The answer to perfectly shaped rocket fin edges

I've only been messing around with this rocketry thing for less than a year, and I've been particularly obsessed with sanding, filling, shaping, etc. the balsa parts. One of the most difficult parts for me has been getting the edges of the fins perfectly rounded with even margins for the entire length of the side of the fin. I found a good solution at a woodworking store. They were a little bit amused that I was at their store because i was wanting to do something as strange (to them) as shaping the edge of a rocket fin, but once I described what I was doing, they were eager to help. They had these little hard rubber sanding blocks with concave edges in several different sizes. All you have to do is wrap the sandpaper around so that the ends are held as you grasp the handle. Then you press the concave part onto the handle of an exacto knife to smooth the sandpaper up into the concave section. Then it is super easy to shape the edges of the fins perfectly. Forgive me if I'm all excited about something that EVERYONE but me knows about. Pictures below of the sanders and my entire rocket output for this year so far.

Very impressive, SF! By the looks of your hangar-you haven't taken on the easiest birds first. Beautiful roster!

Very cool-looking fleet there! I especially like the shuttles - very nice.

I had never thought of a shaped sanding block for fin edges.

I went looking online and found these:

https://www.leevalley.com/us/wood/page.aspx?p=20183

though for just LPR it seems to me only the smallest (1/4") concave and maybe a couple of the smallest convex ones (for sanding fillets etc.) would be useful. Not that, at a buck apiece, the full set of fifteen would break most banks.

In fact for fin edges I'd think a smaller radius concave would be preferable.

Nice find! I bet a little work with a router and the proper bits and someone could make their own rounded and angled sanding tools. All you would have to do is glue some sandpaper into the grooves after routing them.

What I've done for a numbers of years is to mark the leading and trailing edges with a black Sharpie marker before any sanding. Then I take an equal number of light strokes on either face of the fin in order to maintain a visible centerline. Rounding the leading edge is a fairly easy but the trailing edge is much more tedious and requires good eyesight in order to create the right taper without creating a "knife edge" which easily chips. Leaving a thin black line identifies the stopping point.

The original posters idea definitely has merit, especially for forming a true taper on trailing edges.

Rich Holmes said:

I went looking online and found these:

https://www.leevalley.com/us/wood/page.aspx?p=20183

though for just LPR it seems to me only the smallest (1/4") concave and maybe a couple of the smallest convex ones (for sanding fillets etc.) would be useful. Not that, at a buck apiece, the full set of fifteen would break most banks.
In fact for fin edges I'd think a smaller radius concave would be preferable.

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I was actually able to buy just the two smallest sizes individually at a local store that specializes in all things for high-end woodworking, so not a big expense. I also don't simply swipe the sander with it centered directly above the edge of the fin. I found that it's better to swipe along each corner of the edge at about a 45 degree angle; carefully and slowly a few times. Then I'll sand it just a tad on the very top. To get it right, it still requires a little care and finesse, but it is so much more predictable than any other method I've found.

tmacklin said:

What I've done for a numbers of years is to mark the leading and trailing edges with a black Sharpie marker before any sanding. Then I take an equal number of light strokes on either face of the fin in order to maintain a visible centerline. Rounding the leading edge is a fairly easy but the trailing edge is much more tedious and requires good eyesight in order to create the right taper without creating a "knife edge" which easily chips. Leaving a thin black line identifies the stopping point.

The original posters idea definitely has merit, especially for forming a true taper on trailing edges.

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That is an excellent idea to mark the fin edges. I will definitely be doing that from now on.

Zeus-cat said:

Nice find! I bet a little work with a router and the proper bits and someone could make their own rounded and angled sanding tools. All you would have to do is glue some sandpaper into the grooves after routing them.

