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Pretty much all model rockets have fins for stability, but when I think about it, I can't seem to come up with a full scale, commercial or government rocket that has fins. I'm probably forgetting one really obvious, but does anyone have an example?

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    $\begingroup$ That would be an interesting project - A model rocket without fins, but with vectored thrust and an IMU. $\endgroup$ – Steve May 26 '16 at 20:55
  • $\begingroup$ @Steve I was thinking the same thing and found this interesting design: youtube.com/watch?v=xO0D0KEGc2E $\endgroup$ – cylo78 May 28 '16 at 15:57
  • $\begingroup$ You're definitely forgettng quite a few ICBMs, and other missiles. $\endgroup$ – SF. May 28 '16 at 18:50
  • $\begingroup$ I believe some fireworks are spin-stabilized... the border between these and amateur rockets is somewhat blurred :) $\endgroup$ – SF. May 29 '16 at 17:57
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    $\begingroup$ @Steve Vectored thrusts and the like is usually a big taboo in the model rocket community, for fear of opening up a can of worms with the differentiation between "toy model" and "guided projectile." $\endgroup$ – Sarah Bailey May 31 '16 at 20:20
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Sure, lots of sounding rockets, like these from NASA's Wallops Flight Facility:

stable of sounding rockets

This one had pretty darned big fins:

space shuttle

Similar fins on this one:

pegasus

This one had small fins, but it really didn't need them:

saturn v

The old Scout rocket had small fins too:

enter image description here

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    $\begingroup$ +1 That's the first time I noticed the Saturn V fins are labelled. It "didn't need them but they wanted somewhere to label the engines." heh heh. $\endgroup$ – Level River St May 27 '16 at 0:51
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    $\begingroup$ The Saturn V did not actually need them, but they were put in to increase safety in case of an abort. $\endgroup$ – Polygnome May 27 '16 at 6:12
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    $\begingroup$ Same comments about incidental stability and steering applies to the Saturn V. For shuttle on the other hand, those were quite impactful aerodynamic surfaces during ascent, providing lift as well. Anyway, it's all just a matter of interpretation. $\endgroup$ – Mark Adler May 27 '16 at 17:48
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    $\begingroup$ Just checked. I was incorrect. It flies at a non-zero angle of attack (hence why I thought it was lifting), but that is to zero out the lift so as to limit the structural loads on the wings. Otherwise they would break off. This does not minimize drag, so the reason you quote is also incorrect. Also the whole point of wings is that you can get more lift than drag. $\endgroup$ – Mark Adler May 27 '16 at 18:38
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    $\begingroup$ If the wings were strong enough, they could have been used to provide lift on ascent. The Pegasus does that. It was the original plan to use the space shuttle orbiter wings to improve ascent performance, until they realized that the wings would break off at max-q. It was too late to go back and strengthen the wings, so they figured out the right angle of attack so as to not load the wings on ascent. $\endgroup$ – Mark Adler May 27 '16 at 19:10
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As for the reason you see fins on model rockets but not on most large launchers:

Rockets that require precision guidance to a specific trajectory have to be actively steered; the two obvious ways to do that are by moving fins or vectored thrust.

Vectored thrust has two big advantages: one, it works outside of the atmosphere, two, fins produce drag. So big guided orbital launchers go with vectored thrust.

Passive fixed fins like those on model rockets help stabilize, but not guide, the rocket's flight. If you have active steering, that gives you stabilization as well, and unless the aerodynamics of the rocket are very poor, you don't need fins at all.

Mark Adler's exceptions fall into two main categories:

  • Sounding rockets -- generally unguided, fin or spin stabilized
  • Vehicles that need aerodynamic surfaces for lift at some point in their flight (shuttle, Pegasus).

Saturn I and V are outliers; as Mark notes, the fins weren't needed in normal flight because the engines were gimbaled, but they would improve stability in the event of a catastrophic first-stage engine problem, providing a longer time window in which to abort safely.

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SpaceX's Falcon 9 uses grid fins to help stabilize the rocket while returning back to land.

Grid fins perform very well at subsonic and supersonic speeds, but poorly at transonic speeds; the flow causes a normal shockwave to form within the lattice, causing much of the airflow to pass completely around the fin instead of through it and generating significant wave drag. At high Mach numbers, grid fins flow fully supersonic and can provide lower drag and greater maneuverability than planar fins.

Falcon 9 grid fins

Falcon 9 grid fins

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Ariane 1-4 had fins. Ariane 4 is an odd case, having fins on some versions and no fins on others:

Ariane 4 with fins on the 40, 42L and 44L variants, no fins on the 42P and 44LP variants

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  • $\begingroup$ Never noticed those before. $\endgroup$ – Mark Adler May 29 '16 at 17:02
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The V2 definitely couldn't gimbal and used the fins and control vanes in the rocket exhaust for stability and little spoilers on the wingtips for guidance too

V2 rocket in

Note: I took this photo in the imperial war museum in London

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    $\begingroup$ The V-2 had control vanes, which served much the same function as engine gimbaling. $\endgroup$ – Mark Adler May 29 '16 at 6:42
  • $\begingroup$ the Imperial War Museum is fantastic! $\endgroup$ – Sarah Bailey Sep 2 '18 at 20:05

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