Volume on aircraft is cheap. Weight is very, very expensive. In particular, increasing volume by a factor x means growing the skin by only x^1.5. Tankage is on

Volume on aircraft is cheap. Weight is very, very expensive. In particular, increasing volume by a factor x means growing the skin by only x^1.5. Tankage is only a small fraction of aircraft volume. So, even though space in the wings, where fuel is put today, would go unused, the amount of extra weight for new tankage would be minimal. If, as I expect, tankage is slung under the wing alongside the engines, airframe design would not need to change, and existing aircraft could even be retrofitted. Lifting bodies are supposed to be more efficient and have lots more room inboard, but efficiency might get less important as fuel cost plummets. Ultimately we expect aircraft to move to electric-driven turbines, attached to fuel cells. Tankage and fuel cells could be anywhere convenient on board. It seems like there would be safety reasons to keep the tanks outboard, as the kerosene is, mostly, today.