[powderpig]

Worth noting the use of titanium at Mach 3, Mach 3.5, is really at its limit. Any faster and you'll start to see serious problems with thermal management. This pushes you into using super nickel alloys or even ceramic matrix composites, which would be necessary for use cases at Mach 5+ for sustained flight. Acknowledging planes in the last have gone faster like the X-15 but this such plane's architecture wouldn't stop for longhaul flight distances. CMCs and super nickel alloys will be very very expensive.

[stametseater]

The version of the X-15 that got the most impressive speed records was coated in an ablative paint designed to burn away while in flight to protect the plane.

With a rocket engine, reaction control system and ablative coating, the X-15 almost had more in common with a space ship than an airplane.

[zbrozek]

Have we gotten good enough at jet engines to be able to generate enough thrust in the upper atmosphere to turn planes into ICBMs for an ex-atmospheric inertial jaunt?

[stametseater]

With ramjets/scramjets maybe, but developing the inverse of what you suggest is where the money is being spent. Missiles on ballistic trajectories are easier to see on radar and have less ability to maneuver, so they're easier to intercept. So instead of starting out breathing air and then pulling into a ballistic climb at the end, the idea is to start out with a rocket powered ballistic flight, reenter the atmosphere, and then glide/fly through the atmosphere, possibly with a ram/scramjet, to the target. This way the missile is much harder to detect and intercept.

Like this image depicts: https://en.wikipedia.org/wiki/Qian_Xuesen#/media/File:Tsien_... (This sort of missile flight is sometimes called a Qian Xuesen trajectory, after the man who first came up with the idea.)

[zbrozek]

Shame; those dollars aren't going to improve my lifestyle the way that faster passenger flight would.