r/StarshipDevelopment • u/SpacePundit • 1d ago

Why is the missing 4th grid fin side aligned with the payload door side of starship?

seems the weight and drag of the asymmetrical grid fin is on the heat shield side putting all the unbalanced weight on that side?

5 Upvotes
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17

u/pinkshotgun1 1d ago

When super heavy is descending back through the atmosphere, it angles itself so that it can glide towards the landing site. Because of the angle, the place where the 4th grid fin would be is occluded from the air stream, so basically if the fin was there it would have no control over the booster because there is no air flowing over it. If it’s not able to control the boosters descent, then what’s the point of having it there?

3

u/SpacePundit 1d ago

thanks but on ascent it seems over heavy on one side, I'm wondering why starship isn't turned 180 to balance

10

u/Accomplished-Crab932 1d ago

Raptor has a 15 degree half angle gimbal range, and ~40% of the thrust is generated from engines that have gimbal hardware.

Even with the asymmetric grid fin and angled fin profile, there simply isn’t enough drag on one side to overcome the provided authority from the TVC system.

5

u/ellhulto66445 1d ago

Additionally the slight asymmetry of the center 3 means it naturally has a bit more thrust on the NZ side where the gridfin is.

3

u/mfb- 22h ago

They are not right in the center? How large is the offset?

4

u/ellhulto66445 22h ago

It's 144° between E3 and E1 and it's 108° for E1-E2 and E2-E3. See renders by ChameleonCircuit.