There are a lot of people commenting on how difficult this will be to achieve, but it’s worth keeping in mind that even the “conventional” roadmap for improving aircraft efficiency is full of enormous challenges.
Some of the proposed technologies include variable-pitch turbofans, laminar flow wings, blended wing body aircraft designs, all of which are risky R&D projects which might cost tens of billions of dollars to bring into production.
Just an incremental improvement like the last GE9X engine cost several billion to develop, for a 10% fuel efficiency improvement.
While these hydrogen concepts are incredible risky, it’s worth putting into context how difficult it will be to get to the next level of efficiency in aviation even without them.
Sometimes it can be better return on investment to try a new approach than to thoroughly optimize every detail of the existing approach.
Edit: but, of course, complete failure is now possible when trying a new concept: it may not work at all. On the other hand, when making 1000 small optimisations the 100 unsuccessful ones can be left out.
Nah. A new approach is not what the investors want to hear. They want their dividends tomorrow. Better to squeeze an old design until the last drop of profit was extracted from it. No matter if crashes a few times in the process. You can always boast about the Jedi tricks you played on the (corrupt) "certification" authorities, and blame the crash on poor 3rd world piloting skills.
Your definition of complete failure is something making it to production and catastrophically failing. I would have imagined complete failure in this case as being significant R&D expense with no returns.
Exactly. This is how we progress, by identifying things that are really worth doing, despite being hard and risky, because the payoff is worth the investment.
I am very skeptical they will be able to get the efficiency numbers to a place where it makes sense in the timeframes they're throwing around unless oil goes up like 4x in price. Storing hydrogen is really hard and the related tradeoffs are much tougher than with a room temp liquid hydrocarbon. I imagine burning it in turbines also comes with some new issues but I know less about that. I think it's a decent path forward if we can figure it out, and I imagine the engine related problems are likely much more surmountable than the storage issues.
There's also the related fuel infra that will need to happen before this is viable. It's far away unless something truly drastic changes the calculus on fossil fuels. (which would be great)
Storing hydrogen underground is, in fact, very easy and cheap. And, storing liquified hydrogen is likewise cheap. We can expect hydrogen to be produced on-site at large airports from power delivered over transmission lines, stockpiled against low-wind low-sun periods.
Once hydrogen-fueled aircraft enter a particular air freight market, kerosene-fueled craft will be unable to compete, because with LH2's much better mass-energy density, the fuel mass dispensed with may be replaced with paying cargo. Furthermore, hydrogen fuel produced on-site electrically will be cheaper per unit joule than mined, refined, and transported kerosene.
I am surprised to see designs with inboard LH2 tanks. I would expect to see LH2 tanks in wing nacelles, minimizing piping to engines.
Producing hydrogen is not very energy efficient. Electric power would have to get much cheaper to make hydrogen cost competitive with fossil fuels. While hydrogen has sightly higher mass-energy density, the tankage is larger and more complex so the total mass savings is limited and there's a significant drag penalty.
> While hydrogen has sightly higher mass-energy density
It’s not slightly, it’s three times as much, or more.
Per wikipedia [1], Hydrogen has an energy density of 120 MJ/kg vs 43 MJ/kg for kerosene. That’s if you burn it outright. If you put Hydrogen in a fuel cell, and the output of the reaction is liquid water rather than water vapor, then the energy density is 142 MJ/kg.
The mass density of hydrogen is very much negated by the fact that liquefied hydrogen needs to be in a 700 bar pressurised tank whereas kerosene is happy at atmospheric pressure. The more valid comparison is "hydrogen in a hydrogen pressure vessel" to "kerosene in a tank".
I know cars are different, but the Toyota Mirai is actually heavier than the Tesla Model 3 even though it only needs a measly 5 kg of hydrogen to go as far as the Tesla.
"FlyZero has concluded that green liquid hydrogen is the most viable zero-carbon emission fuel with the potential to scale to larger aircraft utilising fuel cell, gas turbine and hybrid systems. This has guided the focus, conclusions and recommendations of the project."
And what happens if ATC directs the airplane to hold on the runway for a few hours? The space shuttle used cryogenic fuel, and after loading they had to continuously replenish the liquid hydrogen due to leakage and evaporation for several hours until launch. That can work for a rocket with a huge operating budget on a dedicated pad but I don't see that working at a busy commercial airport.
