> Surely the primary issues are that the efficiency would be dreadful and that the price would be ridiculous even for “the very wealthy”? I guess you're thinki

> Surely the primary issues are that the efficiency would be dreadful and that the price would be ridiculous even for “the very wealthy”? I guess you're thinking that drag increases with the square of the speed, and so for very high speed the fuel consumption should be horrendous. This is only if all things are equal. And they are not. Two things change when you increase speed: you decrease the cross-section (via decreased angle of attack, and shorter wings) and decrease the air density (because you fly higher). A regular commercial jet flies at about 10 km height, where the air density is about 3-4 times lower than on the ground. The airplane discussed here is supposed to fly at 50km where the air density is more than 1000 times lower than at ground level [1]. By the way, I don't think this particular project has any chance of ever becoming reality. The main problem with hypersonic vehicles is the aerodynamics, not the propulsion. These guys are simply working on the wrong problem. [1] https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html https://www.engineeringtoolbox.com/standard-atmosphere-d_604...