My internal pedant compels me to note that a turbine-based engine (which all aviation jet engines are, AFAIK) is still an internal-combustion-engine. Not all in

My internal pedant compels me to note that a turbine-based engine (which all aviation jet engines are, AFAIK) is still an internal-combustion-engine. Not all internal combustion designs rely on reciprocating pistons. Though you are quite correct that turbine-based designs can achieve efficiencies that most piston-based designs typically can't. 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.