Jet fuel is much more stable. It can sit in simple fuel tank for months with no special cooling or containment measures.
LH2 in tanks boils off and is vented. There will be different procedures for LH2 vehicles on hold than today. That might involve plugging in somewhere for active cooling while on hold. Or, it might just mean you load on enough LH2 to make up for losses while holding.
The practical merits of Jet-A fuel will be traded off against the value proposition of LH2 fuel. LH2 will win easily where volume is high and profits are greatest. So, marginal routes and uses will be the last to switch over.
The space shuttle, by the way, could not afford insulation on its external LH2 and LOX tank. We may assume that airliners' tanks will have vacuum- or aerogel-insulated tanks.
Efficiency of electrolysis will only ever improve. In the meantime, it can be produced and banked preferentially during periods when power is cheapest.
I don't think they burn the hydrogen at all. Rather, it's turned into water and free electrons by some chemical process whose details I totally forgot.
One idea I've seen floating around would be having small scale nuclear plants near airports producing hydrogen from water and electricity (the reverse process of what happens in the plane turbines).
You’re talking about fuel cells, which are only used with the smallest (prop) design. The two larger jets are intended to have hydrogen turbines, burning hydrogen and producing water.
I love the idea of small modular reactors (SMRs) being used to produce the hydrogen from water. We also need a vast buildout of safe, clean, dependable modern nuclear reactors to provide the developing world with enough energy to make the demographic transition (via a higher standard of living) to better environmental practices.
Yes this has already been tested in existing commercial turbine engines and works fine. It's safe and won't require any changes to aircraft or aviation infrastructure.
The sticking point is the cost of manufacturing the fuel. It will only be economically viable with some major government intervention.
Synthetic kerosene as jet fuel seems to me to be a far more pragmatic approach, it does not require the development and airworthiness certifications of entirely new fuel handling and storage systems, and it does not make obsolete the entire stock of existing aircraft.
With all these “green” airplane startups I’m so completely unconvinced by the “electric” battery powered and hydrogen powered ones. The energy density for the battery’s and safety for hydrogen just isn’t feasible.
I believe the only carbon neutral route forward for air travel is a carbon natural synthetic hydrocarbon fuel that is as close to current fuels as possible. It gets us the energy density needed and builds upon the 100 years of development the engines have had.
Siemens Energy and Shell are both putting significant energy (pun intended) into R&D around these.
I’m working on my private pilot license. Recently learned the most common AV fuel in the US is 100LL. 100 octane Low lead. That’s right, it has lead!
Commercial jet fuel doesn’t, but all those small planes you see running around (and that I’m training in) are running leaded gas. It was supposed to be phased out by 2018, but reasons…
As others have mentioned here, it is difficult to keep contained, and because it's not very dense at room temperature/low pressures. So the example FTA given is cryogenic cooling. Because of the extreme cold, this can lead to new failure modes for materials.
The other issue is that hydrogen when it burns has a very pale blue flame that can be difficult to impossible to see during daylight. It also apparently has a low radiant heat, which makes it difficult to feel from a distance.
This makes firefighting and safety a major concern. The first you might know of a hydrogen fire, is that you're suddenly on fire yourself.
In confined spaces, mixed with air within a range of concentrations, it is explosive.
So, hydrogen systems will need continuous positive airflow around them so that the partial pressure of any leaked hydrogen is kept at all times well below the point of ignitability. You start with the assumption that hydrogen will be leaking everywhere (because it does) and design so that it doesn't matter.
> Although the fuel cells only emit water as a byproduct mixture of liquid and vapor, the ATI adds that a water storage system was added. This was done to avoid the potential for creating precipitation during initial climb or leaving liquid water on the runway.
What's the problem with liquid water on the runway? Can't most planes fly in heavy rain?
Takeoff and landing distances are based on surface conditions, and those conditions have to be known for pilots to make accurate decisions for the type of aircraft they are flying. Presumably unexpected water on the runway (which might later freeze, too) would cause the state to deviate from what was expected.
For a particular plane, there are some runways that are long enough for takeoff when dry, but not (say) safe for takeoff with water/ice because you cannot abort safely.
