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On 2020 January 19 SpaceX launched the Crew Dragon capsule and tested an in-flight abort, where the capsule separated from the Falcon 9 shortly after launch.

Many headlines claim that the Falcon 9 was intentionally destroyed.

Isn't it more likely that the Falcon 9 was just left to its own fate after capsule separation, instead of intentionally triggering a self-destruct mechanism? After all, SpaceX was collecting telemetry from it until it exploded. SpaceX wouldn't have learned anything useful from destroying it before it fell into the ocean.

Secondarily, if it was left to its own fate, what caused it to explode suddenly? Will we have to wait for analysis of its telemetry?

A summary of recent comments and hints therefrom:

Quoting another answer, SpaceX expected

a Falcon 9 breakup... After thrust termination, the Falcon 9 trajectory would be uncontrolled and would be expected to start departing from the nominal trajectory. In this nominal scenario, the propellant is expected to be consumed in the deflagration or aerosolized. ... Off-nominal Scenario 2: Violation of autonomous flight termination criteria results in commanded destruct of Falcon 9."

Another answer, now deleted, surmises that

the Falcon 9's Range Safety System consists of explosive charges that "unzip" the fuel and oxygen tanks, allowing all the propellant to come together and combust rapidly in a huge fireball. The charges are detonated by command issued by... an autonomous onboard computer system.

Scott Manley's video analysis says:

"They didn't activate the flight termination system" (8:57 in his video). Had they done so, it would have "unzipped" and destroyed the second stage as well. But it was intact (SpaceX's video shows it from 3 to 10 seconds after the explosion; Scott at 9:18 shows a photo of it approaching the ocean), and it contained fuel and oxidizer that exploded upon ocean impact (10:22). At 9:48 Scott proposes that the booster split just below stage 2, tearing open stage 1's oxygen tank; a later rip in the fuel tank plus a hot spot would detonate stage 1.

So was the Falcon 9's destruction

  • manually commanded?

  • automatically commanded by the AFTS automatic flight termination system, after thrust stopped and trajectory departed from nominal? (SpaceX's video shows that it didn't slow down much relative to its "contrail." Neither did its attitude change enough to call its flight sideways. But its ballistic trajectory must have dropped below what an engine-on trajectory would have been.)

  • uncommanded, and just a side effect of (for instance) asymmetrical aerodynamic loads letting fuel and oxidizer mix?

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They didn't blow it up. They simply knew it would break up. A rocket is a very frail thing. It can only maintain it's structural integrity in forces it was designed to handle. When the Dragon left, the Falcon no longer had an aerodynamic nose cone. So supersonic wind forces pitched it sideways and the body of the rocket could not stand that force from that direction. Rockets don't go sideways. Here is a Russian Proton going sideways at much slower speeds. (shot in slow motion no less)

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    $\begingroup$ +1 "A rocket is a very frail thing," sometimes no more than a flying noodle; 1, 2. "Rockets don't go sideways"; when they try everything goes sideways. $\endgroup$
    – uhoh
    Jan 20, 2020 at 4:37
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    $\begingroup$ looking back at the footage it looks like it was blown up by the automated range safety devices after detecting deviation from the nominal flight path by more than a set margin. The failure of the booster was too symetrical to be caused solely by aerodynamic forces. $\endgroup$
    – jwenting
    Jan 20, 2020 at 4:46
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    $\begingroup$ "Here is a Russian Proton going sideways" - Actually the 'going sideways' part seems a lot less stressful on the rocket's structural integrity than the 'hitting the ground' part. :) $\endgroup$
    – aroth
    Jan 20, 2020 at 9:51
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    $\begingroup$ @jwenting if the FTS had been fired, the second stage would have blown up too. It did not. The failure looks exactly like what would happen if the common bulkhead on the first stage were to fail, allowing the LOX and RP1 to mix before combusting. $\endgroup$
    – Tristan
    Jan 20, 2020 at 14:56
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    $\begingroup$ @aroth: The structural integrity was already quite thoroughly compromised by 1:20 in the video; rockets aren't supposed to bend and fold and fall apart in flight, either. The big kaboom at the end was just icing on the cake. $\endgroup$
    – Ilmari Karonen
    Jan 20, 2020 at 16:10

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