Bjorn’s Corner: Air Transport’s route to 2050. Part 6.

Bjorn Fehrm

January 24, 2025, ©. Leeham News: We do a Corner series about the state of developments to replace or improve hydrocarbon propulsion concepts for Air Transport. We try to understand why the development has been slow.

We have covered why the progress of battery-based aircraft is slow and also described what to expect at the end of this decade and the beginning of next.

Now, we look at hybrids, an inherently more complex design. Upstarts are changing to hybrids after realizing that battery-only aircraft will not have useful range this side of 2030.

Figure 1. The Heart Aerospace ES-30 has passed the phases in the article. Source: Heart Aerospace.

Hybrid eAircraft and eVTOLs

In last week’s Corner, we examined the operational utility of battery-only eAircraft and eVTOLs. The designs were restricted to VFR trainer missions near the airfield, nine-place commute hops of 100nm, and eVTOLs to the 10-minute Airport to City Center shuttles in fair-weather environments.

As upstarts realize there is no way around these problems this side of 2030, which usually takes about two to three years (we call it the battery-electric to hybrid incubation time), the next step is to look at adding a thermal engine + generator “Range Extender” to the design.

This is the path of almost all battery-eletric upstarts, so also Heart Aerospace, Figure 1. It takes a year or two for the upstart to realize that one range extender reduces the design to a single point of failure, so two are needed.

Dual Range Extenders

Once it has applied two range extenders and accommodated the doubled systems of a complete battery-electric propulsion system combined with a dual hydrocarbon-fueled gas turbine system, which includes dual generators with charging regulator electronics, the aircraft’s complexity, weight, and cost become problematic.

When the aircraft efficiency calculations then show the complex, heavy aircraft has higher sales and operating costs than what it replaces, despite a low electric energy cost (which is not that low once the airport charging infrastructure is amortized over the energy price), the search for a better alternative starts.

The reason is that the battery-electric eAircraft or eVTOL is a serial hybrid (Figure 2), which is inefficient by design.

Figure 2. The different battery and hybrid propulsion concepts. Source: Leeham Co.

The path from the Jet-fueled gas turbine over the generator and battery charger/inverter into the battery and then back through the propulsion inverter and motor to the propeller has a path loss of ~20%.

So, the range extender cannot really be used as a “range extender,” as the CO2 emission values and total efficiency tanks.

Why can’t the gas turbine generator be coupled directly to the electric motor? It can, but then it must have the same size, including the path losses as the cruise propulsion system, and you are better off coupling it directly to the propeller. This is what you do in the next concept.

Parallel hybrid

So, after a “Range Extender” hybrid incubation time of about two years, the design team looks at a parallel hybrid, Figure 2. A parallel hybrid’s design efficiency can be higher than that of a serial hybrid. But the complexity increases, forcing development and production costs above the aircraft it shall replace.

As long as the parallel hydrocarbon-fueled part is used to provide mission reserves, the aircraft has the low emissions of a battery-electric aircraft. However, the gains in this mode are marginal compared to a simpler battery-electric concept.

The reason is, once again, the batteries. Theoretically, you can now empty them on the mission part to 0%, as you don’t rely on them for reserves. But batteries shall not be emptied below 25% State Of Charge (SOC), or you need to replace them several times a year, to the tune of around $75,000 for a battery modules renewal for a 30-seater like in Figure 1.

So, regardless of how hard the upstarts calculate different mission profiles with the gas turbine part on or off, the operational economics in real-world use, where the gas turbines have to be on and to consume fuel (thus generating emissions), the complex and heavy hybrid aircraft fail to be the low emission deal. And its operating economics fails to convince.

The turn to hydrogen.

The above trip through the realities of the hybrid for eAircraft and eVTOLs has made many projects move on to hydrogen-fueled propulsion concepts after yet another incubation time. We will look at these in the next Corner.

Note: So why do hybrids work for cars and not eAircraft or eVTOLs? In the car case, you have energy recovery when braking for the corner or stop light. There is no energy recovery phase in a flight mission.

37 Comments on “Bjorn’s Corner: Air Transport’s route to 2050. Part 6.

  1. Too funny to see how down to earth aerospace professionals like Bjorn start to recognize patterns in the development, failures and next steps of e- & H2 aerospace entrepreneurs.

    It is indeed possible to see trends, because there are so many start-ups and almost all of them fall victim to an environment of denial & tech optimism realizing their dreams and convincing investors.

