Concorde was commercially viable at Mach 2.2 in supercruise (although there's a common misconception that it was not).
However, its overheads were very high, and its applicability was severely limited by fears around the sonic boom (most particularly in the US, which banned supersonic flight overland, possibly largely because they wanted to kill off foreign competition).
Also, this doesn't scale down to Mach 3-4 and under. This thing uses scramjet, or supersonic combustion ramjet. It REQUIRES intake air to be at high supersonic speeds for it to work.
This is why I am highly sceptical it can be part of a commercial supersonic passenger jet: how do you get from subsonic -> supersonic without also tacking on some kind of conventional jet engine?
Case of China's got them, and can't rely on the Orange Emperor and his heirs to have their backs.
Given there will at some point be the need to deliver competing cruise missiles for this platform, and after the crisis of the US not being able to keep demand with Israel's and Ukraine's orders they greenlighted SK and Japan to enter the European defense market, to answer your question yes, this is of course a defense related project.
There's been an industry request to develop native defense components on these matters within the EU following pressures and contrasts with the US (on a report to the EC for the ReArm campaign, EU's biggest playes of aerespace industry made a joint report estimating 60-80% of their components and tech are sourced from the US).
One is not exclusive to the other.
Skylon was expected to use air breathing engine up to Mach5+ and switch to rocket engine beyond it.
You can probably do the same for a suborbital airliner if you are insane enough.
Nominal SECO for the last starship mission was at ~8 minutes and it took ~20 minutes from deceleration started (well, from air resistance outweighed the forces of acceleration) to landing. So basically 30 minutes of flight is just the "getting up to speed" and "slowing down" part. Both account for some distance traveled, but still. ~45 minutes is probably a good bet.
Do note however that you may have to go around the world "the wrong way" to get some places due to launch constraints. But living in a world where going around the world "the wrong way" is the easier path is interesting. Imagine that.
A suborbital craft won’t be travelling at that speed.
It was proposed as nuclear warhead delivery method though: https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment...
You can also reduce peek deceleration forces by using aerodynamic lift to stretch out the reentry over a longer period.
But yeah, if it is going down almost vertically then this will not be enough.
Unless one has seriously variable aerodynamics, the vehicle will have to swerve to nearly horizontal over a distance of about 1 scale height of the atmosphere, which is about 10 km. The exponentially thinning atmosphere goes from "too thin to matter" to "brick wall" over a short distance.
The acceleration for turning is v^2/r; for v = 5000 m/s and r = 10 km this is 250 g.
Acceleration also limits how rapidly one can reenter from beyond Earth orbit. At > LEO velocity, the vehicle has to use (downward) lift to stay in the atmosphere, and if v is too high the required acceleration is too high.