Test to Destruction

[Rich Holmes]

Here goes, my first scratch build. "Test to Destruction" gets its inspiration and its name from the fact that I want to try out a few building method ideas that I haven't used so far. And I don't want to try them out on a beautiful kit I dropped 20 or 30 bucks on! If I end up finishing off a build with a ball peen hammer, some lighter fluid, and a match, I want it to be one of mine.

It's pretty much a bog standard 3FNC. LPR, BT-60. The fins are a little different in their shape but that's about it. The parts are mostly ones I had in my spares box, basically leftovers from an Estes Patriot I bought and raided for parts. I bought a nose cone and a baffle kit for this, and will have to scrounge a thrust ring from a motor mount kit.

OpenRocket file and picture of a possible paint/decal scheme follow.

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[chadrog]

Be careful, you might become more attached to this than those kit builds. I like your fins.

 

[Rich Holmes]

Be careful, you might become more attached to this than those kit builds.

Good point. I'll hide the hammer.

I like your fins.

Bet you say that to all the rockets.

 

[chadrog]

Bet you say that to all the rockets.

Most of 'em - guilty.

 

[Target]

Good point. I'll hide the hammer.

Bet you say that to all the rockets.

Great humor.

 

[Rich Holmes]

Step one: Clean up the workbench. No really. Check it out.
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That half of the workbench hasn't been so uncluttered since about three days after I built it.

No, I'm not showing you the other half.

I even vacuumed around the miter saw.
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About fifteen years ago I swore I was going to organize my screws and nails. It still hasn't happened. BUT I'VE STARTED.
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OK, no actual building yet, but I'm almost ready.

 

[Rich Holmes]

Right, let's get started. Fins. I have a fin template. Thank you, OpenRocket. I also have 3/32" balsa. I bought this for a scratch build my son was considering but that was weeks ago and he hasn't touched it yet so it's fair game.
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I marked the corners with a compass point
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and drew the edges with a pencil. Some of the pencil lines are kind of questionable. Doesn't matter. I'm just using them to keep track of which hole connects to which.
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I cut the fins out with a hobby knife. Metal ruler on the fin side of the line to keep me honest.
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Yay fins.
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There was a rather shameful amount of mismatch between the three. Never mind, that's why God invented sandpaper. On day three.
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One of the things Test to Destruction is supposed to test is laminating fins with paper. I'm proceeding similarly to Chris Michielssen in his blog posts starting here. One difference is because Chris knows better than to make nonconvex leading edges he's able to roll his paper over that edge. I'm just going to have to treat it like the other non-root edges.

I'm doing just one fin first. If I like it I'll do the other two the same way. If not, it's hammer time.

I traced the fin twice onto 20 lb copy paper, flipping it over in between.
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I put an even coating of glue stick on the paper, laid the fin on it, flipped it over, and burnished it with a Sharpie barrel.
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Despite italics, I still missed it here where Chris says "Let the fins dry overnight in a heavy book before trimming." I used scissors to trim the first side close to the balsa edges. Immediately. D'oh!

Lather, rinse, repeat on the other side.
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Yeah, I trimmed that side too. I am so going to Hell. And then I applied the heavy weight.
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And now the heavy wait.

 

[Rich Holmes]

I'm also not filling the balsa before papering it. Chris says he didn't on the previous rocket and you could see the balsa grain. I look at the picture and say, "You can?" Maybe, but I'm not building for close-up inspection.

 

[Rich Holmes]

Lift the books next morning, take a few minutes to recover, and then check the fin; what do we find?
View attachment 141861
Not warped, not wrinkled, paper's sticking down. But it's a bit stained from stuff on the workbench. Lesson learned: Work on a cleaner surface. I don't expect this to be visible after priming and painting, though.

For the next step I moved to my remote facility:
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I applied CA adhesive to all edges but the root,
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spread it with a cotton swab,
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and put the fin up to dry.
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That's all for this morning.

 

[Scotty Dog]

Hey,we got a really cool build thread going on here....I likey the fin drying rack.
I call this a "Big Brother" thread...Im keeping a watchful eye on it.

 

[Rich Holmes]

Evening: I sanded the CAed edges of the fin. I decided some of the paper was overhanging the fin by too much, so trimmed back with a hobby knife before sanding. For now I left the root edge alone.
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Trouble! At one corner both layers of paper have come up.
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I put a droplet of yellow wood glue between paper and balsa on both sides
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squeezed the corner and wiped off the excess glue
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and sandwiched the fin in wax paper before applying the heavy weight again. This time I focused the weight on the fin by putting a wood block, a little larger than the fin, between the fin and the books. Don't ask me why I didn't think of that last night.
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Now we wait some more.

 

[Rich Holmes]

Hey,we got a really cool build thread going on here....I likey the fin drying rack.
I call this a "Big Brother" thread...Im keeping a watchful eye on it.

Big brother is watching! Will he keep me out of trouble, or just tell Mom what I did?

