Rolls-Royce Developing Turbogenerator Tech For Hybrid-Electric Flight


Rolls-Royce announced on Wednesday that it is in the process of developing turbogenerator technology including a new small engine designed for hybrid-electric flight applications. According to the company, the turbogenerator will “recharge batteries after take-off or power propellers directly, enabling aircraft to switch between power sources in flight.” It will be scalable to serve a power range between 500 kW and 1200 kW and is being designed to complement the Rolls-Royce Electrical propulsion portfolio.

“Rolls-Royce will be the leading provider of all-electric and hybrid-electric power and propulsion systems for Advanced Air Mobility and will scale this technology over time to larger platforms,” said Rob Watson, Rolls-Royce Electrical president. “As part of our strategy, we are looking at offering the complete sustainable solution for our customers. This means extending routes that electric flight can support through our turbogenerator technology.”

Watson also noted that Rolls-Royce is planning to build out its existing network to offer maintenance services for electrical systems. The turbogenerator design process, along with work on system integration, is taking place at Rolls-Royce locations in Germany, Norway and Hungary. The German Ministry for Economic Affairs and Climate Action is providing some of the funding for research and development.

Kate O'Connor
Kate O’Connor works as AVweb's Editor-in-Chief. She is a private pilot, certificated aircraft dispatcher, and graduate of Embry-Riddle Aeronautical University.

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  1. This will work, but is unnecessary and solves a problem that does not exist. At least it is extremely complicated and certainly eye wateringly expensive.

    The problem with electric motors in aircraft is not the motors, but the batteries.

    I have asserted that if the motors are powered by HFC or a diesel or turbine driven generator, the range and capability would be acceptable, but at what price for what reason?

  2. @ William. Indeed. Battery-ICE hybrid aircraft do not appear to make much sense or certainly not sufficient sense to make it worth spending vast sums of money developing. Unlike a road-based ICE vehicle, where the engine RPM is often varying wildly as the journey unfolds and therefore the ICE is rarely able to operate at its most efficient RPM, once at altitude, an ICE aircraft is trimmed, likewise the engine – and at an RPM that well suits its most efficient operation – and stays there for pretty much the entire duration of the flight.

    In a road-based vehicle, the engine is *only* operated at its most efficient RPM and is used (usually) to charge a battery. Once charged, the ICE switches off. The rest of the drive train is then essentially identical to a battery-electric vehicle. Of course under heavy load, both engine and battery provides power.

    Hence, what the Dickens is the point in all that added weight, complexity and cost for a hybrid aircraft?

    I put this to a regular flyer friend recently and he could only suggest that you might gain some efficiency using a battery to accelerate for takeoff and climb out where the aircraft engine is at or near max output. Well, can’t help thinking that some sort of catapult system could easily take care of the first bit at least even f it were an inductive power transfer system embedded in the runway, or something equally whacky.

    Realistically, all-electric aircraft are already completely viable – for short range use (say, up to an hour or so’s duration) – and at the rate battery technology is improving it seems very unlikely to me that a hybrid system will make any sense at all in a relatively short time-frame.

    Meanwhile, whilst theoretically possible, hydrogen-based fuel cells have so many practical issues that the same argument will do for them, too (see for a thorough explanation of why it all simply won’t work).

    In my view, in the long term, short distance aviation will be electric, medium distance will be a combination of electric and (electric-powered) dirigible ( and long haul will almost entirely stop altogether (and what remains will use synthetic jet fuel) or will transition to near space. High speed trains (possibly supplemented by a hyperloop technology) will add to the mix making medium distance travel time and cost comparable or better than jet aviation (especially once security etc time implications are taken into account).

    • Trains, sans airline-style security?
      Sure. Right until the first major terrorism event occurs on a train. Then?
      What does the T in TSA stand for…

      As for inter-city “high-speed” passenger rail… Consider how many flights are conducted per day, and how many passengers are enplaned on those flights. Now, consider the math of trying to move a like number of passengers. How many trains? On HOW many tracks? WHAT tracks? Eminent Domain, you say? How many TRILLION dollars, you say? Good luck with that, in 21st-century “What’s in it for me” America.

