Honest John fin profiling (AFTER THE FACT)

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dixontj93060

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I started with build of the Madcow Honest John 4"/5.5" with the initial goal of just getting it together quickly so I could move on to other projects targeted for LDRS 33. But, as I got more involved with the build and did some research, on the missile and what others were doing with their builds, I have gravitated more toward bringing out some detail features of this scale build. A few things I have done is 1) purchase a better set of decals from Stickershock (of course), 2) source detailed spin pods from TRF member JMZawodny (see pic) and, 3) cut three rings from 5.5" FG for application on outside of booster (see pic detail).

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Problem is, in my haste to get this done, and while focusing on other complexities, like how to make this thing dual-deploy, I moved forward on the fins weeks ago. With my design changes, the booster can now take motors up to 32" in length, so L-range. This, along with the fin profile simulated in FinSim led to choice to laminate fins with CF (again, done weeks ago). So now, after the fact, I'm considering adding the wedge fin profile (see pic in first post) to the now flat (and finished) fins. Clearly the correct strategy would have been to use balsa and add the profile shape before laminating, but, hey better late than never, and I have found in a dozen years building HPR, you can almost always recover from mistakes/oversights.

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I first considered adding lite ply or balsa to the CF but I didn't like the idea of all the sanding and especially didn't like the idea of a soft surface that had to get additional multiple layers of laminating. Second thought was to use a bunch of filler, like SuperFil, and again sand to shape. The worry I had with this method, besides again, all the sanding, was that I could see the filler chipping/chunking off over time. It was then I remembered I had faced a similar problem of getting a secure base of large, thin fill areas when I did the re-finishing of an antique dresser for my wife. Better, yet, I still had some of the US Composites Kleer Koat sitting on a shelf down in the basement.

Before starting on the fins themselves I wanted to do some testing of the epoxy to see if it would fit the application. Two issue with use on the fins are: 1) would the epoxy layer adhere, especially in the areas where it transitions to an ultra-thin edge and, 2) would it be stiff and shatter or would it be flexible. I felt somewhat comfortable with #1 above after my use on the antique dresser, but less confident in #2 as the prime application for this epoxy is bar and table countertops which are clearly fixed surfaces with little/no flex.

So to test, I took some scrap pieces of plastic packaging from a tool I had bought and cut it up into pieces so I could simulate ~1/8" thick fill that transitioned into thin leading/trailing edges. I tried two pours, first the epoxy only with no filler as it is meant to be used (i.e., clear table top application). I also did a second pour with phenolic sphere fill as this filler typically will provide the finished epoxy with more flex. I left the epoxy overnight and took a look this morning. What I found was:

1) The epoxy cured solid with no filming and/or soft spots. In the picture below you see the two main samples, with and without fill. Note the one without fill did have a bit of overspill as can be seen.

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2) The epoxy cured into a nice sharp thin edge on both samples.

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3) I was expecting to be able to pop the epoxy out of the plastic mold as most epoxies don't adhere to plastics. This epoxy is not one of them. I had some extra of the filled sample and put it in a deeper mold--in this case I cannot even cut/rip the mold off around the epoxy. So the adherence is great--good news for my fin application.

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4) Finally the last issue I was worried about was flex. Again, presently surprised. Both samples flexed well, with the filled sample performing better, i.e., easier to flex across a greater distance. In the extreme case, I am able almost to make a "C" shape with the thin edge of the sample (medium bend shown below).

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So, after this morning, I am happy with the application testing of this epoxy. Also nice to be able to use the US Composites Kleer Koat as I had nearly a gallon left over from my first project. Never thought I would find an application for it! In the next few days I will begin working on the spine pieces and begin filling the Honest John fins so "more to come."
 
Voila spine, splines or whatever you'd call them. Yes flattened bamboo skewers. I thought about using little foam wedges, but I liked the idea of a harder wood material that I could do final fin shaping without it falling apart.

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Attachment done simply with a bead of 5 minute epoxy. Then clamped straight and tight.

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After getting all the spines attached today starts the fun part. As expected, the idea is to pour the fin bevels. First need to set up/clamp booster to achieve as close to level as possible on each successive pour area. Given the fin profile you don't achieve perfect level from every angle but that is OK because the Kleer Koat has some viscosity that helps it get into and stay in the fill areas.

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Then mixing the Kleer Koat well with fairing filler to get a thick syrup consistency. I then pour and "squeegee" with across the fill area. Do a little bit of overage clean up and then wait(!). There will be some bubbles during cure which are easily removed with a heat gun to give you a near glass smooth surface when done. As can be seen in the bowl in the background, I mixed way too much--will have to adjust on the next section.

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I did want to address one question that came up--epoxy weight in fin area as negative. This statement is true in general, but in this case, after evaluating the alternatives, I thought it was a small price to pay to get an easy solution with a nice finish. Each application two sides, two section adds about 10g, so 40g per fin, 160g or ~5.6 oz total. Two reasons this is a non-issue, 1) with a 54mm mount in a stocky 4"/5.5" FG airframe you could never really call this a "high performer" to start with, and 2) with the changes in the design to add dual deploy in the nosecone it makes the rocket considerably over-stable already so adding a few ounces to the aft is instead some help.

