Peter,

I'm not positive about it, but, even at 80,000 feet, to go 2100 mph requires an enormous amount of fuel. The air may be much thinner, but, you still have to push it aside. Just look at the space shuttle after it reenters the atmosphere at something like 140,000 feet. The thing has its wings and underbelly glowing nearly white hot from the friction. The Concorde grows 14 inches in length from the skin heating up at Mach 2 and 60,000 feet. Just imagine the friction at Mach 3!

Do we know, for sure, what it's range is at Mach 3? I cannot believe that it is much more than 2500 miles or so. If this is true, it is less efficient than the Concorde. But, Mach 3 is way more than the Concorde's speed of Mach 2. Speed costs fuel. There is no way around it.