      • There already are commuter trains, including:
        – Olympia to Everette WA
        – Mission BC to Vancouver BC
        – probably some in Toronto-Ottawa-Montreal area
        – some on Long Island into NYC
        – probably some on east coast of US

        You speak of many more, and raise the excellent point of security.

    • You left software out of the equation (s). When Toyota started selling the Prius, a very soft hybrid, everyone in the industry laughed — how would they ever make money on a car with two engines, extra weight batteries and so on.
      Turns out they did so with clever software.
      Twenty years on nearly all the hybrid systems, be they serial or parallel do very nicely for the makers, and customers, thank you. Hardware has improved but the huge advances have been in software.

  3. This makes zero sense and was probably pursued solely to gain the grant from the eco-fanatics in the German government. You can see they are using the right buzz word “sustainable”. So how is burning Jet-A to recharge a battery any more or less “sustainable” than burning the same Jet-A in a pure turbofan or turboprop Aircraft? And what is “sustainable” anyway? Plants and animals die. Their remains turn into the wonder-fuels, coal, oil and gas. How more sustainable is that? They should read Julian Simon’s “Ultimate Resource”. Frack everywhere!

  4. I see some future in this, if we stop thinking we must ELLIMNATE fossil fuel burning completely in the short term. Electric motors are 85% or more efficient. Turbines on the other hand are 50% or so.

    If you had a small turbine capable of generating enough power to fully sustain an electric motor flight then efficiency will go up. So total fuel burn would go down because the electric portion adds efficiency to the total system.

    I see these new turbines gaining 2% efficiency but I think a smart hybrid system could gain 20-30% or more. That would help with the fossil fuel burning, eventually we will make it to not needing any fossil fuel burning……in the future.

    • “If you had a small turbine capable of generating enough power to fully sustain an electric motor flight then efficiency will go up.”

      Um… That’s not how physics and chemistry work. If your charging turbine operates at 50%, and your turbine-powered electric motor operates at 85%, then your overall efficiency will be 42.5%.

      • That is the engineering problem to be working on.

        Not this I can fly with unicorn tears that we seem to like to hear about.

  5. This ain’t rocket surgery. Every time you change energy from one form to another, you lose some of it. The first law of thermo says that the best you can do is break even. The second law says you can’t even do that, so changing kerosene into heat, then into electricity, then into spinning a fan involves one more step than just turning it into heat, then into spinning a fan.

    I cannot believe that RR does not have engineers who know all that, so the obvious answer is that they are pandering. We cannot solve technical problems with emotional or political solutions. But enough about Greta.

    • Absolutely correct, the physics simply don’t make sense for a “hybrid airplane”. I suppose there could be some niche application, for instance the precise control and maneuverability advantages of multi-rotor electric lift might possibly be sufficient to make some sort of hybrid powered “Skycrane” type utility lifting craft marginally viable. But I wouldn’t invest.

      As to what RR is thinking, since lightweight turbogenerators can have numerous other applications, they may feel there could be worthwhile payback for the R&D beyond the green PR points they gather.

  6. Is it viable to produce electric current onboard? Granted, there are clumsy and costly ways to accomplish this, but is there an advantage? It is true, nothing is free. How you use what you have makes the difference. Remember the complex domino sets that have been made? We don’t live in a world where just one domino can only knock over one more do we? The same is true with the production of electric current. There will be a display at Oshkosh this year to show how this is done.

  7. Some stats for thought. Car engines, 4-strokes, are approaching 40% efficiency; 60% of fuel is wasted as unrecoverable heat going out the exhaust pipe. Jet engines are between 30%-40% efficient; 70%-60% is lost from unrecoverable heat energy – exhaust. State of the art electric motors are between 85%-90% efficient with 15%-10 wasted energy as heat.

