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Rocketmaniac

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I was wondering if the engineering brains (or rocket experts) could answer a question;

When a rocket has two sets of fins, what effect does it have to have the forward fins larger than the rear fins?
 

vjp

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In short, any fin area ahead of the rocket's CG will have a negative effect on stability.

Therefore, if your forward fins are of any significant size, your rocket will more than likely be unstable. The only way to make said rocket design stable again would be to add significant mass to the nose, to bring the CG forward.

To simplify, you _can_ make a rocket stable with large forward fins vs. small rear fins, but only at the expense of adding lots of weight.

The only time forward fins are 'necessary' is to replicate a scale design (like a sidewinder) in which active control fins were used, or for aesthetic (and/or possibly functional) purposes such as canards on a space plane design.
 

Rocketmaniac

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The rocket I was thinking of when I asked the question is the Exocet........ I was looking at several of my un-built kits and saw my Exocet from "The Launch Pad" ........ It has pretty good size forward fins and real small rear fins.....

I was thinking that it is going to be real easy to have one the rear fins break off when it lands.......
 

vjp

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Originally posted by Rocketmaniac
The rocket I was thinking of when I asked the question is the Exocet........ I was looking at several of my un-built kits and saw my Exocet from "The Launch Pad" ........ It has pretty good size forward fins and real small rear fins.....

I was thinking that it is going to be real easy to have one the rear fins break off when it lands.......
Which Exocet, the M39 or MM40?

From the look of these kits, the forward fins are probably right around the CG location, which would make them neutral in terms of their effect on the CP.

As long as you follow the mfr's instructions and make sure the CG is where the kit recommends, you should be o.k.
 

jflis

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basically, forward fins move the CP forward.

The larger the fin size, the greater the movement.

The further forward, the greater the movement.

To keep your model stable, this would have to be offset with additional weight forward, to move the CG into a stable position again (1-2 calipers forward of the CP)

jim
 

rocket trike

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Our new release has large forward fins. They still on the back 1/2 of the rocket. I have not add any nose weight and it flies great. I hope that this will help a little. Here is a picture for you to take a look at.
 

teflonrocketry1

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I have flown a design with large forward fins. If the forward fins can pivot about their leading edge so they are non-steering, they don't affect the CP very much. Fixed forward fins can shift the CP forward and cause unstable flight. Here is the link to a thread where I show the RockSim simulations for this design:

http://www.rocketryforum.com/showthread.php?s=&threadid=3738

Bruce S. Levison, NAR #69055
 

SecretSquirrel

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The Squirel Works has a kit coming out soon that has small canards but they are very far forward.

In this kit we have added .2 oz of nose weight even with the large rear fins. The rocket is very stable on a C11 without wieght, but on D12's the CP is just barely 1 caliper behing the CG. The nose weight increases that to 1.8 calipers.

http://www.squirrel-works.com/ffe.jpg


I hope this example helps answer your question.
 

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