The SpaceX Autonomous spaceport drone ships have longer and shorter direction, both for the flat, apparently land-able area, and total footprint in the water. The ships have vertical projections at the extreme ends.

There are several potentially meaningful directions in the problem, including:

  1. General direction of the waves; affects pitching of the ship
  2. General direction of the winds; affects rocket's trajectory
  3. Direction of rocket's return orbit, often from the West from Florida, but not necessarily so, especially from California (more polar orbits).

For landings on Mars there are elongated landing ellipses. But on Earth there is so much more live, constantly updating information available (e.g. GPS, meteorology) and of course some significant propulsive control at certain moments. Still, for Earth propulsive landing, wind direction and speed will vary along the path back to the surface and there can be gusting. I don't know if the ASDS had them, but laser doppler anemometers capable of profiling wind speed exist. When all is said and done, Is there anything like a landing ellipse for the Falcon 9?

At the time of landing, are the ASDS ships generally oriented with their longer direction parallel to the direction with the largest residual uncertainties in the landing location, or parallel to the direction of oncoming waves to minimize the amplitude of wave-induced pitching, or does it not really matter?

below: A SpaceX Anonymous Spaceport Drone Ship (ASDS), from here.

enter image description here

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    $\begingroup$ That spaceport drone ships use an existing barge model, see mcdonoughmarine.com/assets/mcd-spec-sheets_v8-marmac_300.pdf These barges are rectangular but have a dedicated bow and stern section like traditional ships. If there are no waves at all, the direction may be choosed free, but with waves any responsible captain will prefer to hold the bow into the waves. $\endgroup$ – Uwe Jun 25 '17 at 15:48
  • $\begingroup$ @Uwe even a responsible "autonomous captain" :) Thats a nice cross-section, thanks for the pdf! $\endgroup$ – uhoh Jun 25 '17 at 15:52

The general consensus amongst the barge-stalkers in Reddit and the NSF forums is that the ASDS aligns itself long wise into the waves. This allows it to minimize rockings as it is longer than the wavelength of most waves. If the waves are that far apart, odds are good they cannot land in that weather.

They do apparently attempt to land on the X, as the BulgariaSat-1 landing, which came in quite a bit off center showed, as Musk tweeted that a sudden gust (or some malfunction) hit at the very end of the landing burn to push it off the center.

  • $\begingroup$ So I'm thinking in calm seas with gusty winds... there might be a tendency to provide a few more meters in the direction where a gust could induce an error. I don't mean try to land "upwind" - of course target the center, but in calm seas, why not orient long side parallel to gust-axis? $\endgroup$ – uhoh Jun 25 '17 at 16:00
  • $\begingroup$ @uhoh speaking about the sea... You can't have calm seas and gusty winds. Winds * fetch = waves. Fact, seas are big, so they tend to have large fetches. $\endgroup$ – Aron Jun 26 '17 at 3:12
  • $\begingroup$ @Aron I'm referring to the expanded discussion of wind in my question, "wind direction and speed will vary along the path back to the surface and there can be gusting." You can have unexpected gusts at different altitudes, but only the wind very near the water surface affects wave formation. Further, waves which are large enough to cause significant pitching of a ship as large as the ASDS can take a significant amount of time to form. Sudden changes don't make large waves instantly. This is a 4D meteorological problem (3D + wave evolution time) and can't be addressed with a one-liner. $\endgroup$ – uhoh Jun 26 '17 at 4:57
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    $\begingroup$ @uhoh haha all very true. My experience with fluid dynamics had mostly been practical, and I'll admit that my knowledge of high altitude wind is limited. However, I assumed that the length of the ship only mattered for low altitude corrections, near final approach, when the fins have the lowest amount of airflow, and reaction times are low. At least at the last 20m or so the wind is pretty uniform, baring 20m high waves. I will of course concede on your t evolution point... $\endgroup$ – Aron Jun 26 '17 at 5:08
  • $\begingroup$ @Aron It's pretty hard to say anything about the sea or marine weather that doesn't have exceptions. With a first stage height of over 40 meters, it may even be subject to a little bit of shear. $\endgroup$ – uhoh Jun 26 '17 at 5:19

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