system
Given the following post, respond as if you're commenting on Reddit:
human
What’s special about hypersonic speeds?

I understand that characteristics of airflow change when it becomes super sonic (Mach 1), but what is special about specifically 5 times that speed? Is it just that just a human label or is there a physical phenomenon at Mach 5?
gpt
Hypersonics are conveniently defined as any speed above Mach 5, because around Mach 5 a lot of engineering problems change their form.

At supersonic speeds, you can quite easily maintain subsonic combustion (a shock cone or ramp intake will do much of this, check Concorde's intakes which are well documented) for a conventional turbine-based engine.

Your airframe will experience drag heating from skin friction, but this again can be solved using titanium, ceramics, carbon, etc.

Flight controls can be conventional control surfaces, so long as you take into account things like aileron reversal.

As we approach hypersonic velocities, all this starts to change. Subsonic combustion becomes impractical, it causes too much drag, so you need to either not intake oxygen (e.g. a rocket) or do your combustion supersonic (e.g. scramjet).

Airframe heating is more intense, but that's manageable (orbital craft re-enter way, way faster than mach 5), what changes is that the leading edge is now exposed to reactive oxygen species (ROS), oxygen plasmas, atomic oxygen, ionised oxygen. Oxygen attacks metal just sat in the open, ROS are types of oxygen turned up to 11: Atomic oxygen can oxidise fluorides. Coming back from orbit, we can use disposable ablative heat shields, and large blunt-bodies to push the shock back. In flight we can't do this if we want to remain flying very long.

Flight controls are different too. If you actuate an elevator, you lose your elevator and empennage, so your turn radius is extremely large. Hypersonic craft get their lift from inertia (ballistics) as much as from the air, and from the air it's less wings as much as lifting bodies.

This is partly the sheer speed of oncoming air, and partly your enormous inertial momentum. You go in a straight line, which becomes a ballistic trajectory at hypersonic speeds, aerodynamic flight is a secondary concern.

So in summary, hypersonics are where the engineering challenges change substantially, and it's conventionally accepted to be around mach 5 where those challenges are the primary ones.