Boeing has selected mach 5.0 as the top speed for a hypersonic passenger jet because that greatly simplifies the structural materials and the propulsion. Standard titanium alloys used in aircraft and jet engines today are strong enough to survive the surface temperatures ranging up to 600°C (1,100°F).
A mach 5 hypersonic plane would also use standard Jet-A fuel. Liquid methane or some combination of those fuels are also options for the commercial aircraft.
A mach six aircraft requires a supersonic combustion ramjet (or “scramjet”) engine, a technology that still isn’t mature after decades of research and demonstration. A mach five might be able to just modify the lockheed spyplane engine technology.
The Lockheed SR-71A spyplane flew at speeds up to Mach 3.2 using two Pratt & Whitney J58 engines. The J58 featured a unique configuration called a turboramjet. The engine functioned like a turbojet up to about M2, then diverted air from the compressor into ducts that emptied in the afterburner.
Boeing’s hypersonic airliner also would use a turboramjet configuration, with some variations compared to the J58. Instead of ducting only a portion of the airflow around combustor over M2, Boeing’s concept might bypass all of the airflow around the engine core at higher speeds.
A mach five hypersonic passenger airliner could cross the Atlantic four or five times a day with the same crew versus twice with a mach 2 supersonic plane. The increased utilization rate could make the hypersonic airliner more economical than a supersonic jet.