I wonder if Space X’s difficulties with landing on the barge might be due to any perceived motion of the target? That is, is there any movement, roll, pitch, and/or yaw in the drone ship that adversely affects how the rocket perceives the distance and attitude to the target?
I’m also reminded that Apollo 11’s Lunar Module had information overload errors and couldn’t keep up with the stream of information the landing radar was feeding it. Maybe something of the sort is happening with the Space X vehicle, too, and the guidance systems can’t stick the landing.
Looking at the good video of the landing, the ship does appear to rocking quite a bit, and it’s not that big of a target. As the rocket enters the top of the frame, the main engine, which is controlling lateral movement, is rocking back and forth (gimballing) quite a bit. (I’d say it was a having difficulty staying on target.) For the last few hundred feet there doesn’t seem to be any steering motion and the rocket begins to tilt quite a bit. As the rocket gets close to the deck, the engine gimbals again and steers back toward the center of the deck, but isn’t able to completely get the rockett back upright again.
First thing I thought was why don’t they spin it when it lands sorts like how it spins or rotates after takeoff for stabilization? Then again I’m no rocket scientist.
This is a minor symptom that could contribute, but in the vid you can see a last minute trajectory correction to zero in on the barge and get vertical. This imparts a extra horizontal motion which the craft has trouble correcting. There's not much time for this - it has to be dead on due to a major constraint on the flight envelope - the engine is too powerful at minimum throttle. Even when throttled down to minimum it still produces more thrust than the weight of the craft. Therefore it needs to decelerate to zero velocity at the moment of landing but not before, aptly entitled "hoverslam". I suspect the barge is a bit too small, there's not quite enough time to null out the horizontal velocity before landing. Probably it would have no trouble landing on a field like the Ellipse, for example, which is within cross-range on ISS rendezvous launch I bet, heheh.
Maybe they should launch from a barge instead and arrange things so the landing field is properly down range. That new spaceport in Brownsville probably offers a sea of options for booster recovery as well.