I honestly can't remember what I made the fin drying rack for originally. Possibly not even for fins. But it's handy here. Just four miter saw kerfs in a scrap of wood.

 

[Rich Holmes]

We have happy build days and we have not so happy build days.

I took the fin out from under the books. Looked okay.

I decided I wanted to round off the non-root edges. So I marked guide lines
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and sanded the edges with an emery board
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and... look.
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It's the same corner where the paper lifted up before, though I don't think there's a direct cause and effect there. Maybe that corner can be repaired. And maybe the stains on the paper won't show, and the miscellaneous dings on the surfaces will get taken care of by the primer. Or maybe this fin gets the hammer.

My guess as to what's happening is that the CA-soaked paper being much harder than the balsa requires a lot more force to sand, but then you catch the balsa corner and tear it apart.

An obvious solution is to not round the edges. I've referred to Chris Michielssen's blog post here, where he says "Only the leading edge of the fins was rounded" and that was done before papering — the paper was rolled over that edge. I don't have that option, so either I have to sand the paper and endanger the balsa, or I have to not round any of the edges... or if there's a third possibility, I haven't thought of it.

I'm thinking I'll probably throw out this fin and make all three with square edges. But not tonight. I'm taking the rest of the night off from this. Tomorrow too, probably; busy night.

 

[Scotty Dog]

Im not totally sold on papering fins.
I have had mixed results.
I guess it has its place like everything else.
I have some old blood in me veins, I like and appreciated wood/balsa thats been worked to give a mirror finish.

 

[Rich Holmes]

I have some old blood in me veins, I like and appreciated wood/balsa thats been worked to give a mirror finish.

I need to work on that; if my balsa fins have a mirror finish, it's a funhouse mirror. Makes me look like I have even more wrinkles.

 

[rocketgeek101]

Great build so far!

 

[Rich Holmes]

All truly great ideas originate in the shower. So where did the idea of the shower originate?

This morning in the shower I realized a fair amount of my problems with my fin were due to its shape, in particular the 2-segment, concave-angle leading edge. Makes it hard to sand, and impossible to roll paper over that edge.

So if you have a design that's too complicated to allow it to be executed easily, what do you do? That's obvious. Make it more complicated.

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The original odd fin shape was purely for visual interest. By splitting the fin into two parts I think I've doubled down [see what I did there?] on the visual interest; at the same time, instead of one difficult leading edge, I now have two easy ones. They can be rounded first, then papered over. I can leave the other edges square. Problems solved.

Well, that's a theory, anyway.

A lot of really stupid ideas originate in the shower too, you know.

(OpenRocket says this design has more stability than the previous one, by about 0.2 cal. I'm very skeptical. I've played around enough in OR to come to believe it's not necessarily handling multiple fins correctly. In particular if you butt the two fin sections together, making in effect a single fin, the CP isn't in the same place as if you put in a single fin the same size and shape. Perhaps it's assuming end effects that aren't correct for fins close together.)

 

[Rich Holmes]

It's been a day and a half and no one's given away my location to the local psychiatric hospital, so Operation Split Fin is a go! I cut three fore and three aft fins and sanded them to matching size. (Pleased to report I did a more consistent job of cutting them out in the first place this time.)
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I put two pairs of fins aside and worked on the third. I rounded the leading edges.
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Then I traced around a fore fin on a piece of 20 lb bond paper, drew in a generous area for the other side,
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and cut the paper out. I applied glue stick to the paper, then put the leading edge down,
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rolled the paper over that edge,
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and flattened from root to trailing edge.
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Finally I burnished with a Sharpie barrel.
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Then I did the same with the aft fin.
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Fore and aft fin were sandwiched in wax paper and put under books with some wood to focus the weight.
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Stay tuned.

 

[krusty]

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I like the split fin

Krusty

 

[Rich Holmes]

The WPI Alumni Association will no doubt be pleased to hear that, having given up on finding the cutting mat I know we have someplace (besides, technically, it's my wife's), I've bought a new one.

 

[Rich Holmes]

... which of course I left behind at the office.

Anyway, I trimmed the paper close to the balsa edges
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and applied CA along the non-leading, non-root edges.
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(The aft fin sort of has two leading edges, or one leading edge in two segments. The one that will go directly behind the fore fin is square and uncovered, the one that sticks out beyond the fore fin is rounded and covered.)

 

[apburner]

And I don't want to try them out on a beautiful kit I dropped 20 or 30 bucks on!

I have a friend that absolutely refuses to build a kit. The only time he comes close if he builds a scale that hasn't been done. But everything else is his own design.
I like scratch building too but don't have the aversion to kits that he does. I will build the occasional kit.

 

[Rich Holmes]

Yesterday I sanded down the edges on the fins. They'll do. Today I glued paper to the remaining two fin sets. That may be all I do this whole week. I'll be out of town for most of it.

