> but how much we can't know until someone builds a modern supersonic aircraft.
You really think modern aircraft manufacturers are surprised by the fuel consumption (or any other performance characteristic) of their aircraft prior to them actually being built?
Oh, by the time they start to assemble physical parts they of course know exactly the performance figures, but that is pretty close to the end of the design process. Only after the design process you know these and I don't know how much of that is being done now.
That's true but there's enough history of supersonic aircraft design that it's not really much of a mystery what the general numbers are going to look like given speed, range, capacity, etc. targets. Sure, the designers won't know at a detailed level until they actually design the aircraft to some degree. But it's pretty much a certainty that they have (at a minimum) a back of the envelope idea of how things will look.
You really think modern aircraft manufacturers are surprised by the fuel consumption (or any other performance characteristic) of their aircraft prior to them actually being built?