[trhway]

one of the things that really can be resolved by technology here is re-fueling in air. You don't need solar panels and batteries if your drone hanging at 20km can be automatically supplied by refueling missions of another drone.

Say 40kg of batteries would contain 14KWh, ie. equivalent of 1.5 liter of gas or with adjustment for the thermodynamic efficiency of a gas turbine - 4 liter. Thus 30kg (lets allocate 10kg for the gas turbine and other stuff) of gasoline is a 10 day supply of fuel. Thus your re-fueling drone would need to make 1 trip/week.

[pavel_lishin]

But if you can build a drone that can stay up nearly indefinitely, why bother refueling it?

Would it make more sense to perpetually spend money on fuel (and the fuel it takes to launch the fuel), or to spend more money up front to design a robust HARP that only comes down in case of severe fault, and is cheap enough that it's easier to just replace it?