Opinions on fin thickness

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

qquake2k

Captain Low-N-Slow
Joined
Jan 26, 2010
Messages
13,574
Reaction score
65
I'm playing around with a design for an upscale Big Betty, based on LOC 5.54" airframe. It will have a 38mm motor mount, and the biggest motor I would ever consider putting in it is an AT J575. I was initially planning on using 3/8" birch plywood for the fins, but got to thinking, would 1/4" be strong enough? Max velocity on the J575 sims at 510fps (0.46 mach), so I don't think flutter would be a problem with 1/4" fins. My biggest concern would be breaking a fin. There's a lot of unsupported fin hanging down. What do you guys think?

screenshot02.jpg

screenshot03.jpg

View attachment Whale Betty v3.ork

View attachment Whale Betty v3 quarter.ork
 
hmm, were you thinking some form of dual deploy? something that size looks to me, to be wanting a nice slow final descent rate...I would be inclined to do dual deploy & do 1/4" fins.
Rex
 
I'm playing around with a design for an upscale Big Betty, based on LOC 5.54" airframe. It will have a 38mm motor mount, and the biggest motor I would ever consider putting in it is an AT J575. I was initially planning on using 3/8" birch plywood for the fins, but got to thinking, would 1/4" be strong enough? Max velocity on the J575 sims at 510fps (0.46 mach), so I don't think flutter would be a problem with 1/4" fins.
Should be strong enough for the purpose of the flight stabilization.
May not be strong enough if the rockets lands at the wrong angle on one of the long fins.

YMMV, but I've broken multiple plywood fins. The break was always in the fins themselves, the root edges were epoxied + filleted and held tight, which only made repairs more problematic. In retrospect I wish I did not fillet the plywood fins, allowing them to dislodge from the airframe instead of break in half!
I am yet to break a fiberglass fin, and some of them are as big as yours:

PML_BumbleBee_rocket__69668.1429810091.500.659.jpg


My biggest concern would be breaking a fin. There's a lot of unsupported fin hanging down. What do you guys think?

I vote for fiberglass fins, if you can get the raw materials.

YMMV,
a
 
Last edited:
I never thought about FG fins for your build,have you ever used it as fins or a complete build?I don't remember any of your rockets that were not paper tube builds.
 
I believe 1/4 will be fine , my upscale 4" mosquito is 1/4 ply several flights

last spring I flew in some wind , the wind dragged the rocket across field seemed like quarter mile no issue with fins
 
I never thought about FG fins for your build,have you ever used it as fins or a complete build?I don't remember any of your rockets that were not paper tube builds.
You can use FG for fins alone, or fins and airframe. I've done both on different rockets.

FG is nearly indestructible, easy to prep for paint, and ocassionally comes impregnated with color and not requiring any painting at all. I've never broken or dinged a FG fin.

The downsides to FG are additional cost and weight. Cost is not THAT much more, but if you are going for max altitude, you might have to forgo FG.

a

Sent from my Pixel using Rocketry Forum mobile app
 
hmm, were you thinking some form of dual deploy? something that size looks to me, to be wanting a nice slow final descent rate...I would be inclined to do dual deploy & do 1/4" fins.
Rex

I'll be using a chute release with a 78" chute. It sims at 18fps descent rate.
 
Should be strong enough for the purpose of the flight stabilization. May not be strong enough if the rockets lands at the wrong angle on one of the long fins.

YMMV, but I've broken multiple plywood fins. The break was always in the fins themselves, the root edges were epoxied + filleted and held tight, which only made repairs more problematic. In retrospect I wish I did not fillet the plywood fins, allowing them to dislodge from the airframe instead of break in half!
I am yet to break a fiberglass fin, and some of them are as big as yours: I vote for fiberglass fins, if you can get the raw materials.

YMMV

I never thought about FG fins for your build,have you ever used it as fins or a complete build?I don't remember any of your rockets that were not paper tube builds.

You can use FG for fins alone, or fins and airframe. I've done both on different rockets.

FG is nearly indestructible, easy to prep for paint, and ocassionally comes impregnated with color and not requiring any painting at all. I've never broken or dinged a FG fin.

The downsides to FG are additional cost and weight. Cost is not THAT much more, but if you are going for max altitude, you might have to forgo FG.

