The grid fins on Starship are serrated, why? Does it help in transonic maneuvers?
Source: Starbase Factory Tour with Elon Musk [Part 1], YouTube, at 30:28
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3$\begingroup$ Maybe Elon's having Fred Flintstone over for dinner later and needs to tenderize a couple tons of brontosaurus steak? Anyway, welcome to Aviation.SE -- please be sure to take the tour and read over the FAQ to get a better idea how Stack Exchange sites work. $\endgroup$– Zeiss IkonAug 4, 2021 at 18:20
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1$\begingroup$ Wouldn't this be a better fit on Space.SE? $\endgroup$– Ralph J ♦Aug 4, 2021 at 18:36
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5$\begingroup$ @RalphJ It's specific to the aerodynamic reasons for this shape, which affect only atmospheric operation. Firmly in a gray area, IMO. $\endgroup$– Zeiss IkonAug 4, 2021 at 18:58
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9$\begingroup$ @RalphJ For questions about the rocket, I would agree. But since this is about the aerodynamic design of the grid fins, I think the question also fits here. $\endgroup$– BianfableAug 4, 2021 at 18:58
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3$\begingroup$ Well those fins are useless in space, and they are an aerodynamic apparatus used to guide flight, so definitely a question best suited here. $\endgroup$– Jpe61Aug 4, 2021 at 20:00
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2 Answers
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Such design, with hyperbolic leading edges, has been invented by MBDA (Airbus branch for missile systems) represented by BAE Systems (defense contractor), and is described in the European patent 3 599 442 A1 filed on July 2018:
The curve helps reducing drag, especially at supersonic velocities, an important factor for missiles and rockets, for which fuel is very limited.
Having a hyperbolic curve for the leading edge planform shape provides for a lower drag, in particular a lower wave drag. This means that the grid fins could be used as efficient lift and control devices/surfaces for supersonic flight vehicles.
Fins in place at the top of a 70 m-tall Super Heavy Booster 4:
The booster is used for the SpaceX Starship program. Curved leading edges are pointing at the ground, in the position required for booster return to its launchpad (video of the return trip). Fins are indeed used for the atmospheric flight. In space attitude is controlled using thrusters or reaction wheels.
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5$\begingroup$ Since those fins are used in the descent phase, isn't it counter-productive to minimize their drag? After all, the rocket needs to decelerate when those fins are deployed. $\endgroup$ Aug 4, 2021 at 20:42
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5$\begingroup$ @PeterKämpf: They are not used to decelerate, but to maintain the stage vertical for engine efficiency. The engine is used for deceleration. $\endgroup$– minsAug 4, 2021 at 20:45
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4$\begingroup$ @PeterKämpf: My assumption is that it's more efficient to have the engine burning in the right direction than to try to add some deceleration with the fins, at the cost of having them less reactive, so having a thrust vector component perpendicular to the desired deceleration axis, and burning more fuel, which is very limited. Just a guess. Note the first use of curved fins is for missiles, this use here is a second application. $\endgroup$– minsAug 4, 2021 at 20:58
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6$\begingroup$ @PeterKämpf: A booster doesn't go straight up, it goes quite a long way down range. If you want it to fly back to the point of origin, minimizing drag means less fuel needed for the flyback, so more can be devoted to launching payload. $\endgroup$– jamesqfAug 5, 2021 at 1:27
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4$\begingroup$ Minimising drag also minimises friction. Originally SpaceX’s grid fins were made of aluminium which on its own couldn’t survive without an ablative coating. Newer fins were subsequently changed to titanium which doesn’t need the coating, but I imagine that you’d still want to reduce heat if you’re planning to reuse them many times. $\endgroup$ Aug 5, 2021 at 11:39
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Edges like these reduce drag. 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.
@ElonMusk. Rotating the pointy edges to face upwards during launch will likely save drag on the way up too. Then rotate them down at the azimuth for return. Alternatively, sharpen the top edges the same, to cut both ways and maybe even save more mass? Then if the drag on the way up is reduced, might that imply a reduction in the mass of the mount-points as well?
Afterthought. Boosters will be reused a lot (maybe 100 x). (Mass and drag) saved by booster optimization will yield mass-to-orbit scaling of the product: (#launches x #boosters). Seems worth it.
