New downscale project

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JRThro

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I started my first downscale project today, after doing some scale calculations in an Excel spreadsheet. It's also my first scratch project, though I have bashed a kit or two.

It's going to be a 1:4.1 downscale of the <a href="https://www.rocketreviews.com/reviews/kits/dga_armageddon.html">DG&A Armageddon</a>. I really like the look of the Armageddon and mini-Armageddon, but I've stated repeatedly that I'll never ever get into high-power rocketry. I've also said that I don't plan to ever use anything bigger than an 18 mm motor.

So... what to do? Ah, downscale, of course!

The body will be an Estes BT-50 tube, which is what determined the scale factor. The Armageddon uses a 4" OD BT versus the 0.976" OD of the BT-50. I've got an Estes PNC-50YR nose cone, so that's what I'll be using. It's almost *exactly* the right scale length, which is a nice plus (scale NC length is 4.098", actual NC is 4.125" long).

The fins will be made of 1/16" basswood, because I have a 3" x 24" sheet of it that I bought a few months ago and haven't used. The root edge of the fins will be 7.75" long, so this big sheet of basswood will be perfect for cutting out 4 of them. This will be my first project where I've had to cut out fins from scratch, too.

I'll post progress reports as progress occurs, and hopefully I'll remember to take pictures once in a while, too.

Stay tuned!
 

JRThro

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The 4 main fins came first. Dimensions were from the spreadsheet, downscaled from the Armageddon RockSim file that I downloaded from EMRR.

I drew a fin template on 1/8" quad-ruled paper, using a plain No. 2 pencil. Then I flipped the template over and scribbled on the back of it to transfer the pencil lines on the front to the basswood stock. I did this twice and cut out two fins with my Exacto knife (actually an Excel brand knife I bought yesterday at Hobby Lobby). Then I used one of those two fins as a template and traced the outlines for two more fins on the basswood stock. After cutting out those two fins, I have four fins that are actually nearly the same size!

Now the fins just need to be trimmed a little with the hobby knife and then sanded as a "sandwich" so they're all the same size and shape.

Just a side note: the basswood grain is parallel to the root edge of the fins, not the leading edge. This probably won't be a problem with basswood. Also, since the fins have two edges that could be considered the "leading" edge, one edge would not be aligned with the grain no matter what I did.

Pictures soon, I hope. I'm at work right now and don't have my camera. I cut the fins out during my lunch break.
 

JRThro

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Also in this model, I'll be using the prototype FlisKits BT-50-sized ejection baffle that Jim Flis sent me!

Thanks again, Jim!
 

gerbs4me

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this sounds like a really cool project:)
don't forget consrtuction pics
 

JRThro

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Here are some of the parts as they are today.

The fins are cut from the basswood stock but not yet sanded. Then there's the BT-50 tube, and the FlisKits ejection baffle from Jim Flis.
 

Stymye

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the picture is kind of fuzzy but the grain looks wrong, it may just be the picture tho
 

JRThro

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Originally posted by stymye
the picture is kind of fuzzy but the grain looks wrong, it may just be the picture tho
The grain is parallel to the long/root edge of the fins. Cutting the fins that way seemed to make the most sense, since my stock was 3" x 24" basswood and the fins are each one single piece. I'll be hardening the edges, at least, with CA.
 

jflis

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If the grain is parallel with the root edge you are going to want to laminate those fins or they will most likely break on landing (perhaps even during boost). Even with basswood there is much greater weakness in the grain direction.

Let me know how that baffle works!
jim
 

JRThro

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Originally posted by jflis
If the grain is parallel with the root edge you are going to want to laminate those fins or they will most likely break on landing (perhaps even during boost). Even with basswood there is much greater weakness in the grain direction.

Let me know how that baffle works!
jim
Laminate with typing paper or something like that, you mean?

How about if I just coat the entire surface of the fins with CA and then sand them smooth after it dries? I've done that with balsa fins and it seems to work well.

Also, hey, the grain gets filled at the same time!
 

JRThro

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The main fins got their edges trimmed and sanded as a set, so they're all the same size and shape now.

More pics when progress warrants them.
 

JRThro

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I glued the main fins to the BT last night. They're on pretty straight, and pretty perpendicular to the tube, but there has GOT to be a better way than putting them on by hand. I used medium CA so I'd have some time to adjust them before they were stuck on for good. Starting tonight I'll follow that up with white glue fillets.

One strange thing: There's a noticeable bulge in the BT-50 tube at each motor mount centering ring. I've never seen this happen before. If I remember right, these are Custom centering rings that I bought at a hobby store, in a BT-50 that I bought on eBay.

No pics of this progress, but I'll post 'em as soon as I have 'em.
 

Fore Check

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By your description of these "bulges" at the centering rings, I assume that you used white or wood glue to hold the engine mount?

