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They are reporting that the three engine relight in the upper atmosphere worked. But the relight of a single engine, lower down for pretend landing on the water generated too much torque and the fuel (in the mostly empty tanks) centrifuged away from the feed line and the engine ran dry.

Why did it torque out of control?

Momentum from the turbopump?

Did the RCS run out of fuel to counteract/control it?

The three engine re-light, might make sense that was probably center and two side engines, so could gimbal two of them to counter any rotation.

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For the time being, there likely won't be more information than what Elon Musk tweeted two hours ago:

Rocket booster relit twice (supersonic retro & landing), but spun up due to aero torque, so fuel centrifuged & we flamed out.

This was an attempt at testing the rocket's ability to reignite its engine to reduce its velocity prior to hitting the Pacific Ocean, part of the reusable Falcon 9 rocket system to reduce the cost of future launches, but wasn't the launch's primary objective. The rest of the Falcon 9 launch went nominal though, and it successfully deployed its cargo into LEO (Low-Earth Orbit).

Musk later tweeted:

Between this flight & Grasshopper tests, I think we now have all the pieces of the puzzle to bring the rocket back home.

From the comments, thanks @Chris, another tweet from Jeff Foust, space industry analyst and writer, provides a bit more perspective on the matter:

Musk: Lower stage 3-engine relight went well, reentered. Single-engine relight went well, but exceeded roll control of ACS.

So, according to all of these tweets, after the first stage successfully launched and separated from Falcon 9 second stage, Attitude Control System (ACS) wasn't capable to correct for the roll of the first stage due to atmospheric torque centrifuging the fuel and choking the ACS.

Edit: And at the 3:30EST press conference, they mentioned that the landing legs are designed to be aerodynamic for some extra control at landing time.

Since I don't have anything more, here's a nice photo of the Falcon 9 lifting off from SpaceX's pad at Vandenberg Air Force Base in California on September 29, 2013, carrying Canada's Cascade SmallSat and Ionospheric Polar Explorer satellites (or CASSIOPE) to orbit:

    Falcon 9 lifting off from SpaceX’s pad at Vandenberg, carrying Cascade SmallSat and Canada's CASSIOPE satellite to orbit

Video of the launch is now available on YouTube.

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  • $\begingroup$ Apparently, at least according to ArsTechnica, another secondary objective didn't go quite according to plans, and the second stage engine restart: "... encountered a condition it didn't like during repressurization and the process was shut down." This wasn't a critical part during this launch, though. $\endgroup$
    – TildalWave
    Sep 29, 2013 at 22:52
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    $\begingroup$ @TildalWave But the second stage restart was crictical for next couple of GEO launches. So that is probably the most important failure. They have tweeted that they know the root cause and have a fix already, now they have to convince the insurers and SES and Thaicom to trust them. $\endgroup$
    – geoffc
    Sep 29, 2013 at 23:15
  • $\begingroup$ @geoffc - Agreed, it is important and cheers for the edit! I didn't want it to appear as unimportant at all, just said that it wasn't critical for this particular mission. For any orbit where LV first assumes a transfer orbit, this would be crucial. I actually wanted to edit it into the answer, but have been still searching for more things to add, new info is coming in literally every half an hour. Elon Musk said the landing legs on the Grasshopper would act as fins and stabilize the reentry, that's what he meant with the second tweet I posted. ;) $\endgroup$
    – TildalWave
    Sep 29, 2013 at 23:23
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    $\begingroup$ @TildalWave This has been a totally awesome day, I am enjoying all of it. Interesting how large the payload hit will be for recovery of the first stage though. $\endgroup$
    – geoffc
    Sep 30, 2013 at 1:16
  • $\begingroup$ One begins to see the point of the Falcon Heavy, with much more payload capacity than current customers need. But if the first three stages are resuable, then who cares what the penalty is? It would still be way cheaper than a non-reusable Falcon 9. $\endgroup$
    – Mark Adler
    Sep 30, 2013 at 3:08

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