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How big to make fins?

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Seyser

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I am building my first rocket from scratch and i am just wondering what size to make my fins. Do i make them in any type of ratio to the body size or something like that? I am using a program called open rocket and the fin size i made on there seems like a good fit for my rocket. Any one have any thoughts?
thanks!
 

shreadvector

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See the sticky thread above in this forum and find the "Classic Collection" link. It will explain what makes rockets fly stable. This is what determines your fin size. it is very simple and the Classic Collection will explain it simply with pictures.

And feel free to explore the rest of the information linked in that thread. it will answer an insane amount of questions and help you build better rockets that last longer (and avoid many crashes).
 

JDcluster

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Years ago I bought an Estes starter kit it came with a little booklet that had the information you're asking for. Any how, a quick rule of thumb is;
root chord should be at least 2x the body tube diameter , fin span should be 1x the tube diameter & forgot the tip chord though.



JD
 

adrian

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The figures used by a friend of mine who has been running rocket construction workshops for a long time are that root should be 2 x diameter, span should be 1.5 x diameter, tip should be 1 x diameter. This is for a basic 4FNC rocket. The rockets then need a little clay in the nose to make them stable, but that's partly because the nose cones in the workshop rockets are rolled from heavy paper and are probably lighter than equivalent plastic or solid balsa cones. :)

A rocket with only 3 fins will need them to be a little larger, or need more nose weight.
 

mjennings

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If in doubt build it, and before painting load it for flight, wadding, 'chute, motor, find the center of gravity, tie a long string 6-10 feet if the rocket is less than 2 feet or so and do a swing test Spin the rocket around you on the string horizontally. If the rocket stays pointing forward you're good, If not make new bigger fins or add nose weight. The swing test does not work for tall rockets, because of the angle of attack relationship, I once tried a swing test on a mean machine and it did not go well.
 

Seyser

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haha, yea i would imagine that would'nt go so well. A freind of mine a I just got done building the mean machine. Then we decided to have a little competition with making our own rockets. this is my first attemp at building one without instructions, but i have built so many in the past, i think it will go well.

Thanks for the responses, really helped a lot!
 

Handeman

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I use a freeware program called VCP. Link Here

You can add mass to all the parts and calculate the CG, but I usually start with weightless parts and just the rocket shape to get the CP. You can play with fin size and shape and see where the CP ends up. Once you know that, you can adjust nose weight if needed. It will give you a real good idea of whether the rocket will be stable.
 

powderburner

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I am building my first rocket from scratch and i am just wondering what size to make my fins.
If you are taking the dive into DIY design work, it's time for you to also learn about the Barrowman stability model.
http://my.execpc.com/~culp/rockets/Barrowman.html

This is the basic stuff that is programmed into many stability programs. You're going to need this info if you want to continue designing your own rockets, so you might as well learn now. It's not as hard as it might look, give it a try.
 

DexterLB

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I am building my first rocket from scratch and i am just wondering what size to make my fins. Do i make them in any type of ratio to the body size or something like that? I am using a program called open rocket and the fin size i made on there seems like a good fit for my rocket. Any one have any thoughts?
thanks!
OpenRocket is a gourgeous piece of software. I've found out that its simulations are pretty accurate and I've always used it for CP prediction. I suggest to design your rocket in openrocket, make sure it's stable in the simulations, build it, then see the predicted CP in OpenRocket, mark it on the real (physical :D ) rocket and see if the CG is where it's supposed to be, e.g. 0.7-1.7 BT diameters in front of the CP

Please, correct me if I'm wrong.

Terms used above:
CP = Centre of pressure
CG = Centre of gravity
BT diameter - the diameter of the body tube
 
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