Jet engine efficiency has not increased by the order of magnitude necessary to make commercial mach travel feasible, and jet engine noise reduction has focused on high-bypass designs that are not applicable to supersonic travel. Going fast is fucking expensive. For reasons we need not get into I got pretty deep into calculating the costs of ownership of this guy: That thing is the VW Beetle of fighter jets, acclaimed for only burning 600l of Jet-A per hour at cruise. Right now, at historic lows, Jet-A is $1.70 a gallon. So cruising, at 45,000 feet, at about 0.8 Mach, my friend the Gnat costs $270 an hour just in gas... and it only holds enough juice to do that for two hours. The Concorde burned a ton of fuel per passenger per trip. That's 17l/passenger/100km. Read it and weep: https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft A 747 is 2.59l/passenger/100km. So until supersonic travel isn't basically an order of magnitude less efficient than subsonic travel, supersonic travel will remain a pipe dream.