Where Are the Fins? (2009) [pdf] | Hacker News Reader

Where Are the Fins? (2009) [pdf]

apogeerockets.com

45 pointswaynenilsen14 comments

There's another reason the Ares 1 doesn't have fins, which doesn't seem to be discussed in the OP, namely: big rockets have really thick boundary layers. The boundary layer is the layer of air that a vehicle (rocket, airplane, ship, etc) "pulls" along with it as it moves through a fluid, due to the friction between the skin of the vehicle and the fluid. Basically, you don't get free laminar flow right at the skin of the vehicle. If you want to use a control surface it has to be big enough to stick far enough out into the fluid to deflect enough air (or water) to be effective.

Boundary layer thickness doesn't scale linearly with size, which is why small scale model rockets can make effective use of fins, whereas the full-sized vehicle cannot.

Combine this with the fact that, as the OP discusses, a rocket is a really unstable system and fins don't really become effective until you get a high enough speed, and that explains why the Ares 1 needs a gimballed attitude control system in the first place.

Interestingly, really early rockets, like the V-2, had control surfaces that were directly beneath the rocket nozzle, in the exhaust stream. They were, effectively, gimballing the exhaust stream by deflecting it instead of pointing the nozzle. It needed them in part because at altitude the atmosphere was thin enough that the fins weren't effective. You can see them in the picture of the V2 in the Air and Space museum:

http://airandspace.si.edu/webimages/collections/full/A196003...

The little red things close to the rocket nozzle are control surfaces.

Rumor has it that the Saturn V only had fins because Werner von Braun decided that a rocket without fins just didn't look right, and ordered the engineers to add them, even though they were so small with respect to the size of the rocket stack that they were very solidly inside the boundary layer and had no effect whatsoever.

You can see them clearly here: https://www.flickr.com/photos/nasacommons/4858567248/

> Rumor has it that the Saturn V only had fins because Werner von Braun decided that a rocket without fins just didn't look right, and ordered the engineers to add them, even though they were so small with respect to the size of the rocket stack that they were very solidly inside the boundary layer and had no effect whatsoever.

Hah! Thanks for this tidbit, I had never stopped to think through their presence on the saturn 5, and absenece on other vehicles.

According to von Braun's Popular Science AMA it was a safety feature:

"It is in the area of crew safety that fins come in handy. In Saturn V the booster fins are not to provide perfect aerodynamic stability under all conditions - that would take fins of excessive size. But the fins reduce the aerodynamic instability enough to make sure that the astronauts can safely abort, no matter what technical trouble may afflict their space vehicle."

( https://books.google.de/books?id=MiYDAAAAMBAJ&pg=PA184 , continued from https://books.google.de/books?id=MiYDAAAAMBAJ&pg=PA68 )

Both could be true at once, just like when bicycle helmets are worn more as a fashion statement than out of confidence in their protective value. Or maybe he was just squelching rumors.

Answering this accurately would require a lot more information about the aerodynamics of the Saturn V than I have. But I suspect it was the latter - he was justifying a design decision after the fact.

I say that because for the fins to do any good at all, boundary layer or not, the launch vehicle has to be moving at speed. I don't think a successful crew capsule abort would be likely past a couple of hundred miles per hour. If that.

> I don't think a successful crew capsule abort would be likely past a couple of hundred miles per hour. If that.

That's not true at all. Every manned space launcher has provisions for bringing the crew down safely on a suborbital trajectory if orbit can't be reached. You would hardly give up on your astronauts just because, say, some stage failed to light. It's not like with an unmanned satellite, where it either gets to orbit or crashes/burns up.

Apollo certainly had provisions for crew abort all the way to orbit, which are discussed here: https://en.wikipedia.org/wiki/Apollo_abort_modes

I agree that they had those provisions. I'm just skeptical about them working if the primary stack ACS went south. The crew capsule didn't have its own ACS, IIRC. It would have likely entered a high rate tumble shortly after an abort separation.

It would have been worth a shot, just like jumping out of a speeding car about to go over a cliff. But you probably die anyway.

The russians have the correct solution of simply adding rockets vaugely looking like finns instead of fake fins!

Thanks for some very clear and interesting commentary! This is everything that I like about HN.

How does the boundary layer scale with size, if not linearly?

With the square root of the length.

Also, apparently, the rocket exhaust actually recirculated up the sides of the Saturn V inside the boundary layer, well past the point where the fins attached. See the picture in the following paper, where it appears that the exhaust is originating from a point about halfway up the first stage.

https://www.nas.nasa.gov/assets/pdf/Kiris_Gusman_PlumeCFD.pd...

Which tells you something about the size of the Saturn V boundary layer.

Launch of a small model rocket with active stabilization.[1] This is common now, since every quadrotor drone has similar control hardware.

[1] https://www.youtube.com/watch?v=kTND_wot9zI

My first thought: https://en.wikipedia.org/wiki/Finland_does_not_exist

"The closest thing to a fish's ass."

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