dassault, Saab and BAe knows how to design it)

  8. Is this effort worth the potential benefits? Commercial aircraft generate about 2-3% of total CO2. Assuming hydrogen turns out to be viable power source it won’t put a dent in that number until decades in the future.

    • Not quite, they put out a lot of CO2. Each pound of Jet fuel puts out 3.2 lb of CO2 and we burn up to 100,000lb per flight. Aviation represents 2-3% of the world’s CO2 problem, this is where the 2-3% comes from. As such, yes it would be smarter to start somewhere else where the gains are easier and cheaper to achieve, but as we have discussed air transport is so visible, it’s in the public eye.

  9. I don’t get this push towards hydrogen. Elemental hydrogen does not exist on earth. It takes copious amounts of fossil fuels to generate hydrogen. Then it takes fossil fuels to pressurize and store hydrogen. You also lose hydrogen over time in transport. Then you use the hydrogen to generate electricity. There are multiple conversions going on here, none of which is 100% efficient. BEV will win out, especially if you cover the outside of the plane with solar panels.

    Hydrogen eventually always escapes containment, which is why rockets are fueled up just before launch. If high tech aerospace hasn’t solved the hydrogen storage problem, I just don’t see it happening on a consumer level.

    Then, you essentially have the most explosive substance on the planet a few feet away from people, what could possibly go wrong.

    Hydrogen is a just a way for big oil to remain relevant and keep their refineries and logistics infrastructure operational.

      • Perfectly right!
        Solar panels have an awfully low efficiency, around 100W/sq meter (even weaker as panels are not perpendicular to sun light)
        Solar impulse could fly because it was awfully light, with huge albatros wings, and it flew at a very slow speed
        Could not fly in storms, or carry passengers anyway

    • Check back on earlier parts on the ‘explosion issue’
      Similar arguments were made at the dawn of the electric age about AC power in homes and ‘the danger ‘ … and electrocution is an ongoing problem, but its not a deal beaker

    • @ Luje
      It might help your understanding if you stopped viewing hydrogem as a “fuel” and instead viewed it as a mobile and convenient “energy storage medium” that doesn’t suffer from the significant limitations and materials-related disadvantages of batteries. Hydrogen is already being rolled out for ground transportation in Europe, and it is quickly gaining traction and interest.

  10. I agree with the figure abose stating that commercial aircraft generate only 2 to 3 % of total CO2 ( total means: for the entire world).
    We must keep aware that these 2 to 3% , as far as I know, is only for the flying of a small group of “wealthy travellers”, mainly in “western countries”.
    This figure will much increase when other people, from presently less developped contries ( India, Africa, South America, ..) will be able to reach the same level of developpment and will demand also their “right to fly” (as they are certainly entitled to).
    In my opinion, developement of CO2-free commercial planes, despite all drawbacks and technical difficulties, is required.
    This task must also include the manufacturing of LH2 with CO2-free despite the low energy efficiency: use nuclear or even wind power / solar (where the problem of intermittence is less crucial)

    • Building traditional planes is expensive enough without integrating exotic tech. Developing hydrogen power will certainly increase the cost of flying enough to depress air travel for the wealthy (I guess that includes me who depends on a paycheck) and especially those who are not.

      And to what end? Realistically, what are the chances of reducing from 3% to 2%, including building and maintaining the required infrastructure, in the next 50 years?

  11. Bjorn, you continue to imbibe the anti-human climate catastrophist scam that humans are causing runaway climate warming that is not happening, did not happen in the Medieval Warm Period when Vikings farmed southwest Greenland, and cannot happen because the saturation effect of the overlap of absorption-emission spectra of carbon dioxide and dihydrogen monoxide limits temperature increase to a small amount most of which has already been realized.

    Why?

    • Even the IPCC agrees with the saturation effect, but theorizes that there will be a positive feedback from the small rise in temperature – but the atmosphere is not obeying the IPCC’s theory, and climate was stable in past warm periods.

      (Note that the claimed 2 degree threshold of disaster was an arbitrary figure from an alarmist decades ago, activists keep burping it up but even the infamous Phil Jones of the CRU of East Anglia university agrees it was arbitrary. There’s been sleight-of-hand recently to justify the figure by not going back far in history.)

  12. Just wait for the first crash. 150 people having been frozen rock solid will not inspire confidence. It will make the Hindenburg seem like a campfire!

  13. It would be interesting to know what the reasons for propsing this design and not using a canard like on the Piaggio P.180 or Beech Starship. All I can find is a reference in the study presentations that nonconventional configurations does not have decisive benefits but no further reasoning. Maybe we’ll get there in the later parts of this series..
    It seem to me these configurations would reduce or remove the necessary downforce on the stabilizer.
    Also these planes being pushers would put a heavy part, the engines, behind the tanks effectively placing the tanks more to the middle..
    The third flying wing concept shown by Airbus would also have similar benefits.

  14. NASA studied the behavior of 1500 to 1800 gallons of LH2 released suddenly in a spill. It formed a ground pool from 18 to 25 feet in diameter, and evaporated in under 40 seconds.

    About a third of the latent heat of vaporization comes from the air, dramatically lowering the local air temperature. The rest comes from the ground (or other objects in contact), meaning there is a substantial freeze hazard.

