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Wouldn’t having the grid fins extended from launch to booster separation just cause extra drag?

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    $\begingroup$ Starship doesn't have any grid fins. The Superheavy booster does, and those can't retract and aren't meant for hypersonic velocities. $\endgroup$
    – Erin Anne
    Commented Mar 16 at 1:38
  • $\begingroup$ I was talking about the ship as a whole. The Starship booster has much larger grid fins in the same location as a Falcon 9 boosters. Falcon 9 does not launch with them extended. Why would the Starship booster have them extended at launch and through the ascent? The drag through supersonic flight has got to be horrendous. Trying to understand why you would want all that extra drag. $\endgroup$
    – PNW Mike
    Commented Mar 16 at 4:48
  • $\begingroup$ @ErinAnne - a SpaceX commentator referred to them as hypersonic grid fins (T+6:20 in the broadcast). Possibly a holdover from Falcon 9 which often exceeds Mach 5 prior to MECO. Starship at MECO was going 5,700 km/h at 67 km which does sort of squeak in as hypersonic based on the speed of sound at that altitude (5,400 km/h according to one source that I looked at). Although it still seems a bit of a misnomer as neither Falcon 9 or Starship are actively using their grid fins during that phase of launch, they use them during descent at about half that speed. $\endgroup$ Commented Mar 16 at 6:27

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The four grid fins on the Starship booster weigh 3 metric tons each (6,600 lbs), which is about twenty times heavier than the Falcon 9 grid fins, based on an estimate that I found of 140 kg for Falcon 9.

Grid fins Starship grid fin (Starbase Tour with Elon Musk)

I don't think SpaceX has said why they don't deploy the grid fins on Starship like they do on Falcon 9, however the widely accepted explanation is that it is simpler and saves weight, as stated in this answer (to a different question) on Aviation Stack Exchange:

They originated on Falcon9, where they fold away on ascent and fold out on return. But on Starship they don't fold away, saving the mass of the folding mechanism AND incurring the drag of flat edges on the way up.

In the video linked above Elon Musk stated that the motor that rotates the grid fins is essentially a Model 3 motor.

The grid fins actually don't present that large of a cross section when flat, as seen in this photo of the recent Starship test flight:

Grid fins top view

Compared to the drag created when tilted:

Grid fins flat and tilting

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    $\begingroup$ Wow, the size of the fins is impressive. $\endgroup$ Commented Mar 16 at 14:02
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    $\begingroup$ Elon Musk mentioned they are very over-engineered. Or, in some sense, not engineered at all. They know how grid fins work from Falcon 9, so they decided to go with grid fins in order to accelerate development, and basically make them large and thick with the option to reduce or replace them later when they gather more data. He mentioned they could probably be made lighter, even staying with the same profile, but most likely, they could also be made smaller. You'd also only need three, not four, and the third one could be a lot smaller than the other two. $\endgroup$ Commented Mar 16 at 17:48
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    $\begingroup$ ...and by not folding, there is one less thing to go wrong. $\endgroup$
    – sdenham
    Commented Mar 17 at 13:15

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