Rolls-Royce Announces New-Tech Small Turbine Engine Program


Rolls-Royce (R-R) announced yesterday, June 19, it is ready to begin testing on a small, as yet unnamed gas turbine engine designed specifically for hybrid-electric flight. The target market includes electrical vertical takeoff and landing (eVTOL) aircraft for urban air mobility, as well as regional “commuter aircraft” seating up to 19, according to R-R. The gas-turbine engine is said to be part of a developmental turbogenerator system.

“The turbogenerator system will complement the Rolls-Royce Electrical propulsion portfolio by delivering an on-board power source with scalable power offerings between 500kW and 1200kW [roughly equivalent to 670 to 1,600 shaft horsepower], enabling extended range on sustainable aviation fuels (SAF) and later, as it becomes available, through hydrogen combustion,” R-R said in an announcement released during this week’s Paris Air Show.

Rolls-Royce currently holds manufacturing rights for the M250, originally developed in the late 1950s as a helicopter (turboshaft) engine. The company said new technology applied to its latest project engine enables a “step change in efficiency” for small gas turbines. “The turbogenerator can be used in serial or parallel hybrid applications,” said R-R, adding, “It is well suited to recharge batteries as well as provide energy to electrical propulsion units directly and therefore enables aircraft to switch between power sources in flight.”

Trials will be conducted at Rolls-Royce’s test facility in Dahlewitz, Germany, near Berlin. Testing will also include running the engine on sustainable aviation fuel SAF “in the coming months.” The German Ministry for Economic Affairs and Climate Action is contributing to funding the research and development of the turbogenerator hybrid-electric technology involved in the Rolls-Royce project.

Mark Phelps
Mark Phelps is a senior editor at AVweb. He is an instrument rated private pilot and former owner of a Grumman American AA1B and a V-tail Bonanza.

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  1. Might work for commuter hybrid aircraft, but doubt it will fit with quad rotor (or more) urban taxis — 670 HP needed for a taxi and burning 100 litres for 100 km? Get out of here…
    They really need either quick charge, with all the risks, or battery swap tech and as Renault and others have found, that is harder than it sounds.

  2. Can someone (preferably who knows what they are talking about) *please* explain what the point of hybrid (ie turbine/electric in this case) drivetrain on an aircraft is? In a road-going vehicle, particularly one in an urban or semi-urban context where it is doing a lot of stop/go driving or going up and down hills, levelling the energy demand from an ICE to its most efficient RPM and leaving it there makes complete sense. The excess energy the engine then provides over what the vehicle needs – when it is running – is used to charge a relatively small battery which is later used for periods of high energy need eg accelerating or going up hills. If the engine is not needed, it turns off and all the motive energy comes from the battery.

    In an aircraft, you climb out, set your throttle for whatever speed you want and it pretty much stays there for the entitreity of the flight; your engine power matches your drag for a given altitude and that’s that (-ish). So, what the Dickens is the point of a hybrid ‘drivetrain’?

    • In theory you get better energy performance with electric motors during cruise too because they have better torque profiles than ICE ones. And if you are twisting a prop, torque is what counts more than HP.
      Plus if you have one ICE motor linked to a generator, you can turn lots of electric motors from it, and ICE motors turn most of the energy in fuel into heat, not power, while electric motors run cooler for the same power.
      But you have the weight of the ICE motor, its fuel, generator, control boxes, batterie (s) cables, electric motors and props, so it takes some working out…

    • John Patson is right. In other words, there is always some kind of coupling between the engine and the rotor/propeller. The coupling has its own efficiency. In some cases mechanical coupling makes great losses, and becomes inefficient. (E.g. on multirotor aircraft, because of mechanical complexity.) Then engineers choose the “electric clutch” that is the electric generator+motor configuration. This coupling has a known weight, and a stabile efficiency value. (The latter is usually around 90%. For very high power this figure may further improve.) It is a relatively easy calculation if you shall, or shall not pick the electric clutch.
      E.g. the military love, and have been using electric clutches for many decades. For trucks and tanks. (And submarines.) RR technology is quite likely to produce a version of the system that is both light and powerful enough for aviation.

    • Bottom line, the electric clutch/transmission concept would seem to be an advantage only when applied to the various multi-rotor (or propeller) designs. You wouldn’t gain any efficiency or design advantage with a single propeller aircraft of conventional design where, as noted, the objective is simply to couple the shaft of an engine running at optimum RPM to the prop.

      There are, of course, other considerations. I think back to a non-aviation military system I worked with that was powered by a portable turbine generator. Even in the associated shelter the noise prompted people to start using earplugs, and conversation outside was nearly impossible. Translate this to a bunch of multi-rotor “urban mobility” craft…gulp!

    • I agree with [email protected] – if hybrid powertrains for aircraft made sense then there would be data that says so. The above article makes no attempt to describe the improvement of hybrid versus conventional. There should be a full accounting of masses – (engine+batteries+motors+propellers+fuel) versus (engine+propeller+fuel), but again, as in the past, there is no accounting for why/how a hybrid improves utility of aircraft. Certainly, it can be argued, a more complex system has more failure modes. Also noteworthy is the trend for turbine engines to be less-efficient as they get smaller.

