
Where Are the Fins? (2009) [pdf] - waynenilsen
https://www.apogeerockets.com/education/downloads/Newsletter248.pdf
======
GlenTheMachine
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...](http://airandspace.si.edu/webimages/collections/full/A19600342000d19.jpg)

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/](https://www.flickr.com/photos/nasacommons/4858567248/)

~~~
le-mark
> 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.

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

~~~
GlenTheMachine
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.

~~~
yaakov34
> 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](https://en.wikipedia.org/wiki/Apollo_abort_modes)

~~~
GlenTheMachine
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.

------
Animats
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](https://www.youtube.com/watch?v=kTND_wot9zI)

------
radious
My first thought:
[https://en.wikipedia.org/wiki/Finland_does_not_exist](https://en.wikipedia.org/wiki/Finland_does_not_exist)

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

