I did skip that one, as the topic was engines (gas fueled or electric), not overall efficiency. But since you mentioned this, here's yet another big one (known

I did skip that one, as the topic was engines (gas fueled or electric), not overall efficiency. But since you mentioned this, here's yet another big one (known by very few people): just go faster. Much faster. The lift-to-drag ratio goes down, but the time spent in the air goes down more, so you might end up burning less fuel overall. In cruise mode, a Concorde quite efficient, but it was horrible at takeoff and climbing; because you need fuel to carry fuel, the problem was compounding and overall Concorde was not economical. If someone can solve the takeoff problem (e.g. by using catapults) and climbing (maybe using detachable and reusable boosters?) then a Mach 5 jet could be very competitive. I think this is what the Boom guys had in mind when they say they aim to get to where anyone can travel to any destination on Earth in 4 hours for only $100 [1]. [1] http://edition.cnn.com/travel/article/boom-supersonic-four-hours-100-bucks/index.html http://edition.cnn.com/travel/article/boom-supersonic-four-h...