    Sate of the art battery packs have never been developed to approach the energy density of gasoline, diesel or jet-A fuel. Gasoline; 120,000 btu per 6lbs/gallon. Diesel; 137,000 btu per 7bs/gallon. Jet -A; 135,000 btu per 6.8 lbs/gallon. Lithium battery; 0.583kwh/lb. 1 kwh=3412 btu, 1,989 btu/lb.

    An equivalent gallon of Jet-A fuel@135k btu would require 67.87 lbs of lithium battery. It can be surmised that trade offs are made in every configuration. The devil is in the details.

    • So need thermoelectric generation of electricity?

      Common at very small scale, including in spacecraft with nuclear isotope making the heat.

      Weight at large scale I do not know.

      • The isotope generators put out relatively low power levels but just don’t run dry. Good for operating instruments for a long range space mission; not at all viable for propulsion.

    • I believe your final math is off a bit.
      Assuming the rest of the numbers are correct, at 40% efficiency, 135,000btu/6.8 lbs/gal Jet-A = 19,853 btu/pound = only 7,941 btu of useful work per pound. And at 85% efficiency, 1989 btu/1 lb battery = 1690 btu of useful work. That means the useful work done by 1 pound of Jet-A would require 4.7 pounds of battery.

      That’s still less effective energy density than Jet-A, but battery density has increased by around 8x in the past 12 years, so it’s not an insurmountable obstacle, assuming battery tech continues to improve at roughly the same pace.

      I think the bigger obstacle will be charging time and rethinking W&B (since battery weight doesn’t change based on charge level). And also loss of battery efficiency over time, and in extreme temperatures.

  8. Actually this does make sense for a vertical takeoff and landing aircraft. I have always said that if I won the lottery I would blow it all on developing a flying car with similar technology.
    This results in basically a turbine powered aircraft, but it can have a much smaller turbine engine that can be sized to be efficient at the required cruise power while a relatively small battery pack can handle the 10 minute or so peak power demands for vertical flight.
    Keep in mind that efficiency drops off steeply below peak power levels on gas turbines.
    Of course this assumes the FAA having a habit of being rather counter productive towards new technology doesn’t demand a battery pack with so much endurance that it weighs more than the fuel it saves.

  9. The above comments regarding the loss or efficiency at each step in the process are correct and that alone damns such projects. Further have any of you picked up a fuel powered electric generator? They are very heavy. So between the loss of efficiency, added weight, and complexity, the only rationale for such projects is virtue signaling and obtaining others (tax payers) money to play with.

  10. Oh good grief. Stupid, stupid, stupid. Burn jet fuel to charge batteries for electric motors so you don’t burn jet fuel for turbine engines to make the plane go? And that, for what reason again? To solve a problem (Climate change/global warming) which doesn’t exist. By the way, if we get too good at getting rid of carbon dioxide, which is not a pollutant, you’re not going to like the resulting world we’ll have to live in. Plants, trees, and crops need CO2 to thrive, and produce life sustaining oxygen. Photosynthesis 101. It’s not nice to fool with Mother Nature.

  11. It always amazes me that some apparently smart people who appear to understand basic physics nevertheless still don’t grasp the obvious science of anthropogenic climate change. Sometimes, even when their own house is on fire!

    Luckily, Rolls Royce engineers don’t fall into that category.

    • @Tim B. Yeah there’s a lot of that here on AvWeb apparently.

      I don’t think anyone is suggesting this is some magic bullet that will enable massive fuel savings suddenly. Yeah, it still burns Jet-A. What this Hybrid Electric generator could enable is powering electric motors for eVTOL concepts, as Michael C mentions above. Further, one could use this on a more conventional fixed-wing aircraft, replacing relatively large (and drag-producing) nacelles for turbine engines turning the props with electric motors of a lower aerodynamic cross section (and probably lower maintenance costs). With the generator placed within the fuselage of the aircraft, the total airframe drag could be significant lower. Again, the devil is in the details of the implementation here. But I hope to see this become viable in the next 10 years or so.