    • It’s not for efficiency, it’s for longer duration flight with distributed electric power. I find that people look at aircraft like the Joby or Archer and get wrapped around the axle about limited battery duration, not to mention weight.

      But they miss the advantages of distributed power. If the Joby had, say, a three-hour range it would be quite a capable aircraft. VTOL and faster than a conventional helicopter. Now they have find that sweet spot between weight, payload and capability. Might be doable.

    • I think it’s all about multi rotor and eliminating the expensive, and sometimes smelly, part in the cockpit.

  3. These hybrid electrics need a turbine engine to power the electric generation system as stand alone electrics can’t cut the mustard. Where exactly is the advantage in these boutique aircraft? This a supposed response to the climate hoax.

  4. Rolls-Royce’s decision to market onboard turbogenerator systems is a real Volltreffer. Is not hard to predict Lilium becoming one of their first customers. Good news – congrats to you, both!

  5. Lots of thoughtful characterizations of the use case here for the RR-hybrid system. If you want to observe the benefits in the form of a real-life implementation then go drive a first-generation (no plug in for the battery) Toyota Prius for a few days. The engine kicks in when needed to top off the battery, accelerate aggressively (TOGA Power) or to regeneratively charge the battery (think about a descent from 20,000 feet during which your only speed brake is spinning prop recharging the battery. As the system evolves the engine will be used less and less until the energy density of batteries is sufficient to cover the entire mission. Pretty excellent research platform IMO.

  6. I’ve been flying 40 years, and driving a plug in Prius the last 5, Marc nailed it. I’d like extra power on climb out, and the ability to regen on descents, with no extra weight gain of course! One other benefit of e power often seemingly overlooked, is there is no loss of power at altitude. At least that is my experience in riding my electric Talaria dirt bike to above 9,000′.

  7. The turbine engines,say a turbo prop,can run on different fuels.Once the turbo prop i was flying was fueled(accidentally)with avgas instead of Jet A,didnt notice any difference(it was certified to run on avgas for a couple hundred hours between overhauls).Maybe they are trying other fuels at RR

  8. I think we are missing it. The Hybrid not only enables EVTOL because watts/kg needed for true EV are still far off on the horizon to be viable but so much more. And that is not just the fact that turbines are omnivorous, for any new e-fuel or even gaseous fuels. The big one will be the addition of a Recuperator to increase fuel efficiency up to 30%, that what no one in this arena is discussing. It’s probably the thing they are keeping close to their chest. This is on top of all the other fuel saving technologies that can be incorporated that have been utilized off and on for years in their Turbofan brethren such as, fuel pre-heat, brush seals, oil free bearings, etc that may not have been incorporated in the APU arena.

  9. Here is a way a hybrid piston/electric configuration could make sense, increasing efficiency significantly but not in a revolutionary way: This works for very clean, well designed airframes with high aspect ratios. It could apply from a Lancair-like GA aircraft up to mid-sized commuter aircraft whose missions cannot take advantage of high-altitude turbine engine efficiencies. As an example, for the Lancair you might replace the 350 HP piston engine with, say, a coupled 200 HP piston+150 HP electric motor, and add a high power density battery sufficient to power the electric motor for, say , 20-25 minutes. You use the electric motor for pre-flight ground opps and taxiing, both for TO/Climb, the piston engine(ideally turbo-normalized) for cruise. The electric motor could probably be designed to operate as a generator as well, opening the option of slowly recharging the battery during cruise and/or recovering energy during the descent, possibly arriving with enough juice to refuel and repeat. The commuter could have the added advantage of swappable battery packs allowing arrival with a nearly discharged battery but still a quick turnaround.

    • Looks like editing is no longer possible. First sentence should have said “could make sense for fixed-wing aircraft…”

  10. Thanks to all who contributed to answer my Q. I am left (distinctly) unmoved and – given that if Israel’s Eviation (in the form of the Alice) is to be believed – a 400+ mile-range battery-powered commuter aircraft is already feasible, and given that that is a quarter of the global aviation market, all this amazing (and amazingly complex and expensive) technology is going to be simply left behind by the incredibly rapid advances in battery technology (along with equally complex, expensive and impractical hydrogen).
    What short haul airline is gong to want all this engineering complexity when an e-plane has 1/10th the down time due to servicing needs compared to turbine-based aircraft, not to mention the cost (nor the cost of the ‘fuel’)?
    In the short term, short-haul aviation will transition quite quickly to battery electric just as road-going vehicles are, leaving long haul to ever more expensive (due to the drop in demand) kerosene. Even this would achieve a *dramatic* cut is emissions from aviation, enough I would think to keep the tree-huggers happy (as long as they continue to have to balance their conscience with the desire for a sunny holiday once a year).
    As for the long terms and medium distance air travel? Well, there’s dirigibles… and long haul, near space…