    Over the last 10 years many pointed out and substantiated batteries are and will be the problem. That didn’t stop billions being invested in battery powered aircraft and people who know better singing along.

    Nobody wants to be a nay-sayer, show stopper, old school pessimist. Entrepreneurs, innovators attract respect and money! Poor stupid investors..

  2. Yes with hydrogen fuel cell and/or hydrogen burning engine (jet or piston) you can use a smaller, lighter and cheaper battery. (In theory it can be ejected if failing and burn up on the way down). You probably start with Toyota Mirai types of H2 tanks or Mercedes/Volvo coming Truck hydrogen tanks. The fuel cell can provide heat for cabin and anti-ice and feed the same electrical motor as the batteries feed. Still certification rules are not yet developed for this powertrain setup I assume, maybe after Airbus coming hydrogen burning Passport flight trails.

  3. With respect, I think this entire line of discussion is mistaken. Current and imminent battery energy densities can overcome the obstacles Bjorn finds, if the designer first pays far more attention than he assumes to improving mass and lift/drag, using proven methods and technologies. Kindly see Moore & Fredericks of NASA Langley (2014), “Misconceptions of Electric Propulsion Aircraft and Their Emergent Aviation Markets,” American Institute of Aeronautics and Astronautics 2014–0535, 52d Aerospace Sciences Meeting, Jan 2014, https://doi.org/10.2514/6.2014-0535. The just-retired chief scientist of NASA Langley, Dennis Bushnell, also surveyed some of the design space in 2021 at https://ntrs.nasa.gov/api/citations/20210021985/downloads/NASA-TM-202100219851FINAL.pdf.

    These theses are elaborated in my annual Stanford lecture on “Fossil-Free Flight” (latest 28 May 2024, https://static1.squarespace.com/static/661ed88a9b053e2b6cb43b04/t/66aae003fb619572d082fc56/1722474510503/StanfordExploreEHouse_FossilFreeFlight_85m_28May2024v.pdf, next scheduled for 1800 20 May 2025). Stanford’s two most senior professors of aeronautics/astronautics noticed no errors in the 2024 lecture.

    Bjorn is repeating the trajectory of the many car experts who said two decades ago that electric cars would be too small, short-range, costly, and unmarketable to succeed. Now, with >40M on the road, they’re a fifth of the world auto market and set to take the rest. Again, the solution wasn’t just better batteries and scale-up, but whole-platform physics…with another 2-3x improvement still available (please see my SAE paper at https://doi.org/10.4271/13-01-01-0004). Tesla’s EVs like the Model 3 weigh no more than the average US car, despite being made of metal rather than advanced composites (which needn’t cost more, as proven by BMW’s i3).

    Bjorn knows a lot about many things, but isn’t always right. I suspect that if he were right on this topic, ≥80% of the world’s top 20 aerospace firms wouldn’t be working on electric aviation, nor would McKinsey find (a year ago) more electric than fueled planes on order worldwide.

    • As 2030 is not that far off, you can guess its 2040 at the soonest.

      Batteries have been around for 150 years? I don’t expect sudden break through and the people that get excited about it are going far overboard, they buy their own hype.

      Governments are making a push for battery powered cares and in a lot of cases they work fine. But if not for tax breaks, you would not see the sudden adaption.

      It hard to compete against a convenient well built infrastructure like fuel stations (and fast) vs chargers that take an hour or more for a full charge and the parking needed to allow that extended charge up time.

      • Amroys main point is that todays aircrafts are not optimized for short range flights that they fly at great frequency, optimizing the design for 1-1.5hr flights with a good passanger load requires a new design with a much better L/D number using todays technology like carbon fiber and thermoplastic fuselage/wings. Still aircrafts and boats differ to passanger cars that they have a much higher constant load and no regenerative braking charging. One step towards this is the Jetzero/Northrop Z5, still $bn’s required to get a commercially certified aircraft from it. Maybe Embraer could join the team if the numbers turn out to be right.

        • Pedro this confirm the slow pace. People always refer to the most advanced experimental batteries in laboratory circumstances, ignoring all other required specifications.

          We can collect a busload of professors & articles (without actually reading them) to create perceptions and expectations. And we did.

          Commitments and partnership with top Brands further convinced the public/ subsidizing governments.