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One thing I've started doing is simply using the fins that come with the kit as templates and cutting new ones from basswood. With basswood I've been able to round the edges with a Dremel. Good results there. And basswood fills and sands so much easier and ends up as smooth as a guitar pick. I realize that basswood is heavier, but it's much stronger, and so far I've used slightly thinner basswood than whatever thickness are the balsa fins. I'm currently embarking on a new kit of the old Estes Orbital Transport using all basswood. When it's finished I'll weigh the two and see what the difference is. That kit is so wood-heavy that that should be a good test. If the new one is too heavy, I'll just use it as static display maybe. But I can't imagine that it wouldn't still work pretty well. After all, altitude isn't really the point with that one. I'm itching to build a new Commanche 3 using basswood though.

I've been using these black rubber "tadpoles" for about 20 years, when I bought a set from Micromark.

I use the round one for smoothing fillets on big models. The other end of any of the tools is handy for shaping fillets of smaller models.

i've used a chamfered router bit in the past with great success. you'll need a router table and a guide, or you'll have to make a jig. i've wanted to get a shallow chamfer bit for a while now, as most are sold at 45 degrees, but they seem to be up near 80-90 bucks. been holding off until i absolutely NEED perfect bevels.

this method seems promising though. i've ground kitchen knives on wetstone's with guides that look very similar, holding the blade at a steady 18 degrees or so to get that perfect bevel. seems kind of like the same concept.

Never mind the sanding tips - you have a gorgeous fleet! Some real classics there!

cvanc said:

Never mind the sanding tips - you have a gorgeous fleet! Some real classics there!

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Thanks very much. When I was ages 9-11 I was heavily into this, and that was during a more or less golden age. Ha ha. when I was 10 my parents gave me a car battery with a nice plastic carrying box for Christmas so I wouldn't have to rely on the really dodgy D batteries of the era. A little over a year ago, I found a website called "ninfinger" https://www.ninfinger.org/rockets/rockets.html that was a tribute to the old days. I decided to look into what was happening now, and to my utter delight I found that many of the kits I built as a kid were still available. So I decided to build as many of those as possible; three of the ones in the photo are duplicates (albeit better constructed) of those I built as a kid, and I am currently building an Estes Saturn V which is another of my childhood favorites. Even as a 10-year-old, I was pretty damned obsessive about getting the balsa parts as smooth as possible. I got a real charge out of having fins and nosecones that were as smooth as plastic. For some reason, I'm better at it now and with less work, but I'm still looking for ways to do even better; especially with fillets.

I had a good friend with whom I used to launch rockets as a kid. He had the opposite approach from me. He would just slap his rockets together, spray them in a weird pattern with different colors of Rustoleum and call it done. He would launch his conventionally a couple of times, but what he really enjoyed doing was launching them sideways into a old barn that was on our launch site.

When I was 11, my Dad had become interested in black powder flintlock/percussion cap weapons. He had several very nice recreations, but more importantly he had several large tins of black powder. My friend and I stole a quantity of black powder and sprinkled a handful into the top of a Big Bertha in the hopes that the ejection charge would ignite the powder and the thing would blow up. It did ignite the powder, but the powder wasn't contained enough to actually explode. It did provide a very nice flaming Big Bertha though. Not MY Big Bertha needless to say.

I am so fond of these new rockets I'm building that I have a difficult time bringing myself to launch them. I've launched the big Commanche successfully and that little white one. Also the Orbital Transport. I was lucky that the little glider seems to be constructed so it glides in a wide circle. That was inadvertent, but fortuitous.

ShortFuse said:

I've only been messing around with this rocketry thing for less than a year, and I've been particularly obsessed with sanding, filling, shaping, etc. the balsa parts. One of the most difficult parts for me has been getting the edges of the fins perfectly rounded with even margins for the entire length of the side of the fin. I found a good solution at a woodworking store. They were a little bit amused that I was at their store because i was wanting to do something as strange (to them) as shaping the edge of a rocket fin, but once I described what I was doing, they were eager to help. They had these little hard rubber sanding blocks with concave edges in several different sizes. All you have to do is wrap the sandpaper around so that the ends are held as you grasp the handle. Then you press the concave part onto the handle of an exacto knife to smooth the sandpaper up into the concave section. Then it is super easy to shape the edges of the fins perfectly. Forgive me if I'm all excited about something that EVERYONE but me knows about. Pictures below of the sanders and my entire rocket output for this year so far.

