From: niklas.holsti@tidorum.invalid   
      
   On 2020-07-26 5:03, Alain Fournier wrote:   
   > On Jul/25/2020 at 19:37, JF Mezei wrote :   
   >> On 2020-07-25 14:49, Alain Fournier wrote:   
   >>> Not nearly as important as recovering the first stage, but still   
   >>> recovering the nose cone reduces yet again launch costs for SpaceX.   
   >>> Their fairing recovery ships caught both halves of the nose cone on   
   >>> their launch Monday.   
   >>   
   >>   
   >> How is this done?   
   >>   
   >> is there logic on the fairing to control parachutes to steeer to the   
   >> location of ship with the net?   
   >>   
   >> Or does the ship with the net have high speed capability to position   
   >> itself under the arriving fairing?   
   >   
   > My understanding is that it is the latter, the ships are high speed and   
   > go to where the fairing will fall.   
      
      
   A considerable time ago, there was a video of a failed catch of a   
   fairing dropped from a helicopter. It was evident that the yaw (azimuth)   
   stability of the fairing+parachute was poor, with sometimes rapid   
   deviations left or right, which the ship could not match, because the   
   ship would have had to go "sideways" -- the ship could not turn quickly   
   enough to match the new trajectory.   
      
   This yaw instability probably depends a lot on the amount of turbulence   
   -- gusts -- in the wind. The videos from the recent successful catches   
   suggest a very calm day, with stable, laminar wind flow. Even so, the   
   fairings were caught close to the side edges of the nets.   
      
   Unless SpaceX can improve the fairing+parachute yaw stability, it seems   
   to me that the catch rate will remain low, and catches will succeed only   
   on such calm days.   
      
   Possibly the ship itself, and its motion through the air, are causing   
   turbulence in the wind wake. An aerodynamic design of the ship   
   superstructure might help.   
      
   Or the final approach and catch could be secured by using a drone to   
   carry a towing wire from the ship to the fairing, when the fairing is   
   say 100 m up, and then using the wire to slowly pull the fairing down   
   and into the net.   
      
   --   
   Niklas Holsti   
   niklas holsti tidorum fi   
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