Poll: How to make these fins?

[geof]

Hi again everybody. I'm working on a 18" tall scratch build, max-diam=4in, should total about 30oz and fly on G64. So note that it is somewhat short and stubby, but not extreme because much of the body is narrower than the 4in max. However, weight is a serious issue in this design, especially because much of the mass is aft.

Body construction is 2 or 3 layers of 6oz S-weave glass layed over a temporary mandrel, then removing the mandrel when cured. The FG will constitute the entire body...there is no underlying tube.

My question is about the fins. They have some radical beveling on them. They will be approx 3/8 in thick at widest parts, but beveled to airfoil cross-section and beveled thinner toward the tips. (Actually, max thickness is 1/4 - 1/2...not sure yet of final dimensions). But the point is that 3/8 max thickness could be quite heavy in ply. (The fin is roughly trapezoid , say 6in x 4in) Also, making pretty shaped bevels will be tough with hard ply or other hardwood stock.

Which option do you think is best for balancing the goals of adequate strength, excellent shaping, and weight control?

Option 1: Construct the fins from balsa. After the first layer of body FG, attach the fins and use 1 layer of 6oz tip-to-tip on the fins, then finish layers on the body.

Option 2: Construct the fins from balsa. After the first layer of body FG, alternate with 2 layers of 6 oz tip-to-tip on the fins.

Option 3: Construct the fins from ply (or basswood if they make 3/8). Use 1 layer of 6oz tip-to-tip on the fins after the first (or second) layer of body glass. In this option, the tip-to-tip is for fin attachment, whereas in the first two options it also is for fin strength.

How do you think I should proceed?

 

[Rocketjunkie]

Balsa, easier to shape. I'd use lighter glass than 6 oz though. 1 or 2 layers of 2 oz would be plenty.

 

[uncle_vanya]

Originally posted by geof
Hi again everybody. I'm working on a 18" tall scratch build, max-diam=4in, should total about 30oz and fly on G64. So note that it is somewhat short and stubby, but not extreme because much of the body is narrower than the 4in max. However, weight is a serious issue in this design, especially because much of the mass is aft.

Body construction is 2 or 3 layers of 6oz S-weave glass layed over a temporary mandrel, then removing the mandrel when cured. The FG will constitute the entire body...there is no underlying tube.

My question is about the fins. They have some radical beveling on them. They will be approx 3/8 in thick at widest parts, but beveled to airfoil cross-section and beveled thinner toward the tips. (Actually, max thickness is 1/4 - 1/2...not sure yet of final dimensions). But the point is that 3/8 max thickness could be quite heavy in ply. (The fin is roughly trapezoid , say 6in x 4in) Also, making pretty shaped bevels will be tough with hard ply or other hardwood stock.

Which option do you think is best for balancing the goals of adequate strength, excellent shaping, and weight control?

Option 1: Construct the fins from balsa. After the first layer of body FG, attach the fins and use 1 layer of 6oz tip-to-tip on the fins, then finish layers on the body.

Option 2: Construct the fins from balsa. After the first layer of body FG, alternate with 2 layers of 6 oz tip-to-tip on the fins.

Option 3: Construct the fins from ply (or basswood if they make 3/8). Use 1 layer of 6oz tip-to-tip on the fins after the first (or second) layer of body glass. In this option, the tip-to-tip is for fin attachment, whereas in the first two options it also is for fin strength.

How do you think I should proceed?

? Maybe....

Very thin (1/8) ply with foam. Shape the foam and then if you must glass it with a light glass. If the shape permits you could build the core from foam and sandwich it between two 1/8inch ply shells and hardwood dowels for leading edges. You can also look at the honeycomb materials for these outer layers if you glass.

 

[cjl]

I'd use a balsa core, and then 2 or 3 layers of 2oz glass. If that isn't an option, I'd go with a single layer of 6oz over balsa.

 

[uncle_vanya]

What about epoxy impregnated fins - look around on the forum there was some info on doing that using acetone (I think - don't quote me until you find the posts) as a thinner for epoxy. Also some have used CA soaked balsa on items that needed stiffening and durability.

 

[cjl]

Simply soaking balsa in epoxy adds far less strength than even a very lightweight fiberglass layer. I would argue that it is much better to use some light fiberglass than just soaking, and you shouldn't need to thin any good laminating resin - it should be pretty thin to start.

 

[ben]

I would just go with balsa and a layer of 2oz. get some mylar and put it on each side then put it between 2 sandbags. Gets you strength and lightweight

Ben

 

[uncle_vanya]

Originally posted by cjl
Simply soaking balsa in epoxy adds far less strength than even a very lightweight fiberglass layer. I would argue that it is much better to use some light fiberglass than just soaking, and you shouldn't need to thin any good laminating resin - it should be pretty thin to start.

While I agree with this... there are others here who have tried impregnated balsa with great success. The key would be the speeds expected and the desired strength. I just think fiberglass is overkill in this case.

 

[cjl]

Originally posted by uncle_vanya
While I agree with this... there are others here who have tried impregnated balsa with great success. The key would be the speeds expected and the desired strength. I just think fiberglass is overkill in this case.

I would bet that impregnated balsa weighs more than balsa with a layer of 2 of light glass on each side, but is no stronger.

 

[FROB]

I would go with a core of thin G10 or FR4 laminated on both sides with balsa, then 1 thin layer FG (3oz or less) on the outside just to make the surface more resistant to dings and give it a smoth hard finish.
The extra strenght wont really be needed on the way up, its more about surviving the landing. Especially with sharp beveled edges, the central G10 makes them unbreakable, with tough leading and trailing edges. Then you can use a small chute and reduce the chances of losing the rocket in the wind. For reference, i did my L1 with a 3" scratch built, 7' long with large fins made of copper clad 0.040" FR4 only(surplus PCB material, about credit card thick, and dirt cheap . Piston jammed and main chute didnt open on first flight, came in hard on drogue only, landed on hard pack dirt road 100' from the pad, not a scratch.
Its also easier to shape the fin profile on each 1/2 fin side before laminating. You can vacum bag them with the outer FG layer, if you extend the G10/FR4 part for thru the wall attachment, no need for tip to tip glassing. If somehow you do manage to damage a fin, its much easier to replace one. Being paranoid, i made 1 extra fin in the original batch, for easy field repairs.

 

[geof]

Thanks guys. I'm going with balsa+fg for its combination of simplicity and familiarity. Some of the other ideas sound good.

My partner sent me the body molds today, so I'll post a build thread when things get going.

Geof