      Given that we’re a very long way from getting battery energy density high enough to make fully electric transport class aircraft feasible, hybrid-electric could significantly reduce fuel costs and maintenance costs. Oh and also (very modestly) reduce CO2 emissions but don’t tell anyone here that!

    • Humans cannot cause runaway climate warming, which is not and cannot happen.

      Earth was warmer and climate stable in the Medieval Warm Period when Vikings farmed southwest Greenland.

      Climate has been warming slowly since the end of a cool era that drove them out, shown by accurate thermometers like weather balloons and satellite sensors, and tide gages.

      The effect CO2 can have is small, limited by the ‘saturation’ effect of overlap of spectra of carbon dioxide and dihydrogen monoxide vapour, most of the increase has already been realized.

      Popular models have major holes in them, including not having granularity to represent thunder storms in the tropics, and demonstrated errors such as water vapour.

      Rolls-Royce company is pandering.

  12. Tim B., please explain to me what caused the Medieval Warming period between 900 and 1300, when temperatures were as warm as they are today, and there was no industrialization. The Vikings colonized the coast of Greenland, and tended livestock and engaged in agriculture. Michael Mann and his hockey stick, which the IPCC bought into and trumpeted to the world, didn’t even try to explain it, and he and Phil Jones of the University of East Anglia even discussed how they should handle that inconvenient fact because it just didn’t fit their theory of anthropogenic climate change and the hockey stick. Remember Climategate? Maybe you could explain the Medieval Warming period to me. You reckon it was just climate change, without the anthropogenic b.s.? Been going on for 4 billion years; and we’re about due for the next coming ice age, maybe within the next few thousand years or so. A mere blip in geologic time. There have been many ice ages in the past and they are much longer in duration than periods of warming, lasting as long as 80,000 years and followed by periods of warming lasting 10,000 years or so. Since it’s been about 10,000 years since the end of the last ice age, we’re about due for the next one. One more point to ponder. Carbon dioxide is considered a trace gas in the atmosphere; composing only .04% of the atmospheric gases. That’s 400 parts per million. In eons past, before industrialization, CO2 existed in much higher concentrations. Cliimate and temperature is influenced by many, many variables. To attribute it all to an increase in CO2 is absurd.

    • And climate was stable in the MWP at higher temperatures than today.

      Yet IPCC activists claim runaway warming if temperature rises more than 2.0C.
      (Proven to be an arbitrary number someone fantasized years ago then activists burped it up endlessly.
      Yes, even the infamous Phil Jones of Climategate agrees it is arbitrary.)

    • One known cause of variation in climate is variation in earth’s orbit due to gravitational effect of other planets whose period of orbit around sun is different than earth’s. That’s been known for centuries, it changes distance from sun thus amount of energy reaching earth.

      • Elaborating, variation in period results in reinforcing or offsetting effect of each planet on earth’s orbit.

        Just as happens with weather – such as the high temperatures on the mid-wet coast last June, Typhoon Freda there in 1962, and the superstorms that hit NYC every decade or three.

  13. The Spirit of Butts Farm model airplane flew 1881 miles on slightly less than a gallon of fuel. Pretty efficient. What I would like to see is a very efficient small turbine turn a very efficient powerful generator to power a very efficient electric motor to turn a very efficient propeller. What can we do? P.S. has any one done this sort of thing? According to google it takes 900 watt hours to fly a human 1 mile, that is around 3000 BTU’s per mile, a gallon of jet fuel has 135,000 BTU’s of energy, a 500 mile trip would need 1,500,000 BTU’s so about 11.2 gallons of fuel. Does any of this work? I am asking, I’m not a know it all, but I love to learn.

    • The problem with turbine is the low torque. It takes a lot of RPM to get power out of them. A plane like a Boeing 747 uses approximately 1 gallon of fuel (about 4 liters) every second. The average alternator in autos takes about 5 Hp. There will be a display at Oshkosh to show the best way onboard generating can be done.