This has happened to me on avery rocket I've built using white and or wood glue. I think it's due to the shrinkage of the glue when it dries (those bulges may actually be depressions that look like bulges. Sounds weird, but it's a bit of an optical illusion.)

Anyway, since I started glueing all of my centering rings in with either thick (or slow) CA and/or epoxy, the visible location of the centering rings went away.

Long story short - that happened because of the glue.
 

JRThro

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I'm about 99% sure they really are bulges, but I'll check tonight. I did wonder if it had something to do with the glue, though.

And yep, I used white glue because... well, that's what all the instructions I've seen say to use, besides which I'm afraid of getting the motor mount stuck halfway into the BT. I'll admit I've never used thick CA, so I don't know how much extra time it would give me to slide the motor mount in, as compared to medium CA.

Btw, feel like driving to Hearne, TX, for the Crossroads of Rocketry 2004 this coming Saturday and/or Sunday?
 

Fore Check

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LOL,

Do I feel like it? Sure!

Is it practical for me? Probably not.

Besides - if the weather in Hearne is anything like it is up here in North Texas, that will be on muddy sucker down there.
 

aksarben10

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Originally posted by JRThro
Laminate with typing paper or something like that, you mean?

How about if I just coat the entire surface of the fins with CA and then sand them smooth after it dries? I've done that with balsa fins and it seems to work well.

Also, hey, the grain gets filled at the same time!
I hope the paper works for you. I cut some basswood fins wrong on a recent project when I saw what I did I would use the paper laminate to reinforce them.

It did not go very well, I lost two fins on landing. They snapped right along the grain even with the paper laminate. I ended up having to remove the fins altogether.

I would really suggest cutting them again or using a lightweight fiberglass on them or they will not last.

Scott
 

JRThro

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Originally posted by Fore Check
LOL,

Do I feel like it? Sure!

Is it practical for me? Probably not.

Besides - if the weather in Hearne is anything like it is up here in North Texas, that will be on muddy sucker down there.
It's raining here in Houston, too.
 

JRThro

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Originally posted by aksarben10
I hope the paper works for you. I cut some basswood fins wrong on a recent project when I saw what I did I would use the paper laminate to reinforce them.

It did not go very well, I lost two fins on landing. They snapped right along the grain even with the paper laminate. I ended up having to remove the fins altogether.

I would really suggest cutting them again or using a lightweight fiberglass on them or they will not last.

Scott
That doesn't sound good. For the time being, I'm planning to proceed with the build. But since it probably won't be ready for an event I'll be attending this weekend, there's more time to consider alternatives.

Thanks for relating your experience.
 

BobH48

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You could try putting a border around the fin to strengthen it.

I have attached a drawing of what I mean.

The heavy black outline is your original fin with the maroon arrow the grain direction.

You could trim a 1/4 inch from the leading and trailing edges and add 1/4 inch strips (red outlines) with the grain in the proper direction (blue arrows).

Let me know if you think this a workable option.
 

JRThro

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Originally posted by BobH48
You could try putting a border around the fin to strengthen it.

I have attached a drawing of what I mean.

The heavy black outline is your original fin with the maroon arrow the grain direction.

You could trim a 1/4 inch from the leading and trailing edges and add 1/4 inch strips (red outlines) with the grain in the proper direction (blue arrows).

Let me know if you think this a workable option.
Bob, thanks for that suggestion. With my limited skills and the fact that the fins are already mounted on the rocket, it probably isn't workable this time. If I run into this situation in the future, though, that might be a good way to go. It looks like it would solve the problem pretty well.
 

JRThro

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After quite some time, here are more construction pictures, showing the rocket with the fins mounted and filleted, plus the ejection baffle that Jim Flis sent me to try out.

First, a pic showing the rocket with the ejection baffle before installation.
 

JRThro

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Here's a picture showing the rocket standing on end, with the Kevlar shock cord sticking out of the top.
 

JRThro

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And finally, here's a shot looking into the body tube, so you can see lots and lots of white glue slopped all over the baffle. The white glue was slopped on with the cotton swabs shown in the first picture. I taped 4 of them end to end so I could reach the baffle once it was installed. I used a 1/2" wooden dowel to push the baffle down into the body tube so that the top of the baffle was 6" from the front of the body tube. I put a thick bead of Aleen's Tacky Glue (TM) around the inner edge of the bottom of the baffle, thinking that some of it might run down onto the inside of the body tube and help to hold the baffle in.
 

JRThro

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One really nice thing is that the baffle came with a good 36" of Kevlar attached to it.

I'm going to harden the front end of the body tube with CA to reduce the chance of a zipper before I ever fly this model.
 

JRThro

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JRThro

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This model feels heavy to me, so I'm going to have to be sure to weigh it before I try to fly it on a C6 motor. It is definitely the heaviest model rocket I've built so far, hopefully it doesn't weigh more than 4 ounces.
 
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