Orbital speeds within the atmosphere, basically.

Edit: I suppose that means the vehicle wouldn't need any aerodynamic lift when going at full speed; indeed, to maintain altitude it'd need negative lift.

Barely within the atmosphere, I assume. Mach 20 at commercial jet altitudes would probably melt the aircraft.

Wonder if this would be much of an issue for a missile. It can melt away on the outside to a degree, as long as the payload gets there intact. And it can get anywhere on the planet in an hour, so perhaps a sacrificial shield would work.

Just build your aircraft out of titanium. *cough*SR-71*cough*

I said Mach 20, not Mach 3.