You guys are being too narrow-minded. Think of the possibilities. Forget the fuel. Work on an engine that consumes so little of it it's almost negligible. We could just use hydrogen for the first little take-off, (Adding some lighter-than-air possibilities) and then use the rotary motion on each engine (In twin engs) to power the other. Sure, difficult, and absorbs a lot of power, but you could say have the fuel at only half the original consumption rate.
And what about lift. The take-off stage uses the most fuel, so you get the picture.
bio-jet fuel sounds like a good idea. But eventually you'll use up all that too. Or at least the basic resources... I'm thinking fill up with your evenings trash then use nitrous oxide to burn so hard you have rocket fuel.
Ah, the sub bit. If the sub blows up in the ocean there's a lot less of a chance that nearby folks will get affected. If a plane blows up over NYC all the beautiful irradiated bits and pieces come falling down and everybody dies of cancer.