[captain_perl]

The issue with cruise above Mach 2 is that you need a lot of fuel, exotic materials and involved processes. None of which are compatible with the civilian airline industry.

From a military perspective, high cruise reduces range, involving multiple refuelings. It also doesn't help in a head-on approach, but missiles do travel at Mach 3.

The costs of Mach 3 outweigh the benefits unless you have an intercept or recon mission.

So the discussion should be ... "we are here, where should we go next?"

I would say re-examine the X-planes results, look at where we are in materials and CFD, and do another X program.

For example, all modern jets use area ruling. What is the next step in supercritical airfoils for airliners? (The current record is around Mach 0.89 for an airliner at cruise.) NASA has a great track record at basic research.

How we can use fuel efficiently and inject a minimum of toxic gases into the upper atmosphere? The SR-71 burnt most of its exhausts, resulting in no soot, can airline engines do the same?