I've only ever used fiberglass fins once. They were 1/8" G10. They ruined my bandsaw blade. I don't have much desire to work with it again. Plywood is so much easier to work with.
 
A wet tile saw does a great job of cutting G10.
If you do use plywood, use good plywood and align the outside grain with the angled leading edge. I’ve had thin plywood break on fins just like yours propagating from the inside corner parallel to the BT. The only thing I thought might have helped would be aligning the grain with the leading edge.
 
I'm not sure that's an issue, since a lot of the strength of plywood comes from the grain on the plies being perpendicular to each other.
 
That’s what I expected too until I saw my fins break along the outside grain. The inner veneers broke across their grain and it started right at the inside corner.

I think I'll just go with 3/8" plywood. It has more plies.

Put one layer of FG cloth on them?

That would be a good idea if I had ever done fiberglassing before. Not sure I want to go to all that trouble for this one build.
 
I never used cloth before,seen builds of others that have and think I might have to practice before trying it.I wonder what the additional cost would be?
 
Might be a project for your Level 3 if you decide it's time.:)
 
No way. I can't afford Level 3, and I'm not physically up to it.
 
I first learned fiberglass lamination for my L3 rocket. It would have much more difficult if I hadn’t watched a VHS tape tutorial (over and over, trying to get over the initial hump) by Dave Triano. It was still intimidating until I actually started wrapping glass cloth around a 7.5 inch phenolic body tube.
It went very well, due to the tutorial and since then I actually enjoy fiberglassing. It’s easier to teach yourself with smaller, less crucial projects. I taught one young man while laminating the body tube of an Estes Big Bertha. It’s much better if you have someone show you and hold your hand a little at the beginning. Aircraft Spruce used to have a starter kit to accompany a set of composite building lessons by Burt Rutan, but there’re no better sources of tutorials than those by Tony Alcocer (TFish here on TFR) or John Coker, who’s also here.

Here’s a composite practice kit from Aircraft Spruce: https://www.aircraftspruce.com/catalog/cmpages/practicekit.php?clickkey=23413
Here’s a link to the Rutan tutorial. I think it would be interesting, but I haven’t seen it.
https://www.aircraftspruce.com/catalog/bvpages/rutancomposites.php?clickkey=23413
Tony’s videos are on YouTube under user TFish38. Here’s an example of glassing a cardboard tube:
https://youtu.be/bjZNyXsd8gw
Here’s a similar video from John Coker.
https://youtu.be/_gTxwEVK6IY
The same techniques work for laminating flat pieces.
An alternative would be to just epoxy thin G10 sheets to both sides of the plywood. I’ve not done that in rocketry before but it would provide you with a finished surface. You could take that a step further by vacuum packing it with a seal-a-meal, but that’s not necessary.



Steve Shannon
 
I would fiberglass its not hard, if your not too concerned about weight you can buy marine seaglass cloth and resin from places like home depot. It's great to practice with atleast before spending alot more on exspensive resins and cloth. Get some good quality aircraft grade 3/16" birch plywood apply glass on both sides, wax paper and sandwhich it between 2 good flat objects, I do the same to make A/V bay sleds.
 
I’ve got all the fixings for doing fiberglass, Jim. Resin, gloves, alcohol, breathing sheet, you name it. Most of my tube fins were structurally enhanced with fiberglass and resin to prevent landing damage to the tubes.

Remember your motto: “If something is worth doing, it’s worth overdoing.”

Easy to glass the fins before they’re installed otherwise you’re going to be doing a tip to tip job. Do everything flat on the bench and use your finishing sander, etc. to get the surface where you like it. Then install the strengthened fins to the body tube.

Let me know if you want to do your fins.
 
I’ve got all the fixings for doing fiberglass, Jim. [...]Let me know if you want to do your fins.

Or, send PML your desired fin profile as a .rkt or "custom fin template" file, and get the desired FG fins by mail:
https://publicmissiles.com/custom_work.html

YMMV,
a

P.S.: I love DIY-ing things (planning, designing, assembling, flying), but frolicking in epoxy is just not my cup of tee. Personally, I would rather spec the parts, buy them off the vendor, and move on to assembly and flying.
 
Back
Top