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Unless you use a hybrid approach, like a super efficient turbine generator + solar cells, so your battery doesn't need to be nearly as big.

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I am not seeing how a jet turbofan is going to be less efficient than a turbine engine driving a generator driving an electric motor driving a propeller or fan.

I don't think there's enough surface area on an airplane to gather significant solar power. And that doesn't help at night or in clouds.

Jet turbofans have an efficiency curve related to the velocity of the aircraft. They are least efficient when the aircraft is stationary, and most efficient when the aircraft is moving at the same velocity as the exhaust.

A jet turbine generator can run at near-constant efficiency.

Or using diesel electric hybrids. That's how train engines work. They're still way more efficient that using combustibles directly because you can run at lower RPMs and buffer the energy (trains run at like 1k ~ 2k RPM).

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