> You would think that by now they'd achieve at least 2x the speed with the same fuel load. No. The problem is not engine efficiency. To my knowledge, Concord

> You would think that by now they'd achieve at least 2x the speed with the same fuel load. No. The problem is not engine efficiency. To my knowledge, Concorde still holds the title of having the best engine fuel-efficiency at cruise. Close to 60%. And although engine efficiencies at subsonic speeds have improved drastically since the 60s, the problem airplanes face when cruising closer to Mach 1 is that they start getting transonic effects around their wing and fuselage. And that greatly increase your drag. It is therefore more economical to stay below the speed threshold at which these effects start occurring. A high Mach 0.8+ could probably achieve that. Aircraft builders have settled for about Mach 0.82 for airliners. Private jet builders, in some cases, care less about fuel efficiency, and still build airplanes flying above Mach 0.9, as for example, the Cessna Citation X.