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The CRS-16 missions first stage, seems to have had a stuck grid fin. Apparently a hydraulic pump failed.

It was spinning fairly wildly and clearly did not try to land at the pad as it was scheduled.

How did they abort to the water if they were so out of control?

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Here is a graphic of the ascent and landing profile

Here is a graphic og the ascent and landing profile that shows the default to water trajectory, with the "corrected" landing if the computer determines all is well.

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  • $\begingroup$ Good find! I remember seeing that pic. Very relevant. $\endgroup$ – geoffc Dec 6 '18 at 2:24
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To be fair, I asked, so I could answer, so it would be ready when people come looking.

SpaceX aims at the water, away from the actual landing point (pad, barge, whatever) and only diverts if all is going well.

They thought about this exact circumstance in advance and it worked well. They did not need to divert away as they were not yet aimed at the landing pad.

This can be seen when the Falcon Heavy center core failed to land on the ASDS OCISLY earlier in 2018 it was aimed off to the side, and ran out of fuel to divert to the actual barge surface.

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  • $\begingroup$ In this particular case it would actually have been benefitial though if it had been aimed at the pad right away, wouldn't it? The landing itself seems to have been soft as intended after all, just unfortunately not on solid ground... $\endgroup$ – leftaroundabout Dec 5 '18 at 23:28
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    $\begingroup$ @leftaroundabout In this case, perhaps. They likely choose the overly cautious approach in the interest of not hitting people or needing expensive landing area repairs, as other failure modes may be less controllable than this one was. $\endgroup$ – ceejayoz Dec 6 '18 at 14:16
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    $\begingroup$ They always aim for a safe crash, and only when things are going well, divert to land. This is the safest approach. Agreed, in this case they might have saved a stage, but not worth the risk. $\endgroup$ – geoffc Dec 6 '18 at 15:01
  • $\begingroup$ I don't agree that the soft splashdown can be taken as evidence a landing was possible. We have absolutely no way of knowing how close the splashdown was to the intended touchdown point, and the fact that the fins were providing an undesirable input that the engine was struggling to null out means there likely was little control over actual touchdown point. Safe procedures kept it at sea, not active guidance. $\endgroup$ – Saiboogu Dec 6 '18 at 18:51

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