Hybrid-Electric Aicraft Engine Targeting 30% Fuel Efficiency (rtx.com)
70 points by r2sk5t 6 hours ago
tgtweak 6 minutes ago
Rtx has an interesting patent [1] on this that highlights some of the novelties of this setup vs a traditional hybrid (planetary motor/generator like in a Prius):
It's a boost-only motor, it doesn't/can't harvest energy on descent.
The patented solution (transient smoothing under auto-throttle control) puts electric motors on both the low spool and high spool, then uses a power-splitting algorithm to route high-frequency thrust changes to the electric motors while keeping fuel flow nearly constant on the thermal engine (turbine). The turbine cruises at a steady operating point with tight compressor/turbine clearances and the electric motor smooths out the spikes that are normally there with turbulence and load changes. Benefits: lower fuel burn, longer turbine life (fewer blade-rub risks from speed variation), and smoother ride quality since the auto-throttle bandwidth improves.
Also disclosed in a previous press release [2], it's only a 200kWh battery so at 1MW peak boost (cited load during takeoff/ascent) it would only run for ~10-15 minutes at the beginning of the flight.
Seems most of the savings are due in part to not using as much fuel during takeoff but also in large part to the undersizing and optimization of the thermal turbine to keep it running in it's peak efficiency zone for more of the flight.
Curious how the safety margins work here - if the battery is depleted on takeoff (aborted takeoff) or there's an issue that requires descent-then-reascent, if the batteries can't be replenished in-flight there could be a power deficit in that window where you'd normally have 2+2MW of gas turbine power for the plane and now you only have 1+1mw of gas turbine power.
[1] https://patents.google.com/patent/US20250296689A1/en
[2] https://www.aerospacetestinginternational.com/news/h55-deliv...
bilsbie 3 minutes ago
I’d imagine the apu could recharge it during flight at least somewhat.
Someone 4 hours ago
Not “30% fuel efficiency”, but an improvement of 30%.
FTA: “The project aims to demonstrate up to 30% improved fuel efficiency for a typical 250-nautical-mile regional turboprop mission”
c0n5pir4cy 3 hours ago
I was so confused by the title - I thought jets were fairly efficient at ~40%-50% of theoretical maximum and turbofans can't be that far behind. It would maybe make sense for a single prop aircraft.
30% improvement makes much more sense.
avidiax 4 hours ago
I think there are startups making a similar sort of engine for general aviation. It's good to see that there is development of the same idea for commercial aviation.
This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.
This means that the battery is quite small and light, having only enough charge to take off and get to altitude.
I suspect that this system probably improves safety as well, if architected properly. If one or both of the gas engines fail, so long as they are not seized, that electric motor can still provide some power for diversion.
kspacewalk2 3 hours ago
>This isn't like a hybrid car. It's a parallel hybrid, where the gas engine is just big enough for efficient cruise at altitude, and the electric motor/generator provides extra power for takeoff and ascent (or go-around power), and then charges slowly during cruise if needed.
Isn't that exactly what hybrid cars (e.g. Prius) are? Extremely efficient gas engine for highway cruising, but insufficient for acceleration, which is aided by electric motors?
Tade0 3 hours ago
The Prius uses a planetary gear set to blend power of the engine with that of the two motor-generators.
Both the engine and motors are used at all speeds. Particularly during highway acceleration the entire assembly rotates in the same direction.
dgfl 3 hours ago
arijun 3 hours ago
The Prius is in series, or something like it.
idontwantthis 3 hours ago
idontwantthis 3 hours ago
Hybrid cars are mostly parallel hybrids. Only the Chevy Volt comes to mind as a serial hybrid.
mikepavone 2 hours ago
Chevy Volt was still a parallel hybrid. The gasoline engine was used for driving the wheels for highway cruise because it was more efficient. I think the range extender version of the BMW i3 was a pure serial hybrid though
projektfu 20 minutes ago
The BMW i3.
SoftTalker 4 hours ago
The fuel burn of take-off and climb substantially lightens the aircraft for cruise. Electric batteries have no such effect, you're carrying all that dead weight for the rest of the flight. This reduces the passenger or cargo capacity of the aircraft, which reduces potential revenue.
And what if you need two go-arounds?
repiret 2 hours ago
Don’t conflate airplanes with rockets.
On an airplane, most of the energy in cruise is spent overcoming parasitic drag, not induced drag. It’s spent pushing the airmass out of the way as it moves forward, not creating lift to stay aloft.
For that reason, a change in weight does not significantly change cruise fuel usage.