 

[Rich Holmes]

I got back last night and will be leaving again today. All I've done is to trim the remaining fins and apply CA. I've done a far less than perfect job on them; I'm particularly embarrassed about one edge I cut at entirely the wrong angle and sliced across part of the fin's surface. Fortunately it was near the root edge, so it'll be covered by the fillet. Anyway, I think I can live with what I have. By the time the edges are filled and the fins are primed and painted I think they'll look fine.
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[cwbullet]

Neat design. Unique fin design.

 

[Rich Holmes]

Back again again. I sanded down the fin edges. Finally I can move on.

Next thing I did was to build the baffle. This was from a Sunward baffle kit. Their instructions say to inset the baffles in the coupler by about 1/8" so I used a small scrap of balsa with some wax paper on top as a spacer.
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I put the bottom plate on the spacer, spread yellow glue inside the coupler, and pushed the coupler over the bottom plate.
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Same with the top plate.
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Finally I added some glue fillets.
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[Rich Holmes]

While the baffle glue was drying I worked on the motor mount. I used the mount from an Estes Patriot kit I'm using for parts, except for the thrust ring, which I used already, so I took that from a Rocketarium motor mount kit, which I guess I'm using for parts. I won't bore you with the details; I built it as directed by Estes, using yellow glue.
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And while that glue was drying, I cut a piece of 100# Kevlar braid, about 11" long, and tied a loop in one end.
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(Later on I realized I wanted an overhand knot in the other end, so I did that.)

I cut out the fin marking guide. Why doesn't OR give you the launch lug line too? I added that, wrapped the guide around the body tube, and marked the fin and launch lug positions.
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Then extended the lines with my aluminum angle. You know the drill.
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That body tube, by the way, is BT-60, 6 5/8" long. I'm using two such pieces, also taken from the Patriot kit. I could have used a single longer piece of BT-60 but decided I'd rather have a long piece in my parts box than two short pieces, and since I'm using a baffle here, it's not like I'm adding weight with a coupler. It'll be more work to finish, that's all.

 

[Rich Holmes]

With the baffle ready, I taped the Kevlar to the aft end of the forward body tube. The position is such that the looped end of the Kevlar is just aft of the forward end.
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Then I applied 5-minute epoxy inside the body tube and pushed the baffle halfway in. The Kevlar is sandwiched between the baffle and body tube, but the coupler still fits in easily enough.
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I painted more epoxy on the face of the lower baffle plate
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and stuck the Kevlar end (with the overhand knot) into the epoxy at the edge of the disk.
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I made a mistake next. I should have let this dry before proceeding. Instead I put epoxy in the forward end of the aft body tube, and pushed that on. The end of the Kevlar got displaced a little and had to be pushed back down with a stick. I think it'll be all right, though.
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For a small rocket like this, I think between being epoxied between the baffle and the body tube and having the knotted end epoxied down, the Kevlar will stay put without having to be tied around anything. In retrospect maybe a better solution would have been to tie around the motor mount tube between the centering rings, then run from there up between the baffle and body tube. But I expect this to work.

Finally I used more epoxy to install the motor mount flush with the body tube end.
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That's that. Fins go on next, and launch lug. Don't let me forget the launch lug, OK?

 

[Rich Holmes]

I attached the aft fins with yellow glue. My Estes fin jig has a side that skews a bit, so some persuasion was needed.
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And then I...

STUPID! STUPID! STUPID!

What?

Ohcrud. I forgot to fill the fin edges first. Why did you people let me do that? You're supposed to be keeping an eye on me, you know. If I screw this rocket up it'll be ALL YOUR FAULT.

So I'll be filling the aft fin edges in place. But I'm doing the forward fins first, before mounting. This is Test to Destruction, remember? One thing I'm testing is wood filler. For my first filled balsa fins I went to Home Depot and bought Elmer's Carpenter's Wood Filler Max (below, left) because it's what they had. More recently I went to Lowe's (ewww) and bought some Elmer's Carpenter's Wood Filler Not Max (maybe it's Norm) (below, right).
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I hope I'll decide Norm is much better than Max, because I don't like Max much. For one thing it costs a lot more. But until I'm fully confident in Norm, I'm proceeding with caution. So, forward fins now, aft fins next.

I applied Norm to the trailing edges of the forward fins,
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let dry, sanded, and applied more Norm. That's it for tonight.

 

[Rich Holmes]

I attached the aft fins with yellow glue...
View attachment 143648

You know, I thought there was something wonky about those fins (I mean wonkier than they were intended to be) but I couldn't figure it out until now.

THEY'RE UPSIDE DOWN.

I built 'em wrong and installed 'em the same wrong way. The aft end of the root edge is supposed to be the forward end. Hoist by my own petard!

I could slice them off and turn them over but then I'd have a squared leading edge and a rounded trailing edge.

I could slice them off and remake them but then this silly rocket wouldn't get done until around the time NASA puts a crew on Mars.

Or I could adjust my OpenRocket file, delete this post, and pretend I meant to do it that way.