View attachment 149265View attachment 149266

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Nice find on the rubber sanding blocks. It's really good that you're “obsessed” with sanding... as you say. Too many rocketeers simply declare “I hate sanding”, and never pursue the skill sufficiently. I read somewhere (I think it was in a “Fine Woodworking” magazine) that craftsmanship is 80% sanding. Sanding is not just a step in the finishing process. It's also used for cutting and shaping.

Too many of us don't pay attention to the ergonomics of sanding. Our methods of sanding are equivalent to holding an X-acto knife blade between our fingers with no handle, and trying to cut. How many of us slap a piece of unsecured sandpaper onto a block of wood, our knuckles turning white, because we're trying to hold the sandpaper in place while we're trying to sand? It's not the way to go.

I have all sizes and shapes of sanding “blocks” to which I attach sand paper using tape or adhesive. I always sand with a handle (unless I'm doing “finishing” sanding, of course). Consequently, I enjoy sanding.

So, persist in your excitement over sanding. It'll continue to make you into a better craftsman, something that the picture of your current rocket fleet already reveals about you. In fact, I think it truthful to say that your current skill level already exceeds a good number of rocketeers many years your senior.

tmacklin said:

What I've done for a numbers of years is to mark the leading and trailing edges with a black Sharpie marker before any sanding. Then I take an equal number of light strokes on either face of the fin in order to maintain a visible centerline. Rounding the leading edge is a fairly easy but the trailing edge is much more tedious and requires good eyesight in order to create the right taper without creating a "knife edge" which easily chips. Leaving a thin black line identifies the stopping point.

The original posters idea definitely has merit, especially for forming a true taper on trailing edges.

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Dang Ted- my Mom taught me that-whaddit yours look like??

himitsu said:

Nice find on the rubber sanding blocks. It's really good that you're “obsessed” with sanding... as you say. Too many rocketeers simply declare “I hate sanding”, and never pursue the skill sufficiently. I read somewhere (I think it was in a “Fine Woodworking” magazine) that craftsmanship is 80% sanding. Sanding is not just a step in the finishing process. It's also used for cutting and shaping.

Too many of us don't pay attention to the ergonomics of sanding. Our methods of sanding are equivalent to holding an X-acto knife blade between our fingers with no handle, and trying to cut. How many of us slap a piece of unsecured sandpaper onto a block of wood, our knuckles turning white, because we're trying to hold the sandpaper in place while we're trying to sand? It's not the way to go.

I have all sizes and shapes of sanding “blocks” to which I attach sand paper using tape or adhesive. I always sand with a handle (unless I'm doing “finishing” sanding, of course). Consequently, I enjoy sanding.

So, persist in your excitement over sanding. It'll continue to make you into a better craftsman, something that the picture of your current rocket fleet already reveals about you. In fact, I think it truthful to say that your current skill level already exceeds a good number of rocketeers many years your senior.

Click to expand...

I always sand with a handle of some kind too, after really botching the job the other way. The first two I built, the Commanche III and the Orbital Transport, are a little too rough for my liking. You can only tell if you get up close, but that bothers me a LOT. I'm sure I'll redo those using bass wood. I've encountered some new filler called "Timber Mate." It's a sort of gluey, whitish mud that you mix with about 10 percent water and then brush on. It is vital that you brush on both sides or it will warp the fin. I've tried this stuff on a scrap fin and after one coat, one sanding, it's pretty damned smooth. One more coat and sanding should do it. I'm kind of amazed. The stuff isn't heavy either. Painting the scrap fin will be the true test of course.

Just started a thread in Techniques section with a video tutorial that illustrates the way I do it. From the Rocket Shop: How to Airfoil and Paper a Fin

dward said:

Just started a thread in Techniques section with a video tutorial that illustrates the way I do it. From the Rocket Shop: How to Airfoil and Paper a Fin

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Excellent video with a wonderful show of craftsmanship!

