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The grid fins on Starship are serrated, why? Does it help in transonic maneuvers?

enter image description here
Source: Starbase Factory Tour with Elon Musk [Part 1], YouTube, at 30:28

Pioneer
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    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. – Zeiss Ikon Aug 04 '21 at 18:20
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    Wouldn't this be a better fit on Space.SE? – Ralph J Aug 04 '21 at 18:36
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    @RalphJ It's specific to the aerodynamic reasons for this shape, which affect only atmospheric operation. Firmly in a gray area, IMO. – Zeiss Ikon Aug 04 '21 at 18:58
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    @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. – Bianfable Aug 04 '21 at 18:58
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    Well those fins are useless in space, and they are an aerodynamic apparatus used to guide flight, so definitely a question best suited here. – Jpe61 Aug 04 '21 at 20:00
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    Wow, the photo really drives home how huge the rocket is. – Michael Aug 05 '21 at 08:22
  • @Michael It's funny, but I was just thinking the exact opposite. It's perspective of time, in my case, I suppose. As a kid I built Apollo rocket models to better visualize the launches I was watching on TV. As far as VtC, Jpe61's comment here says it all for me. VtLO – CGCampbell Aug 05 '21 at 12:49
  • @CGCampbell: For me the thing is that the rocket just looks incomprehensibly huge when put next to humans. But then you put humans next to the grid fins (which don’t look very big when attached to the rocket) and suddenly realize that even those are already huge. – Michael Aug 05 '21 at 13:27
  • Note that those grid fins are for Super Heavy, not Starship. Starship doesn't have grid fins, its aerodynamic control surfaces are flippety-flappety-bits. Unless you mean "the Starship system", which consists of Starship and Super Heavy. – Jörg W Mittag Aug 06 '21 at 19:04

2 Answers2

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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:

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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:

Fins in place at the top of a Super Heavy Booster 4

Source

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.

mins
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    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. – Peter Kämpf Aug 04 '21 at 20:42
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    @PeterKämpf: They are not used to decelerate, but to maintain the stage vertical for engine efficiency. The engine is used for deceleration. – mins Aug 04 '21 at 20:45
  • Yes, I understand that. But if they produced more drag, less engine fuel would be needed. – Peter Kämpf Aug 04 '21 at 20:54
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    @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. – mins Aug 04 '21 at 20:58
  • I'm guessing if they were mere paddles the steering/guidance behaviour would be poor at best (turbulence and such), and maybe somewhat unpredictable. Obviously this is a tradeoff between guidance and braking, and braking lost. Probably the fuel lost in braking was less than extra fuel needed for stabilizing flight with inferior fins (or paddles) – Jpe61 Aug 04 '21 at 21:04
  • I see Mins was faster writing this – Jpe61 Aug 04 '21 at 21:06
  • @PeterKämpf: Re when those fins are deployed -- those fins (on Starship booster) are not deployable, unlike on Falcon 9—Elon explained the reasoning behind it in an interview from which the frame in the question is taken. They'll remain like that and rotate to align with the airflow during ascent. My guess they'll rotate teeth-up during ascent. –  Aug 05 '21 at 00:17
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    @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. – jamesqf Aug 05 '21 at 01:27
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    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. – Robin Whittleton Aug 05 '21 at 11:39
  • @RobinWhittleton "Minimising drag also minimises friction" - was that just clumsy wording or are you indeed trying to say that drag <> friction ? – MikeB Aug 05 '21 at 11:50
  • Clumsy wording. My point was that more drag leads to more heat in the material. – Robin Whittleton Aug 05 '21 at 11:54
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    I think the main issue is heat. Grid fins are made of titanium to keep from melting. More drag makes this more difficult. – Eric S Aug 05 '21 at 13:19
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    @EricS: I understand Super Heavy's grid fins are actually steel. – Fred Larson Aug 05 '21 at 16:09
  • @FredLarson True, they're titanium on Falcon 9. The issue of heat is still the same. – Eric S Aug 05 '21 at 19:27
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    @RobinWhittleton, reentry heating mostly isn't from friction, it's from compression of the air. If you want to minimize heating, you need to maximize drag, which lifts the compression shock wave off the surface of the rocket and causes it to heat the air instead. See https://en.wikipedia.org/wiki/Atmospheric_entry – Mark Aug 05 '21 at 20:51
  • @ymb1 wouldn't teeth-down during ascent generate less drag since they act like conventional trailing edge? (most of the ascent isn't super or hypersonic) – user721108 Aug 11 '21 at 09:30
  • @qqjkztd: First stages generally exceed Mach 1 during ascent, but not for long as you said. Though if they don't during ascent for long, that means they don't during descent for long either! So what's the point? :-) / Anyway it's just a guess like I wrote; maybe it will only be needed when the payload is pushed higher and not during earlier test flights, or maybe the motors don't/won't allow such full-range of motion. –  Aug 13 '21 at 00:02
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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.

wwmbes
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  • Wouldn't rotating the fins just 90 degrees (facing up the narrow edge) on the way up save even more drag? Or am I missing something? – Petr Dlouhý Mar 18 '24 at 09:14