This is less of a thing for planes flying back and forth between large commercial hubs (since those same aircraft fly every day, in whatever conditions, they are generally sized accordingly). For a military heavy-lift transport, though, this becomes more of a thing.
Dumb question, feel free to ridicule me: Most aircraft keep fuel in their wings, but said wings are thin because fuel is not under (high) pressure. Why can't we have thicker wings, or thinner tanks, that can accommodate hydrogen vessels?
Thinner tanks would have to be strengthened and heavier because they're under pressure and the non-spherical shape leads to higher stresses. It'd be like building a main cabin shaped like a square; the volume to weight ratio would suffer. Thicker wings... this would have major reductions in performance. For cruising speeds of today, thinner airfoils are favored for low drag at supercritical speeds. Airfoil design is delicate and minute changes in the shape can dramatically affect the performance (operating cost) of the entire aircraft.
Biggest tradeoff in commercial aircraft design is operating cost vs. everything. Anything can fly with a big enough engine on it, even a lead block. That's not the challenge. Minimizing cost per passenger-mile though is everything.
The answer is air resistance which increases with the third power of the object speed. Airships are fairly slow which allows this type of design. Passenger or freight jets are supposed to be fast, this being their major appeal over ship and rail transport.
I think for safety and convenience, they're likely to use the upper portion of the fuselage for storage. That way, if there is a leak... it goes up and out.
The article describes “dry” wings which allows for more flexibility and some other benefits. The fuel is stored, depending on the model, under the fuselage or at the rear of the aircraft.
Reading NTSB reports on (fatal) GA airplane accidents, most of them involve the plane burning down on impact. I'd say the current state of affairs isn't very glorious in term of fire risk, for planes using AVGAS.
Cryogenic, or some hybrid. If insulation involves some actual gap I'd imagine that reliable methods to detect any cookoff failures would be simple and having access to a near sonic airsteam should give plenty of emergency venting opportunity. You'd still have a serious problem, but more of the ETOPS kind than of the fireball kind.
Not under pressure. Liquified, at atmospheric pressure, and vented to make ignition of leaks impossible. There is a minimum partial pressure of hydrogen that must be exceeded before it can ignite. Keep leaked hydrogen below that, and it is harmless.
Unless you mix oxygen in, hydrogen will just burn, not explode. 2/3 of the Hindenburg crew and passengers made it out alive, despite the visual impressiveness of the fire.
Unlike jet fuel, it won't burn very long. Crashing one into the WTC on 9/11 would likely have done substantially less damage.
> Crashing one into the WTC on 9/11 would likely have done substantially less damage.
That's actually an interesting thought. Had the burn not been as constant, would the towers have survived? And then even if they survived, how in the world would it have been repaired?
The WTCs collapsed because of a myriad of reasons, I doubt very much that if an equivalent hydrogen powered jet had crashed into them the outcome would have been different. iirc the general consensus is that many (most?) skyscrapers would continue to stand after being hit by a plane in the same manner as the WTC towers, and specifically the all-steel no concrete construction combined with some other problematic factors in the design was the reason for the total collapse.
North Tower, probably. Hit upper levels. South Tower, mid-level, not a chance. It’s crazy it survived the impact. If the wing spar had gotten the corner it would have gone immediately (the video shows it tearing through the structural facade on the east side, but missing both corners).
The exterior provided 50% of the structural support.
Between safety factors and the fact that the structures were build before widespread cad and simulation software I would expect 50% or even less to be fine. What really got them was the floor collapse.
You know the famous pictures of the Hindenburg with all those visible flames? Well hydrogen burns with an invisible flame. The Hindenburg disaster was mainly due to other flammable materials.
To be fair a kerosene plane is also a flying bomb.
Some of the proposed technologies include variable-pitch turbofans, laminar flow wings, blended wing body aircraft designs, all of which are risky R&D projects which might cost tens of billions of dollars to bring into production.
Just an incremental improvement like the last GE9X engine cost several billion to develop, for a 10% fuel efficiency improvement.
While these hydrogen concepts are incredible risky, it’s worth putting into context how difficult it will be to get to the next level of efficiency in aviation even without them.