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Let's suppose that the grid fins of the Falcon 9 remain open and the rocket still launches. This is probably never going to happen because the SpaceX team is exceptionally good, but I am trying to see what will happen for a personal project.

So what will happen during the flight if the fins are open even before launch?

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I can think of three principal issues raised by launching with fins deployed; I would guess that none of them would endanger a mission, but I don't have any inside information to support that.

  1. Increased drag from the deployed fins. This would hurt performance slightly, but there is some performance margin in the launcher, and the fins aren't designed to create large amounts of drag. I would not expect the additional drag to compare to the performance loss from an engine-out, for example.
  2. Aerodynamic stability. Aerodynamic surfaces above the rocket's center of gravity reduce stability; finned things want to fly with the fins toward the back (like a fletched arrow). The second stage and payload are above the fins, so the fins aren't that far ahead of the center of mass, and the gimbaled rocket engines should provide enough control to compensate, but if the rocket is normally flown with minimal stability margin, this could push it over the edge, causing it to flip and break apart. (I suspect the engines have plenty of control authority, but I don't actually know for sure.)
  3. Damage to the fins themselves. If I remember correctly, the dynamic pressure sustained during reentry, while the fins are normally deployed, is quite a bit higher than the maximum dynamic pressure on ascent, so the fins should be able to survive easily.
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If you have noticed, the fins deploy by opening upwards. This makes me wonder if they considered your case.

When in normal use, the force vector upon them pushes upwards.

In your case, the force vector would be forcing them down and closed.

So first approximation they would be forced down to a minimum. They would add significant drag in a non-fully open mode. I.e. Would possibly cause less resistance if fully open and inline vs pushed partially down and more of an angle.

Likely they would be able to continue the mission, but it would probably be a bad thing.

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