Engineering360: "NASA Makes Headway in Developing Quiet Supersonic Passenger Jet"

L-M (and, Boeing, I believe) are working on an even more advanced version for military applications (essentially, the next generation Blackbird). My understanding is that it will have three stages of propulsion, all air breathing, to take the aircraft to Mach 5. As a proposed single engine aircraft, the QUE SST (X-Plane) is probably not more than about Mach 2. The ''quietness'' of the QUE SST is mostly due to its ''form factors,'' but may also include some ''quietness'' tweaks of its engine.

Both aircraft (Mach 2 and Mach 5) likely would have a niche in commercial travel, with the Mach 2 in ''short'' hauls of 5k to 8k km and Mach 5 in everything over 8k km. A five to eight year staggered development schedule to get some solid performance and market experience on the Mach 2 version before going all in on the Mach 5 version would make sense.

When one may ask what NASA is doing in the design of advanced aircraft, as opposed to most of what it has done during its lifetime, the answer is obvious in the first ''A'' of its name.