Reviewing the last few seconds of the SN 9 flight revealed an unrecoverable over-rotation which the gimballing of the lone functioning rocket engine could not compensate for.

The "flip" maneuver was started from a falling horizontal position by closing the tail fins, which reduces drag on the bottom end. This rotates the rocket to vertical. The second engine is supposed to control the rotation but:

  1. Would not re-opening the tail fins before the bottom of the arc help slow the rotation?

  2. Does not leaving them closed actually hinder correction of
    over-rotation by presenting a far more draggy "closed cup"

It is known that the crash was caused by the second engine not igniting, but could these Starships be easier to land by flying them aerodynamicly a bit better?

As a final thought, would slowing the rate of rotation, and flipping a bit higher, which would require more fuel to reach 0 rate of descent, but may be more controllable, also increase the odds of landing success?

  • 1
    $\begingroup$ Landing success seems to be quite a low priority for these test flights. Didn't Musk once say about one that he'd call it a success if they didn't destroy the launch site? $\endgroup$ Feb 22, 2021 at 16:09
  • 2
    $\begingroup$ looking at the two maneuvers of sn8 and sn9, the speed seems still high when reaching vertical attitude. Falling backwards at high speed with the flaps open could increase pitch instability, like what happens with airplane's horizontal stabilizer during a tailslide aerobatic maneuver, which tipping over direction can be unpredictable and brutal. $\endgroup$
    – user721108
    Feb 22, 2021 at 16:21

1 Answer 1


Yes, reopening the fins mid-flip would of course increase drag and help to stop the flip.

I'll guess that SN9 didn't do that because the increasing weirdly turbulent drag during reopening is much harder to model than the force exerted by the gimbaling engine. Less predictable response demands bigger safety margins: more fuel, faster gimbals, faster throttle response, slower approach, slower flip, all cascading together in a tangle of cause and effect.

Let's see what SN10 does.


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