    The rapid evaporation of LH2 includes an expansion in volume by a factor of 843. This would displace oxygen and create an asphyxiation hazard, although brief. If confined, the resulting pressure would burst any containing structure.

    If a source of ignition was present, the high flammability range of hydrogen would make fire likely. Detonation is unlikely due to oxygen deprivation within the hydrogen cloud.

    The fire would burn invisibly around the periphery of the plume cone, and accelerate the evaporation and rise of the plume. The flame front will advance through the plume within several seconds, essentially being a flashover event, but being sustained by the ground pool until evaporated. The heat release is on the order of 10 to 100KW per square meter of flame front.

    So for passengers exposed to an LH2 spill, the sequence might be rapid but brief exposure to cryogenic temperatures, freeze burns, and an inability to breath, followed immediately by equally rapid and brief exposure to flame and high heat flux, with heat burns and inability to breath. All taking place over the span of 10 to 30 seconds, after which the hydrogen is dissipated or consumed.

    So it would seem that safety design would require secondary vented containment (perhaps a very tough & tall surrounding tub with internal vertical baffles) that would allow the LH2 to spill, but not slosh, and then rapidly vent vertically from the entire area of the tub top. Perhaps with a tertiary containment bulkhead between the tub and passengers, that was resistant to pressure or penetration if the tub fails or shifts forward in a crash.

    The NASA study showed that contact surface area was the limiting factor in absorption of latent heat of vaporization. So perhaps the tub would have a pebble bed of fixed porous beads that could both source the evaporation heat, and sink any combustion heat, to accelerate the venting. Without being blown out by the rapid expansion of gas.

    NASA also found that wind could translate the (possibly burning) hydrogen plume for 20 to 60 feet from the release point before it dissipated (again a matter of a few seconds). It is also rising rapidly during this time, so the release point must remain above the airframe.

    This makes me think of the A-10 cockpit protective tub, with explosive canopy thread to shatter it prior to pilot ejection. Expose the tub top if there is a crash or spill. Or something on that order.

    The tub would be designed for the worst-case spill scenario, but would also handle the more likely case of a leak or loss of tank vacuum. In that case a venting tube would be sufficient, as shown in the LH2 aircraft images

  15. A well thought out and proper technical paper. But let’s leave aside the technicalities. Look at the reduction in passenger accommodation. That’s hardly what airlines want. But it makes sense to make pressure vessels (as will be required for hydrogen) axisymmetric in geometry and therefore inside the fuselage – the more box like present tanks in the wings are not well suited to taking internal pressure.

    Perhaps all the fuel in the fuselage and the passengers in the wings!? Flying wings?!

  16. Why the tanks can not be part of the fuselage?
    Looks like that is one of those questions really hard to answer on detail, the same than why they require to be pressurized to 3 bar, when the role of that extra pressure just allow to rise only few degreess the boiling point of LH2.
    The only reason I can imagine for those design choices, is if they are following cryo tanks standards regulations that were overestimate for terrestial use, when there are not weight constraints.

      • I already read that, I already discuss on those threads..
        I already read several papers (the ones that are free to download) on liquid hydrogen storage.
        I already made comparison with small liquid hydrogen tanks without pressurization used on quadcopters that fly over 13 hours.
        This mean that just increasing scale and taking advantage of the surface/volume ratio, keeping all the other parameters the same (insulation, pressure differential, etc), it would be able to keep it in liquid form over days.
        Even in the case that you require a tank able to resist up to 3 bar, it would not be much different from the structural requirements of the fuselage, so.. why the tank can not be part of the same fuselage?
        Why evaporation is such a big issue if you can use it to power all the airplane systems with a fuel cell, or just vent or burn the extra hydrogen, which is negligible in comparison with the fuel consumption.
        None of these factors are discussed or explained.
        I can not understand why if you go all the trouble to use hydrogen in liquid form, you can not take all the advantages on tank scale and weight reductions that a liquid state allows you.
        I cant not find the theory explaning why a good designed LH2 tank for an airplane who does not need to keep its fuel over more than 48 hours, needs to be so heavy in comparison to its LH2 content, not from the theory perspective, neither from correlations that we see on practical experiments.

  17. “”why the tank can not be part of the same fuselage?””

    I think a separate tank is not so bad for a start. A sepatate tank doesn’t need to be so heavy, we talked about this already.
    This shouldn’t take 14 more years.

    • not remember talking about that.. but it would take more time trying to mitigate the issues of a bad idea (in my opinion) than implementing a good idea from the beginning, maybe from the legal framework could be some truth of that, but I highly doubt it.

      • I meant you and me, we talked about this and I agree, the LH2 tank must not be so heavy. LH2 needs to be cooled to keep it liquid, sure there is an H2 section in the tank on top of the liquid but it doesn’t need to be kept at high pressure.
        All together, LH2 and tank should be lighter than A1 and wing tanks, so there is something to gain, but we lose fuselage space and in this case cabin space.

  18. ah ok, now I remember.
    Yes.. and in the case there are legal regulations or physsics interactions that we ignore which prevent to improve tank density for LH2, I would love to know them in detail in order to know if that is the limit of the technology or not.

    About the extra volume required, I dont think that is a big issue, taking into account that just increasing a 25% of the tank dimensions, you double the volume.

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