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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?

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    $\begingroup$ the velocity profile of a F9 landing suggests your 6-8000 m/s is about 4 times higher than reality. The answer also suggests the terminal velocity of a F9 booster with landing burn fuel on board is below 400m/s. $\endgroup$
    – user20636
    Commented Apr 12, 2019 at 9:00
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    $\begingroup$ That doesn't consider the case where no re-entry burn is made - however that is fuel inefficient, so it is likely necessary for the booster to survive. $\endgroup$
    – user20636
    Commented Apr 12, 2019 at 9:01
  • $\begingroup$ @JCRM if you watch the ARABSAT-6A MISSION replay at 22:34 it indicates a speed of 5865 km/h at separation. A little below the 6000, and definitely not 8000, and not the typical case, but still, this show that a Falcon 9 booster can reach such speeds. Of course this is in great part "horizontal" velocity. $\endgroup$
    – JFL
    Commented Apr 12, 2019 at 14:09
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    $\begingroup$ It will be constantly decelerating due to air friction. Actual speed on impact will be pretty slow $\endgroup$
    – Innovine
    Commented Apr 12, 2019 at 15:59
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    $\begingroup$ ooops, you were using km/h not m/s -- which is a factor of 3.6 - or "about 4" $\endgroup$
    – user20636
    Commented Apr 15, 2019 at 13:53

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The booster decelerates as it comes down through the atmosphere. From its initial very high speed air friction will provide a larger force than gravity, so it decelerates rapidly, not accelerates.

I don't have numbers for you, but if you look at ballistic and near ballistic reentry capsules like soyuz, mercury and gemini, they all decelerate at several g to subsonic speeds just by air resistance. Soyuz deploys its parachute for braking once it has slowed down to around 200-300m/s. These capsules enter the atmosphere at higher speed than the booster, too.

By looking at the videos of the falcon booster landing, its not going terribly fast when the engine ignites. Probably under some hundreds of kph. Its a big heavy booster, so it'll do quite some damage to whatever it hits, but it's not going to be much different to an airplane crash, roughly speaking.

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    $\begingroup$ Ahh, that may actually invalidate my question - I assumed that without the re-entry burn to slow it, it would accelerate as it descends (and heat up and maybe melt). I didn't realize that the atmosphere slowed down ballistic projectiles that much. Thank you! $\endgroup$
    – sneak
    Commented Apr 12, 2019 at 23:10
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    $\begingroup$ @sneak I did a somewhat similar (and simple) calculation for this answer. Things usually either vaporize, blow up, or if they have good heat shields, slow down substantially before hitting Earth's surface. $\endgroup$
    – uhoh
    Commented Apr 12, 2019 at 23:49
  • $\begingroup$ @sneak, as a rule of thumb, anything that isn't either very dense or very streamlined will hit the ground at less than the speed of sound (possibly after breaking up in midair). $\endgroup$
    – Mark
    Commented Aug 29, 2021 at 7:48

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