Apologies in advance if this is the wrong stack exchange for the question, but I figured it would fit here; if not let me know and I'll delete/move it.
I'm trying to figure out what the damage radius from a Falcon 9 booster would be (~25,600kg empty) if it were to re-enter intact (presumably after a boostback burn) and then simply steered itself toward a point on the Earth with no further deceleration. Would it even survive intact, or would it melt?
I know it's going between 6000-8000km/h at separation. Then there's a post-separation boostback burn, but I'm not sure of the speed of the booster after that. I think there's a second burn for more deceleration. Presumably it's then accelerating until the (third post-sep) landing burn which ends with speed ~0 at touchdown, but what would the kinetic energy be if that didn't happen? How would one go about calculating that?
Also, is there an approximate formula for converting kinetic energy levels to "buildings flattened destruction radius"?
In short, how dangerous is a re-entering booster as an ad hoc WMD, lacking a weapon payload, simply as a kinetic projectile?