Scratch built fins

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Flare

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Hey everyone

Got parts in for a scratch built MPR-HPR build. 3" airframe, 29mm motor mount. Going to fly it on Gs and then if it flies well on a small H for a CAR L1 cert attempt.

The only thing I didn't already buy is material for the fins. I assumed I'd just rip down to Home Depot and get some 1/8" plywood, but niether Home Depot, Lowes, or any other store in town it appears, has any of the stuff. Quarter inch minimum which seems a little chunky for this build.

What is your guys' favourite material to make fins out of? Any easily availble materials you can reccomend? If all else fails I can make my own fibreglass sheets, but that seems a bit overkill for a cardboard G flier.
 
For MP, I prefer aircraft plywood, 1/32” and up, from local hobby shop, Hobby Lobby, JoAnn’s, etc. For HP, I prefer fiberglass or carbon fiber sheet, .5mm and up, from local hoppy shop, Amazon, or EBay. For LP, I prefer basswood over balsa, same sources as aircraft plywood.
 
Big box store plywood is no good. The local hobby store has aircraft plywood in various widths far less than 1/4” and far superior in quality. Hobby lobby has have some craft plywood that may work too.
 
For HPR I either go for F/Glass sheet if I have it to hand. If not, then I go for DIY store plywood in either 3mm (1/8") or 6mm (1/4") thickness which I then laminate with 2 or 3 layers of 200gsm F/Glass cloth each side using the foodsaver vacuum bag process. Aircraft grade ply from the hobby stores is beautiful stuff but soooo expensive!

Second fin being glassed.jpg
 
I use 1/4 ply in almost every rocket. The 1/8th breaks too easy for me. I get all my balsa, basswood and plywood from BMS.
Huh, to me it just seems overbuilt using 1/4 but what do I know.

OpenRocket actually doesn't seem to take off that much performance using 1/4" fins instead. There's no local hobby shop in my town where I can get nice thin sheets, and while I could lay up some fibreglass sheets myself I think I might just go ahead and use 1/4" and power-sand them down a bit if I think I need to. In the end you've got to use the materials that are available.

Now the question is finding the best way to cut fin slots in the body tube.
 
Now the question is finding the best way to cut fin slots in the body tube.
For your smaller MPR and HPR rockets with cardboard tubes, I use a very thin saw that used to be called a "razor saw".
 
Now the question is finding the best way to cut fin slots in the body tube.

Start by going here and generating a fin guide for your body tube:

https://www.payloadbay.com/index.php?page=Tools&action=FINGUIDES
Print the guide and mark your tube.

Get a piece of 1.5" angle aluminum. I shop Lowes; they have it in 36" lengths for a few bucks. File any rough edges off the ends so you don't tear up the cardboard, then lay it along the tube, lining one edge up with your fin mark. Draw your line, then another for however wide your fin is going to be.

Then, I lay masking tape down for a cutting guide. I use a hobby knife. Make several passes, cutting slowly with light pressure, rather than one or two passes cutting heavy. You'll get a cleaner cut.

IMG_0795.jpg IMG_0796.jpg
 
Huh, to me it just seems overbuilt using 1/4 but what do I know.

OpenRocket actually doesn't seem to take off that much performance using 1/4" fins instead. There's no local hobby shop in my town where I can get nice thin sheets, and while I could lay up some fibreglass sheets myself I think I might just go ahead and use 1/4" and power-sand them down a bit if I think I need to. In the end you've got to use the materials that are available.

Now the question is finding the best way to cut fin slots in the body tube.
I use my Dremel with a reinforced cut off wheel. I can do 4 slots in 2 minutes. I just follow the pencil line. Nice and straight cuts. I have reinforced fin tips by sandwiching two more 1/4" ply pieces on rockets with fins below the BT. The dry lake bed is very hard on fins and the rocket in general. If you fly on a grass field you wouldn't have to do what I do.
 
When you are ready to cut the fins, drill holes through the tabs and pin them together. That way they all turn out identical.

Check out the pics in this thread.
 
Thank you everyone for your tips so far. I will be making the fins over the next couple days and will share the results in this thread.
 
So..... here are the fins.

The cuts are a little messy since I didn't have a jigsaw available. These were cut with an angle grinder cutting disk and then cleaned up with a dremel and some sanding. I then beveled the leading and trailing edges a bit. Didn't go crazy on the bevels since I'm trying to keep this rocket a bit low.

1661563358836.png


As for the fin slots: I simply made a paper guide first by wrapping a sheet of paper around the body tube, marking where the circumference was, then dividing that into 4 and marking those locations. Then I just extended lines the length of the fin tabs and made another mark offset by the width of the fins. I then outlined these lines with some masking tape to make it easier to follow the lines with the exacto knife. The whole thing got taped to the body tube and I just slowly cut the slots out with the knife.

1661563579009.png
1661563598911.png

The slots turned out quite nicely.

1661563620315.png

I've made some good progress on the actual build. The 29mm motor mount is in and the shock cord is attached. I also tacked the fins on but have yet to do the fillets. Those will happen tomorrow. The fins themselves fit nicely in the slots and go right up to the motor mount tube. Not bad for plywood cut by hand on an angle grinder! Pretty happy with how my first HPR (MPR for now) rocket is coming together.
 
Fillets applied, used thixotropic silica to thicken some laminating epoxy (fibreglast 2000). Turned out pretty nice just need to clean up the front and back a bit with some sanding. Sanding and painting will happen tomorrow or the next few days.

1661634669866.png
 
Fillets applied, used thixotropic silica to thicken some laminating epoxy (fibreglast 2000). Turned out pretty nice just need to clean up the front and back a bit with some sanding. Sanding and painting will happen tomorrow or the next few days.

View attachment 534509
Two things. Payloadbay.com has templates for marking fin location. And use your Dremel with a reinforced cut off wheel to cut your fin slots. It only takes a minute or two to do 4 slots. A lot easier than a knife. I started do it after cutting and cutting and etc. It will give you nice clean and straight cuts. Just remember to clean the paper dust off the motor tube. Your fins look good to me. Nice work with the grinder.
 
So..... here are the fins.

The cuts are a little messy since I didn't have a jigsaw available. These were cut with an angle grinder cutting disk and then cleaned up with a dremel and some sanding. I then beveled the leading and trailing edges a bit. Didn't go crazy on the bevels since I'm trying to keep this rocket a bit low.

View attachment 534398


As for the fin slots: I simply made a paper guide first by wrapping a sheet of paper around the body tube, marking where the circumference was, then dividing that into 4 and marking those locations. Then I just extended lines the length of the fin tabs and made another mark offset by the width of the fins. I then outlined these lines with some masking tape to make it easier to follow the lines with the exacto knife. The whole thing got taped to the body tube and I just slowly cut the slots out with the knife.

View attachment 534401
View attachment 534402

The slots turned out quite nicely.

View attachment 534403

I've made some good progress on the actual build. The 29mm motor mount is in and the shock cord is attached. I also tacked the fins on but have yet to do the fillets. Those will happen tomorrow. The fins themselves fit nicely in the slots and go right up to the motor mount tube. Not bad for plywood cut by hand on an angle grinder! Pretty happy with how my first HPR (MPR for now) rocket is coming together.
That’s the way I do it only I use cardstock and cut along the lines on it. I don’t use masking tape.
 
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