I did figure out how to give it a rating (which I did), but I do think that something like this should become “sticky”. I can't see how to do that, however. Is that an option only available to the owner of the thread?

himitsu said:

Excellent video with a wonderful show of craftsmanship!

I did figure out how to give it a rating (which I did), but I do think that something like this should become “sticky”. I can't see how to do that, however. Is that an option only available to the owner of the thread?

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I think that Stickys can only be created by moderator types. Being a newbie, I'm not sure how that works though. I have noticed that as a relative newcomer, I am not allowed to upload an awesome avatar. I yearn to have an awesome avatar such as yours.

ShortFuse said:

I think that Stickys can only be created by moderator types. Being a newbie, I'm not sure how that works though. I have noticed that as a relative newcomer, I am not allowed to upload an awesome avatar. I yearn to have an awesome avatar such as yours.

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Just find the "contact us" button (or something like that) and email the web master, and give 'em a day or two to respond. Then, they will give you the ability to update your profile settings (and upload an avatar). It isn't automatic. You have to ask (email) them. I'm really new to this forum too, BTW. I just got "permission" to use an avatar a couple of days ago (in response to the email that I sent).

himitsu said:

Just find the "contact us" button (or something like that) and email the web master, and give 'em a day or two to respond. Then, they will give you the ability to update your profile settings (and upload an avatar). It isn't automatic. You have to ask (email) them. I'm really new to this forum too, BTW. I just got "permission" to use an avatar a couple of days ago (in response to the email that I sent).

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I will agitate for my right to have an awesome avatar. By the way, did you see my post above about a wood filler called "Timber Mate?" I just bought this stuff at a wood working store. It's a water based paste that you thin with water. I brushed it on a scrap fin. After two coats and sanding between them, the fin is remarkably smooth; like a guitar pick smooth. If it takes paint then I've discovered my new filler for real. There's got to be a downside, but I don't know what it could be. I can't find a reference to it here anywhere.

ShortFuse said:

I will agitate for my right to have an awesome avatar. By the way, did you see my post above about a wood filler called "Timber Mate?" I just bought this stuff at a wood working store. It's a water based paste that you thin with water. I brushed it on a scrap fin. After two coats and sanding between them, the fin is remarkably smooth; like a guitar pick smooth. If it takes paint then I've discovered my new filler for real. There's got to be a downside, but I don't know what it could be. I can't find a reference to it here anywhere.

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Yes, I saw that, and I simply forgot to respond. I'll need to see if I can find Timber Mate around here, to see if I like it better (I'm not exactly overjoyed with CWF Max).

I also like to experiment with new products and create new methods.

For example, I'm currently evaluating Elmer's Carpenter's Rotted Wood Stabilizer for use as a penetrating sealer (and strengthener) for balsa wood. It's definitely NOT a filler, but it seems to seal, and strengthen balsa quite well. Plus, it's non toxic (smells a lot like Elmer's glue). To use it, sand your balsa surfaces first (because the balsa wood becomes harder to sand after you apply the stabilizer). Then, put this stabilizer down. Then, use wood filler for the big dings, and high-build primer to get out the surface grain (of course, with a proper sanding in between each step).

I might test to see if Timber Mate could become applied after the Rotted Wood Stabilizer. Why? I like to seal the balsa wood first, so when you apply liquid (water-based, whatever) product, the wood no longer absorbs anything (because it's sealed). Finishing methods sometimes cause our balsa wood to go through convulsions (expansion, contraction, expansion, contraction,...). I think it's best to limit that, if possible, to reduce warping, and weakening (maybe).

However, you did say Timber Mate was a paste, and that you got acceptable results after only two applications. That's pretty good. The sealing step could probably occur right after that, but then I don't think Rotted Wood Stabilizer would work well at that point, another sealer (like regular primer) might make more sense.

Thanks for the tip, and I'm looking forward to seeing your ultra-cool avatar in the not-too-distant future.