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As I see it, Falcon 9 has to land on the barge in a vertical orientation and with no horizontal drift.

It has fins that are ineffective at low speed. It has a rocket motor that can probably vector some, but doing so it gives a rotating (yawing) moment to the craft.

Are there some more tools, like horizontal nozzles at the top that can counteract wind pressure?

Or, can it land at an angle from vertical on two legs only?

New thinking:

Would a dynamic approach be possible? Coming in towards the platform from downwind (against the wind) at a higher speed than wind speed with the base of the rocket pointing slightly into the wind. So the rocket retards to wind speed and rotates to vertical exactly at touchdown.

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  • $\begingroup$ Where did you get the idea that the grid fins are ineffective at low speed? They work perfectly fine at even less than 10 m/s. $\endgroup$
    – marked-down
    Jan 18, 2015 at 0:30
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    $\begingroup$ Any control surface will produce less correction force as the airspeed goes down, ultimately zero force at zero speed, even if they are working perfectly. I got this idea from the book "Aerodynamics for naval aviators". $\endgroup$
    – Wirewrap
    Jan 18, 2015 at 18:20
  • $\begingroup$ While that's true, check out the F9R-Dev video where they test out the grid fins. They are in full control of the vehicle from launch to touchdown. $\endgroup$
    – marked-down
    Jan 18, 2015 at 20:51
  • $\begingroup$ I checked some F9R flight test videos. Seems to be tests flown without grid fins and tests with grid fins. Beautifully stable. Full control with fins and without. So, at least fins does not mess up low speed flight! $\endgroup$
    – Wirewrap
    Jan 18, 2015 at 22:30

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First, its descent speed is very rapid, the deceleration very strong - only the last meter or so is probably below 10m/s. Until then the grid fins work fine. And for the remainder, there are the RCS thrusters. Plus the engine vectoring can keep it angled against the wind and only let RCS and the legs straighten it after touchdown.

OTOH it's a 40-meter mostly empty thin tube on pretty short legs, and possibly on a swaying platform. Once it touches down and RCS is off, it can very much be tipped over by the wind. But if it can keep standing unpowered in the wind, it can stick the landing. It's not a very strong wind it can withstand, certainly not a storm, but the RCS thrusters can help it quite a bit.

And of course if the wind is too strong, the launch will be delayed. It's not like in the several minutes since launch a totally unexpected storm will appear.

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  • $\begingroup$ Is RCS really of any help? Do we have any numbers (or estimates) of their thrust? Without any specific knowledge about it, I kind of doubt RCS is very helpful outside vacuum. The stage weighs about 25t... (I don't remember seeing RCS thrusting on landing aside from the futile attempt to save CRS-6) $\endgroup$
    – radex
    Jun 6, 2016 at 9:54
  • $\begingroup$ @radex: I was unable to find the thrust of the current cold gas thrusters, but they replaced earlier Draco engines for stage 1 RCS, and these had 400N of thrust. Look at the video of a failed landing - how far off vertical the rocket is when the RCS almost manages to right it. $\endgroup$
    – SF.
    Jun 6, 2016 at 10:31
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It probably can't. Where did you see that it can? Launch commit criteria prevents launching if the conditions at the first stage landing site are prohibitive.

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  • $\begingroup$ This seems to read more like a comment than an answer $\endgroup$
    – raptortech97
    Jan 18, 2015 at 3:45
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    $\begingroup$ Well the question is along the lines of "When did you stop beating your wife?" $\endgroup$
    – Erik
    Jan 18, 2015 at 4:39
  • $\begingroup$ Do we know any launch/landing criteria? $\endgroup$
    – Wirewrap
    Jan 18, 2015 at 18:22
  • $\begingroup$ There is also the option of expending a first stage if down range conditions aren't suitable for landing. Maybe the decision would be up to whoever is paying for the launch. $\endgroup$
    – Howard Miller
    May 10, 2016 at 21:58
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In this video, claiming a high-wind landing, the grid fins can be seen as they are moving until touchdown - probably they influence the orientation, albeit slightly.

Besides that, the orientation is controlled by the motor vectoring and some horizontal thrusters, best seen here and in action in this video.

Combined, they provide enough control to counteract the effect of the wind.

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