          Now the unavoidable regulatory & battery realities (finally) kick in and most e-powered aircraft companies file for bankruptcy. Because the expected, hoped for, promised battery improvements and certifications just aren’t there. And won’t be there in the coming years, if ever.

          https://www.msn.com/en-us/technology/tech-companies/mercedes-backed-volocopter-files-for-bankruptcy/ar-AA1wHUa5?ocid=BingNewsVerp

          • OTOH investors rescued Lilium after injecting hundreds of million.

            If we rewind like a century:

            “During the Great Depression, American lost dozens upon dozens of auto manufacturers and fixtures of Americana. […]

            Many others, such as Studebaker (later Studebaker-Worthington Corp.), emerged from the Depression as shadows of their former selves. Once a component of the Dow Jones Industrial Average, Studebaker struggled for decades to compete with the more efficient auto-giant…

            Before the slim times of the Depression—even during the roaring twenties—*hundreds of automobile manufactures started, ran for a few years, and went bankrupt*.”

            Yeah that’s what happened when there’s an emerging technology about to disrupt and open up new market.

            Don’t tell me you’re the one who insisted vehicles with combustion engine were a dead end because many, even hundreds of companies which tried to manufacture ICEV failed and gone bankrupt.

          • Pedro, I think a better parallel is the flying cars of the last 80 years. Lots of enthusiasm, technology, investements, start-ups & expectations.

            Since the last decade they been electrified, renamed eVTOL and the story, dream continuous. Ignoring, downplaying the same showstoppers as 20-40-60 years ago. A real flat learning curve..

            https://www.driving.co.uk/features/will-flying-cars-ever-take-off/

          • I’m not particularly optimistic about flying cars TBF.

            Carl Benz had the first combustion engine in 1879. It took decades for ICEVs to become widely popular. Wright Brothers had their Kitty Hawk first flight in 1903. It took many years for passenger planes to be commercial success.

            We may very possibly have the Kitty Hawk moment for our generation, it’s either pretty close around the corner or even amongst us though not widely publicized in certain circle.

          • Pedro as is standard takes comments out of context.

            Just like Aerodynamics (also a mature industry) there are improvements.

            They are not fast and painstaking research and development go into each one.

            They are not the way forward for aircraft. They certainly work very well in some applications. I bought an Electric Lawnmower, love it. They took full advantage of tech and the deck is composite, very light. For my use its fantastic. Plenty of go pup and longevity for the mission.

            Electric snowblowers work well for two or three care width driveways 40 ft long. They do not do the mission my ICE does, but if all I had was a driveway per above, I would go there in a heartbeat.

            Forcing something that works in one area into another is foolish and billions have been thrown away on the pie in the sky. Equally the every-man chopper. Yea, just like in the 50s, except it was a auto-plane in every garage.

            Electric forklifts have been around for a very long time. They work and they work well. You need counter weight so the battery replaces a hunk of steel hung off the back (or part of it, they are not in the right place for all of the counter weight needed).

            Sadly there are no charging station in the sky.

          • Trans

            Lol. There’s already a certified fully electric aircraft. No, it probably is not good for you.

            ‘One small step for a man, one giant leap for mankind.’

            Unfortunately, those days are gone. From ‘can do’ to can’t possibly done right.
            Boeing’s struggle to put man in orbit again speaks loud.

            “Batteries have been around for 150 years?”

            I doubt I’ve taken your comment out of context. May be you should have taken more time to think through first.

            I’m old enough to remember the EV-1 and how far we (may not include you) have progressed.

          • WSJ was just reporting what a single person said.
            The numbers 5-8% and that’s for MOBILE phone batteries.
            Each one is of course a single battery not a pack of 100s like you would use in a car . Power usage for phone is completely different characteristics to a car.
            Plus cost for a small battery in a high end phone would be astronomical in a car – even if it was the right type for a car usage

          • Did you read the article? Where did that person, as you claimed, referred specifically to “MOBILE phone batteries” or single battery not pack of 100s?

            If you dispute that battery has been improving at 7-8% a year, you better seek professional advice before you post.

          • > “When we started Tesla in 2003, the batteries were just good enough, but what we had noticed was that they got better at about 7% to 8% a year, and had for a long time,” says Marc Tarpenning, a co-founder of the company. “It’s been 19 years, and we still haven’t had a step change in battery capacity—it just ticks along at 7% to 8% per year.”

            Battery for phones but not EVs?? OMG!

          • The WSJ article doesnt background the 5-8% claim- do you think the journalists are battery experts ?

