Three or Four Fins for > Mach 1?

Discussion in 'High Power Rocketry (HPR)' started by Fearoflightning, Sep 1, 2016.

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  1. Sep 1, 2016 #1

    Fearoflightning

    Fearoflightning

    Fearoflightning

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    Hello,

    I am currently designing a 6" diameter all-fiberglass rocket very similar to the ultimate wildman. The biggest motor that I will ever fly it on is the N5800, which, according to openrocket, pushes it to just over mach 2, so it will be designed to have a static stability margin of 1.5 (with that motor) to account for CP shift forward.

    Now this brings me to my question(s):

    Should I use four smaller fins, or three fins that have a slightly longer span to equal the same stability margin?

    Every missile that I have ever seen has four fins, Why is that? Is there some advantage over three in their cases? Is it just due to the fact that they use active guidance?

    To clarify, I am not interested in any aesthetic factor that number of fins provide, and I'm fully aware of the slightly increased weight that goes along with having four fins. I am only interested in any aerodynamic differences. Especially those that work toward my advantage. (i.e. speed, height, both...)

    Thanks in advance! I'm sure this will be interesting.....:)
     
    Last edited: Sep 1, 2016
  2. Sep 1, 2016 #2

    Igotnothing

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    Some have speculated that 4 fin rockets are less likely to roll-couple, or cone. All I know is they "seem" to fly "straighter".
    I do a lot of sims with 3 vs 4 fins and optimizing span. Leading edge sweep is the largest factor.
     
  3. Sep 1, 2016 #3

    SaturnV

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    With three fins until now have no stable rocket. Of course I know that the problem is in me.Under the same faculties of man that makes rocket with four fins produces a stable rocket. This is my practical experience.The problem is probably in the fact that with 4 fins better be seen that they are correctly placed. Otherwise, theoretically if correctly calculated it does not matter 3 ,4,5,6.But in amateur rocketry is very important with limited equipment and tools to achieve results. With primitive tools much easier to plot a stable rocket with four fins than 3 or 5 or 6. For this rockets with 4 fins are the most common.Once the rocket is supersonic smaller fins are preferred. Because they are healthier.
     
    Last edited: Sep 1, 2016
  4. Sep 1, 2016 #4

    SaturnV

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    Otherwise, why not rocket with 5 or 6 fins? The fins are smaller and rocket easier to transport with less diametar. Many military missiles are of 5 or 6 fins. But there have laser technology for positioning of fins.
     
  5. Sep 1, 2016 #5

    Binder Design

    Binder Design

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    All the Max Q fin cans we are making for the military are specified in four fin configuration. They explained to me that they will never use a three finned rocket. High mach numbers do funny things to stability. And a three finned rocket is not always less drag if yaw is induced in flight, ie corkscrewing.
     
  6. Sep 1, 2016 #6

    Fearoflightning

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    Interesting.

    So you're saying that three finned rockets are more likely to yaw during flight and this phenomenon is especially pronounced at higher speeds, correct?

    Any idea on specifically what causes this? Is it just the inherently more symmetrical four fin design or the presence of an additional surface for air to interact with?
     
  7. Sep 2, 2016 #7

    grouch

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    All of my rockets but one have three fins. The one that flies best has four. I am not one to chase high mach speeds but if I were, it would have four fins based on the fact the professionals do it that way.
     
  8. Sep 2, 2016 #8

    SaturnV

    SaturnV

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    B-1000.jpg
    I would not say that there is any significance to stability. If just 3 fins is larger and it is difficult to mount a plane :)In amateur rocketry if you have a good eye and hand and can properly mount 3 fins there are no problems.And because if 3 fins are larger at high speed flutter more easily can break.But if fins are healthy there are no problems to be 3.
     
    Last edited: Sep 2, 2016
  9. Sep 2, 2016 #9

    SaturnV

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    untitled.JPG
    Hard to see if you had it set to 115, 125 and 120. As if 4 are symmetrical and you will immediately notice :)
     
    Last edited: Sep 2, 2016
  10. Sep 2, 2016 #10

    SaturnV

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    In fact, to me is not very good with 3 not for themselves fins. Always on rockets with 3 fins fly with test engine for 1 time ,was not developed strong trust in the beginning. Unstable rocket flew spiral or strange trajectory. Not like that :
     
  11. Sep 2, 2016 #11

    mikemech

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    Every missile that I have ever seen has four fins, Why is that?

    Military missles have active guidance. The fins have moveable control surfaces to steer the rocket in flight. Having matched fins in two planes gives both pitch (up and down) and yaw (left and right) control. It can be done with three fins, but it is much eaiser with four.

    Once the rocket is supersonic smaller fins are preferred.

    True. Smaller fins are stiffer and less prone to flutter. Flutter can occur far below Mach.

    High mach numbers do funny things to stability.

    True. If you're flying at high mach numbers you need that laser positioning system and a PhD in aerospace engineering.

    For model rocketry, take a hint from archery. Arrows have three fletches (fins) and they fly just fine. But at very low mach numbers, as do most LPR and MPR models. Most of my scratch designs have three fins.
     
  12. Sep 2, 2016 #12

    Zebedee

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    Plenty of rockets have done mach 2 with 3 fins (even fins which are close in size to the span of the rocket) - I have personal experience flying a pretty optimized minimum diameter 3FNC rocket to M2.5+.
    Having said that - my next major build will be pushing M3 and I'm planning on 4 fins for reasons discussed above.

    Which is a long way of saying you're probably fine with either and you can choose based on whatever you prefer.

    Zeb
     
  13. Sep 2, 2016 #13

    SaturnV

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    For size of the fins is important not so much the maximum speed. Chart of engine thrust is important too. If the engine from the beginning gives big thrust - can use smaller fins. However, if thrust from start is small, and then grow small fins can play bad joke.The smaller fins are better :) Proved less air resistance and are healthier. But only under the condition that the rocket is stable :) And the rocket is more stable with larger fins :)
     
  14. Sep 3, 2016 #14

    Binder Design

    Binder Design

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    Center of pressure moves forward at high speeds and decreases stability margin. Three fins provide less restorative force that can result in corkscrewing, coning, and increased off axis stress to the rocket. This can offset any drag efficiency gains from using three fins vs. four and will certainly result in decreased altitude via off vertical flight.
     
  15. Sep 3, 2016 #15

    Binder Design

    Binder Design

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    Not true. You just need a fat wallet.
     
  16. Sep 3, 2016 #16

    Fearoflightning

    Fearoflightning

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    Agreed!


    So, to sum everything up (as I understand it), the benefits of four fins really present themselves at higher speeds (greater than mach 2.5 ) so for most of our purposes in HPR it just becomes a question of aesthetics, stability, weight, and the amount of work added by having to attach another fin. Especially if you can align the fins well.

    For rockets that go faster than mach 2, four fins allow for smaller fins to create the increased stability needed. Smaller fins are usually better for supersonic flight because of less flutter risk among other things.

    All of this said, the rocket I'm designing now will have four fins just because I don't want three elephant-ear fins to keep it 1.5 stable.

    Thanks for all of the info and keep it coming!
     

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