Weight is still precious, but that’s because airplanes’ load are more often weight constrained than volume constrained, and capital and operating costs are such that you want to maximize the load.
arijun 3 hours ago
> you're carrying all that dead weight for the rest of the flight
If you're recharging the batteries for extra go-arounds during landing, they are as dead weight as the fuel you would otherwise reserve for that purpose. And if you have 30% more efficient engines, meaning less fuel and smaller engines, it's possible you could come out ahead, weight-wise.
> what if you need two go-arounds
I assume that a go-around requires less sustained power output than a full climb from takeoff, so you will probably get more than one go-around anyway, and we don't know how much over-capacity they're designing for. In any case, any design will require tradeoffs in safety, and having more engine-out capabilities might improve safety enough to overcome the higher risk with go-arounds.
Not saying this project is will work out or that you're even wrong necessarily (this could be the equivalent of a concept car for Pratt & Whitney).
vablings 2 hours ago
dmitrygr 3 hours ago
0cf8612b2e1e 3 hours ago
Energy density of liquid fuels cannot be beat by batteries, so this is not competitive if you are looking to maximize cargo. However, there are plenty of short haul flights: private jets, island hopping, regional routes where you need to move little mass.
usrusr an hour ago
"And what if you need two go-arounds?"
Easy: you don't try the second landing approach before the battery is sufficiently recharged to contain enough energy for the second abort. Chances are this does not take any longer than going through the pattern anyways.
The saving is not just the dead weight of the bigger engine you'd need to do take-off, climb and abort without electric assist, it's also the fuel saved during cruise from running an engine that is completely designed efficiency at cruise load instead of for some compromise between cruise efficiency and sufficient peak power for start and abort.
xattt 3 hours ago
Exactly. A hybrid passenger car can tolerate unpredictable power output that may come with an auxiliary power setup that may or may not be available when stronger dynamics are called for.
A plane doesn’t have this luxury and needs predictable output. The fossil fuel engine either needs a sacrificial “overboost” mode for emergencies (at the cost of wear/long-term longevity), or has to be sized for full power at the ultimate cost of efficiency.
Tade0 3 hours ago
On anything but very short flights most of the fuel is spent on cruising.
dmoy 2 hours ago
hobonation 3 hours ago
Valid.
Perhaps it's not all negative: the electric portion could give a pilot a bit more glide than the gas portion dies.
_diyar 2 hours ago
This is very clever: instead of focusing on electric only flight, just make the existing engine fly in the most efficient window while the electric engine buffers the flight profile.
Like a big-boy prius.
soperj 3 hours ago
Only AI stories show up now, so they had to call it an Ai craft I guess.
krunck 2 hours ago
I wonder if during descent instead of cutting power alot to control speed it can instead use the energy to charge the batteries.
ggreer an hour ago
I'm not sure how that would help. If you're descending it's probably to land, and then you can recharge the batteries with electricity from the ground. That would be more efficient (and cheaper) than burning fuel to charge them.
ck2 2 hours ago
anything that gets rid of leaded fuel on prop aircraft is a win even if 0% efficiency improvement
jabl 30 minutes ago
This is about replacing a turboprop with a slightly smaller turboprop and electric motor/generator+battery. The turboprop burns Jet-A, not leaded aviation gasoline.
Schiendelman 18 minutes ago
If you care about leaded fuel, the culprit now is general aviation - small airports, Cessnas, not commercial flights. If you go work on it, let me know, I'll help you!
mschuster91 3 hours ago
1300 HP electric engine power, now that's an achievement.
I do wonder if prop engines can act as "windmills" (similar to turbine engines, which often in accidents still have been found to provide a bit of hydraulic power), which means regenerative braking could be used instead of speedbrakes.
nradov an hour ago
There would never be a reason to use windmilling props in flight for electrical generation. Even when descending, airliners like the Dash 8 need at least some forward thrust in order to maintain control and stay on the glide slope. They only use reverse thrust for a few seconds during the landing roll.
paunchy 2 hours ago
I've seen references to this capability in reporting elsewhere. It's less useful in an aircraft than a car because you can gradually descend, reducing power proportionally as you're preparing to land. But I'm guessing this is part of where the claimed 30% improvement in efficiency comes from.
soperj 2 hours ago
After they land, they usually sit around for a while. Not really super important to charge the battery on decent I wouldn't think. Would be interesting to get rid of the battery entirely (or reduce the size) and have a beefed up magsafe plug that was attached during lift off, and came unplugged once it was at cruising altitude.
MeteorMarc 2 hours ago
Nothing on recharging the battery in the landing phase, so room for improvement.
repiret 2 hours ago
Not really much room. Unlike a car going down a hill, an airplane descending still wants thrust from the engines, just not as much as in cruise.
bell-cot 2 hours ago
Not an aerospace engineer - but that sounds like a lot of extra cost/complexity/weight, for pretty minimal benefit.