For comparison the energy density of gasoline is 12,200 Wh/kg
Looking things up, I see different numbers for electric motor efficiency, ranging from the low 70s to the low 90s percent. As an overly simplified example, let's assume an electric motor in a car has an efficiency of 80% (in reality, they may be better). At that efficiency, a battery pack only has to get to 5032.5 Wh/kg to achieve the same practical energy density as gasoline, less than half the actual raw energy density. That is probably an easier number to reach than trying to achieve the same raw energy density of gasoline.
Nobody has made this argument in this thread (yet, as I type this), but I've seen it made before and made it myself. Yes, at those energy densities, batteries are highly dangerous if something goes wrong (like a crash) and they release all their energy at once, but so is gasoline. At practical ranges, you're potentially sitting on a pile of high-explosives either way. But, I'd hope a 5032.5 Wh/kg battery is easier to make safer in a crash than 12,200 Wh/kg gasoline is.
[1] https://newsroom.toyota.eu/new-2019-toyota-prius-with-intell...
the other benefit is the weight of gas goes down as the flight progresses which is about 1/3 of the total weight so make the design a bit worse but I believe it would still be compensated by cheapness of solar electricity based charging.
on a related note, IMO this is the reason the whole fuel cell shtick from toyota is BS. Because if it were anywhere near economical & scalable then we would be seeing fuel cell powered drones all over.
I was thinking in terms of passenger cars for some reason (despite the word "airplane" being in the title), where piston engines are far more common than turbine engines. That said, I stand by the main point of my post, which is that you can't just consider the energy density of your energy source in isolation, you have to also account for the efficiency by which you convert that energy source to useful work.
A thing I find myself wondering about now is the viability of an arrangement similar to that of cargo trains. IIRC, the locomotive of cargo trains is a hybrid of sorts. An internal combustion engine running at relatively high efficiency drives a generator, which in turn powers electric motors which drive the wheels. This sound similar to your hybrid idea, perhaps with the addition of batteries.
I think one of the biggest challenges to hybrid systems is to make sure the overall system efficiency doesn't drop below that of the current typical systems which it is intended to replace. Say you have a plane with turbojet engines running at 60% efficiency. And we replace it with a charging turbine-engine at 70% efficiency, coupled to an 80%-efficient charging system, coupled to a 90%-efficient electrical engine. In isolation, each of those numbers sound better than the 60% efficiency of the turbojet engine. However, efficiencies combine multiplicatively, and so 70% * 80% * 90% = 50.4%. That's overall worse than the turbojet we started with. This new system better be much lighter than a turbojet to be worth it.
Turbines are not used in cars because they have high rpm and low torque requiring large transmissions to gear them down properly. They also are not very responsive to throttle requiring spin up, very noisy and have have an abundance of hot exhaust to deal with. You can't muffle them as easily.
Series hybrid diesel electric trains are actually less efficient than diesel mechanical drive trains. They are used because of the need for precise traction control to prevent wheel slip, they sacrifice efficiency at speed though to attain it.
Double conversion from mechanical to electrical back to mechanical will always be less efficient than a straight through mechanical drive once up to speed, this is why nearly all hybrid cars are parallel and go full mechanically coupled at highway speed.
As far as I know, there are simply no rechargeable battery chemistries that have any (even theoretical) hope of achieving this level of specific energy.
Electric motors are lighter than ICE engines per HP and torque unit.
Emrax 268 (electric engine designed for aircraft) - 6.12 HP/lb
GE 90-115B (jet engine used in 777) - 6.10 HP/lb
Junkers Jumo 205A (diesel engine used in Ju 86C-1) - 0.66 HP/lb
Electric also scale down better so for light weight UAV a higher energy density battery will open up a bunch of use cases.
But surely that is largely due to the electric element of the combined powertrain? The ICE part hasn't magically become more efficent?
So yes, the ICE part is more efficient in a hybrid, though it isn't magic.
Here's a single example with the Prius:
1. The intake valve stays open for part of the compression stroke.
2. (1) means that the compression ratio for the compression stroke is lower than the compression ratio for the power stroke.
3. The efficiency of the engine is limited by the compression ratio of the power stroke
4. The compression ratio of the compression stroke is limited by engine knocking (if you compress a mixture of fuel and air too much, it will spontaneously combust)
5. So this engine can be made more efficient than an engine that closes the valve for the entire compression stroke
6. Some (non-hybrid) engines have variable valve timing and can do (1) some of the time, to a small extent.
7. Doing (1) to a larger extent makes the low-RPM torque very poor.
8. Electric motors have excellent low-RPM torque, so (7) is compensated for by having an electric motor run at low speeds.
Over 30% efficiency for gasoline, and over 50% for diesels.
If you can make up in other areas like drag in the design because of the different power plant it's not such a mental gap to close.