            So I looked deeper to find the data to support it, avoiding anything from RMI, the group Amory heads.
            I used a different computer to check the hard data, so dont have the link. But similar all talk about mobile phone batteries only.

            Phones are getting bigger- because they need BIGGER batteries even with the 5-8% More efficent CPU also help , but screen time is the big power hog

    • “Tesla’s EVs like the Model 3 weigh no more than the average US car”

      Thats easily disproved
      ‘Tesla Model 3 Performance with AWD weighs 4,065 pounds — 379 pounds more than a BMW 330i XDrive. A Tesla Model S Long Range weighs 4,560 …510 pounds more than a BMW 540i XDrive, which is no lightweight.
      A Chevy Bolt weighs 3,563 pounds — a huge 829 pounds more than a Nissan Kicks.’

      this of course shows the fantasy electric powered mindset

      Again look at BMW which has EV and ICE versions of the same vehicle
      ‘BMW 4 Series Gran Coupe weighs 1,695kg when you opt for the basic petrol model, however this rises to 2,065kg in the electric i4 with a 70kWh battery. Go for the larger 84kWh battery and you’ll be lugging around 2,125kg.’

      Im sure Mr Lovins other claims are equally dubious

        • What about the other BMWs which are available as EV or IC versions?
          I4 gran coupe vs 4 series gran coupe? Same body

          Tesla M3P with all wheel drive seems well compared with a 3 series with 4 wheel drive.

          • 1. Couldn’t BMW does it wrong by building EVs on ICEV platforms? Did you know why BMW, separately, has a platform for EV specifically? Haha

            2. I wrote about the weight of the BMW M340i AWD, which is about the same as the M3P. 🤦‍♂️

          • What about the other budget models compared .
            And same model in EV and IC versions is perfect comparison

            Of course the EV priesthood thinks that is the devils work. This shows how closed to reason they are.
            Undeniable fact, electric cars are heavier than IC equivalents

          • What the other budget models are you talking about?

            Do you realize the performance of M3P? You think it’s at a similar level of a budget BMW???

    • A business model relying on one fairly old technology to be improved massively by someone else is a recipe for bankruptcy. I wouldn’t invest money into any electric aircraft venture, unless it’s created by a battery developer with a history of breakthroughs.

      Big aerospace relies on wider wings, better aerodynamics, lighter materials, tighter sardine-packing (sadly), better reliability, higher duty cycles, AND more fuel-efficient engines, to keep air travel cost-competitive.

      Computers used to rely on Moore’s law, but now that it’s dead, they still have better algorithms, more cores, NPU, lower-precision models, improved GPGPU integration, cloud computing, storage tiering, and other advances.

      If your business model isn’t viable unless SOMEONE ELSE massively improves a century-old technology, it just isn’t viable.

    • Amory and Pedro,

      This article series is about the gaping difference between your references and your assessment of the situation regarding batteries for aeronautical applications and the reality.

      Now Airbus has joined Tecnam (P-Volt), eViation (Alice) et al. to stop or halt their project because the development and state of batteries are not where they were projected to be and need to be.

      https://www.flightglobal.com/aerospace/airbus-backs-away-from-cityairbus-nextgen-launch-over-battery-concerns/161537.article

      As said in the article series, if things were as positive as you say they are, we should see more real progress after 15 years of investment and development. The fact is we have one light two-seat trainer certified so far…

      • Hi Bjorn,

        Actually more than one AFAIK, just not widely publicized in certain circle as I said before.

        To be clear, it’s early days, I’m not talking about commercial passenger aircraft.

  4. your figure 2 left out the likely most efficient choice where you have an overpowered (relative to prop size) turboprop with a big honking generator feeding a battery which feeds a separate emotor/prop unit.

    this gives you the improved thermal efficiency of fewer larger turboprop engine driving both a generator and a prop rather than a larger number of smaller serial or parallel turbines + electric motors.

    in turbines at the same technology level, larger is invariably more thermally efficient due to lower percentages of friction and seal leakage losses and generally higher pressure ratios.

    • Unfortunately I must say.

      Nobobody is against electricly powering aircraft, contrary.

      But the battery’s just aren’t there to support serious (not niche) operations.

      I’m sure Airbus engineers knew years ago, they just wanted to avoid being looked at as a game breaker, with all enthausiasm, investments, commitment and political support around..

  5. Any realist should be clear-eyed about the growing aviation industry and net-zero target.

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