Placing Rocket Fins for max. hight and stability

[U235Nuke]

Hi, I am just starting to get into rocketry after beeing with pyrotechnics for a little bit of time now. I Dont really know the physics on were to place fins on a rocket, a certain shape, size, weight or material in order to acheive maximum hight and stability. If anyone out their is willing to help please do so. Including picture would help me alot. Thanks you!

[BigBang]

You should place the fins, three if you want maximum performance, at the very rear of the rocket. The trapazoidal, specifically delta, shape its so far been held as the optimum fin shape.

 

Balsa and thin plywood are good choices for fin material. You could use G-10, but its pricy and you will most likely have to have it custom cut for you.

 

For fin attachment, the best glue to use is epoxy IMHO. Stick the fin in place with CA, then use epoxy, say 30 minute, to anchor it. You can go overboard and encase the entire tail in fiberglass cloth like I did with my last rocket. It can in from 3,000 ft, and the fiberglassed area survived intact, without a scratch on it.

[snow]

if there is already a topic about fins

i would like to ask how do you recomend to atach them?

i tryed to ad some glue strips but the fins move and some times with good cam and zoom you can see a spin of the rocket ones a while

becaout the fines moved a little do you have any ideas to atach them stronger but without adding to big wight

thenks

[stunned Psycho]

You could make a small recess in the rocket and glue the fins into that this would provide more strength and stability to the fins

[snow]

I usually make the rocket body as small as posible so it just wouldnt explode so it will have less wight and have better flight preformence so any little recess in the rocket will caouse cato

and i realy dont want to make the body biger.

[dragonman586]

I think he means a recess in the body not the motor.

[paintballtwib]

http://my.execpc.com/~culp/rockets/Barrowman.html

 

 

http://members.aol.com/ricnakk/pix/finattch.gif

 

 

 

The above taken from Richard Nakka, of course.

[Dr Boom]

Paint ball guy has a tried and true design there- well done on the layout of it. Let's not forget (for those of us consistantly reaching greater strides) another design also-

 

Spring Fin Design

 

Good stuff to share-

[ActionTekJackson]

I usually make the rocket body as small as posible so it just wouldnt explode so it will have less wight and have better flight preformence so any little recess in the rocket will caouse cato

and i realy dont want to make the body biger.

If you're wanting to attatch the fins directly to the motor, you could reinforce/thicken the back of the tube with fiberglass tape, then you could make the recessions in the tape so it doesn't actually cut into the motor tube. Just depends on wether or not you wanna use those fins again or not, doubt you'd be able to salvage much if attatched directly to the motor. Anyway, just a thought.

[ActionTekJackson]

Ok, here are some pics of my first real home made finned rocket (i've put store bought fins on estes engines before *lame* lol). I made the fins out of the sides of a soda can, cut off the top and bottom so I had a rectangle sheet of Aluminum. Proceeded to flatten it out, and cut out a fin which I used a grinder to shape. I used this as a tracing fin so I could cut the rest with sissors. I then made small cuts in the engine wall and slid the fins into place and secured them with Gorilla glue. The nose cone is just half of a 3"x5" note card. Suggestions, comments, think it'll work? Either way, I'll test it tomorrow, I'll be sure to get video... :-)

 

http://i29.photobucket.com/albums/c296/ActionJacksonDuCane/Pyro/HPIM2083.jpg

http://i29.photobucket.com/albums/c296/ActionJacksonDuCane/Pyro/HPIM2080.jpg

http://i29.photobucket.com/albums/c296/ActionJacksonDuCane/Pyro/HPIM2079.jpg

[Givat]

Gypsum is not a good choice for end plug. try using cat litter.

[ActionTekJackson]

Gypsum is not a good choice for end plug. try using cat litter.

Who are you talkin to man? I used bentonite....

[Givat]

It looks in the picture like gypsum. Shouldn't bentonite be gray?

[ActionTekJackson]

No... mines a nice cream colour. Probly the flash from the camera that makes it look white.

[Mumbles]

Cat litter stuff is much more grey. If you get pure benonite from say a ceramic supplier it is much less grey. Cat litter probably has some sort of binder in it which may impart the color, or just because it is shitty quality. The main purpose of it is of course for cats to shit upon. Differing from creation of art work. I think you can see the obvious differences in quality requirements.

[ActionTekJackson]

Alrighty, Here's a vid of the rocket I posted pics of earlier. 3/4" x 3 1/2" Finned BP/Hybrid Rocket As you can see the rocket wasn't quite stable. My assumption would be the fins themselves were both not strong enough and thus the air pressure from the rocket rising caused them to warp slightly. And the fins were not entirely straight anyway. However, not a total loss. I will be constructing new fins by using reinforced thin posterboard. Will keep y'alls updated.

[jg74851]

I don't have a lot of rocket making experience yet, but I've been reading up on it. I would have guessed that you're design was unstable. It has to do with where the center of gravity is in relation to the center of pressure. The center of gravity must be in front of the center of pressure. That's difficult to do on such a short rocket. Build a longer rocket, determine the balance point, and tie a string around the rocket on the balance point. Swing it around in a circle with you in the middle. If it stays straight, it'll probably be okay. If it tumbles, it would probably tumble in flight, too. If it is unstable, try putting some weight in the nose, then remeasure the balance point, retie the string, and test it again.

[Exerd]

The above statement is correct.

 

The center of pressure (or the center of your combined aerodynamic drag) is going to be near the front of the fins on this rocket. The fin tips at the rear will act to pull drag towards the rear, while body and nose drag work to pull the center forward a little.

 

Although not entirely accurate, a common theory is that the center of pressure should lie one body caliber (diameter) behind the center of gravity (balance point of the rocket) for acceptable stability.

 

Let's assume that your center of pressure is about 1/4-1/2" back from the forward tips of your fins. If the rocket was completely solid in density, from looking at it this would tell you the balance point would be right around dead center of the airframe. In this case, dead center is about right at the fin tips, so this would actually give this rocket maybe 1/2"+/- between the CG/CP points, or about half a caliber of stability assuming a 1" tube (unstable).

 

In the real scenario however, your rocket more than likely has more weight in it's rear for propellant than it does up front, making the CG even closer to the CP, and even less stable.

 

So what happens then, is you have a rocket whichs leaves very unstable, and as it burns, weight shifts more to the front as propellant burns from the rear, and it becomes slightly more stable through its flight.

 

You can do a few things for this rocket. One trick I use is weighting with bb's. Buy some copper bb's from Wal-mart and weigh out an ounch or more for the nose. Use hot glue to secure them.

 

You can also add weight to the front with body length and additional payload of coarse.

 

If you want to try and bypass adding weight, you can design you fins so that they create more drag in the rear and less up front. First, you may want to try 4 fins instead of 3 if you like the look of their design. Also, trim them back so that they hang rearward more, and you can increase their span as well. Even attaching little streamers from the fin tips would help it become more stable if you ever wanted to try something like that.

 

Hope this helps.