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And now on to section #2...

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This is a really interesting and really cool technique. I would love to try this on a FG Nike Smoke I have that I did not get a chance to bevel the fins on prior to attaching them. I think with two skewers set in parallel down the middle of the fin I could achieve the same idea, but with a flat area in the middle. Very cool concept, thanks for sharing, looking forward to seeing how it turns out.
 
Oh and the really nice thing, US Composites is right around the corner from my office.
 
This is a really interesting and really cool technique. I would love to try this on a FG Nike Smoke I have that I did not get a chance to bevel the fins on prior to attaching them. I think with two skewers set in parallel down the middle of the fin I could achieve the same idea, but with a flat area in the middle. Very cool concept, thanks for sharing, looking forward to seeing how it turns out.

Yes, the Nike Smoke would also be an excellent application for this technique. Really any geometric fin profile with flat surfaces. It just takes a little thinking on positioning the booster to achieve a semi-level set on the section being poured. I say "semi-level" as, 1) you will never get it level all around as you have compound angles coming into play, and 2) you don't really need it all that level--the epoxy mix is much more forgiving than I expected as with the filler viscosity increases and you can pretty much move the slurry around (and also add/subtract in areas) to get almost any fill you need.
 
Was working on other end of this rocket last night and finally completed fit for the "standard" av-bay between the two nosecone pieces. I know it doesn't look like much, but it was fairly tricky adding extra internal airframe sections and airframe shims to get the nosecone assembly "rock solid" where I'd be comfortable with anything I decide to stuff it with.

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Very interesting technique! I've always used the built-up method (as documented in my video), but this is a novel approach. I'm curious what you think of it after everything is complete.
 
Was traveling all last week so nothing done on fins until last day or two. Rounding the stretch now with one fin (well, two fin sides) to go.

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Now one round of filler to smooth out some of the low spots and I'm feeling really very good about this late-stage mod.

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I have the av-bay complete, the recovery pieces built and pretty soon I will be attaching some of the external detail pieces like this mid-ring.

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OK, detail pieces attached (body rings and spin motors); getting pretty close to shooting primer :).

For those that haven't seen my random posts in the "What did you do rocketwise today?" thread the av-bay is located in the nosecone; drogue to the aft, main on top; tracker in tip of NC.

Finally the only real disappointment (so far) is that I have the dA%M rail buttons off by a couple degrees which unfortunately is magnified by them sitting an inch-plus above the body. Won't really effect anything, just hate not get it "just right."

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Well, just have a couple more finish details to complete--black lacquer drops in center of thrust outlets and painting railbutton stand off greens, and then I'm done. Was close enough for taking a couple of shots this morning before heading to work though... The fin mods addressed in this thread came out looking great BTW. One decal is "flipped" from what I had planned but it doesn't really make much difference as in doing research the Honest John was not consistent and seemed to have no rhyme or reason to its ID scheme. Hey, it's sport scale anyway!

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Looks great. A question about the spin motors, are the D printed? I would have loved to had put them on my 7.5" Smokin Rockets Honest John.
 
Looks great. A question about the spin motors, are the D printed? I would have loved to had put them on my 7.5" Smokin Rockets Honest John.

Yes, they are 3D printed. Check post #1 for the source here on TRF. PM him. Quick production and I think he is doing other scale printed parts. Couldn't you do a retrofit on your rocket?
 
Thanks for sharing the post-build diamond airfoil technique. I need to see if this can be applied to my MadCow Ho Jo and/or my Nike Hercules build. This sure looks like it would be a lot faster than balsa and glassing.
 
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Very nice technique. The finished rocket looks great.
I wonder if you could get similar results using rocketpoxy?
 
One of my Books, The Illustrated Encyclopedia of Rockets and Missiles says that in addition to the Spin Motors, the Fins were actually Canted to help impart Spin Stabilization to the Rocket. Is this true?
In all of the Pictures they don't look canted, and I know they are not canted on the Models I see here.
 
I was puzzled by the shade of green you used; not exactly the correct color. You can buy the correct color green from most any Army Surplus outlet (Aervoe Lt. Green, #34151).

Gotta say, I love those 3d printed spin motors. I'm considering ripping the wood ones off my rocket and getting those 3d printed ones.
 
I was puzzled by the shade of green you used; not exactly the correct color. You can buy the correct color green from most any Army Surplus outlet (Aervoe Lt. Green, #34151).

Gotta say, I love those 3d printed spin motors. I'm considering ripping the wood ones off my rocket and getting those 3d printed ones.

By choice I went with an "off color." I have another semi-scale rocket that is the light Army Green. Chose not to have two big rockets match. Also note the gloss finish. Many would argue flat. Again, my choice.
 
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