Super DX3 build

[codysmith]

I'm skeptical. What is it that makes an internal fillet stronger?


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.

A lot of it has to do with the forces during landing. Internal fillets (MMT and BT) spread the load much more evenly than External fillets. I wish I could draw a picture but just imagine a hard landing on a fin. With only external fillets, the joint would flex and break. With internal fillets, the joint doesn't flex as much.

If I'm wrong here someone tell me but this is how I understand it.

 

[mpitfield]

Once the fillets on both sides of the bulkplate in the nosecone coupler had set, I test-fitted the coupler into the nosecone and marked the depth on the coupler at a few different places. I then removed the coupler and verified that the depth of all the marks was the same, at 2.6 inches:

View attachment 176942

This way I can verify that the coupler has seated squarely into the nosecone, rather than being off-axis and making a big mess later on. With that being done, the usual surface preparation, a healthy coat of epoxy on the bonding surface inside of the nosecone, and everything just slid together perfectly:

View attachment 176943


Now if I can just quit being nervous about pouring the external fin fillets....

I have the same bird on my build pile and a very similar BAR beginning as yourself. I just picked up my L1 a couple months ago and recently hit my first big launch, URRF and it was a lot of fun, great group of fliers and some talented builders to inspire me.

I also started off by picking up an Aerotech, but the Arreaux then this Super DX3. Unfortunately I got a bit carried away with purchasing other kits and put the Super DX3 down and built out 4 other rockets. I am just picking the DX3 back up but scrapped the 38mm MMT and ply fins for a 54mm MMT and the G10 fins. I also picked up a GiantLeap glass sock and will be vac bagging the BT with it in the next couple weeks, or as soon as I work out my vacuum issue. One thing I was looking at doing was substituting the stock nosecone for a FW/FG but the VK style, however I was not sure about the fit. I am guessing the coupler for the Ogive and VK are the same diameter but I figured the FG nosecones would fit sloppy as they would be too narrow for the ID of the paper tube...how is your fitting, lose, tight?

 

[Steve Shannon]

Thanks!

You know, I thought about using a forged eyebolt. Welding it myself was out of the question, because I am an atrocious welder. Seriously, we all have our talents, and that is not mine.

But the part you see is what Madcow supplied in their kit... so that's what I decided to use. Hopefully it holds up – I don't see a way I could possibly replace it without essentially starting over?

Under normal conditions it will be just fine! I would not start over. If you fly as well as you build you should never have to worry!


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.

 

[dottore]

Turns out fin fillets go way, way better when you don't overdose on epoxy:



With those taken care of and set up solidly, I put a dose of structural epoxy on the aft surface of the fins, motor mount tube, and body tube, and took one last look inside the motor mount:



After a little bit of sanding, a few taps with a seating tool (read: Schedule 40 PVC fillet shaping tool and a deadblow hammer ), and an external fillet, the aft CR was seated, and the back end of the rocket is all sealed up:

 

[dottore]

Oh yeah... and in the last few rays of sunlight (and after a healthy dose of sanding on that fiberglass coupler!), I can finally test-fit all the components together:



I have a fair bit of body work to do – filling in where the glassine coating separated off the body tube and whatnot*– but it's nice to see it all together!

 

[dottore]

One thing I was looking at doing was substituting the stock nosecone for a FW/FG but the VK style, however I was not sure about the fit. I am guessing the coupler for the Ogive and VK are the same diameter but I figured the FG nosecones would fit sloppy as they would be too narrow for the ID of the paper tube...how is your fitting, lose, tight?

Ha! I wouldn't worry.

The fitting with the unmodified fiberglass coupler and upper body tube was unusably tight. After about 40 passes/square inch with 60 grit over the entire coupler, the fit is snug – it took me twenty or thirty seconds of wiggling and rotating to get the nosecone to seat all the way down.

 

[Steve Shannon]

Great job.


Steve Shannon
Prefect - Tropoli Montana
L3CC, NAR Montana.

 

[Steve Shannon]

Hi Cody,
Rather than hijacking Dottore's thread I started a new thread about fin fillets: https://www.rocketryforum.com/showthread.php?67329-Fillet-Location-and-Strength

 

[codysmith]

Really nice work you are doing here. You definitely learned from your epoxy mistake. We all make them.

 

[dottore]

Had a really busy week at work, so I haven't been able to do much this week.

I have discovered that if something is worth doing, it's probably worth doing three times. And so it goes with wood filler to smooth out the minor imperfections in my epoxy fillets. I thought two passes looked pretty good... so I threw on a coat of primer. Primer, you see, is supposed to make paint stick. That's what they tell you on the can, right? It's a pack of lies. Primer is there to expose every minute flaw in your prep work.

So after sanding most of the first primer coat down with 400 grit paper, I applied some more wood filler (diluted with a few drops of water), smoothed it out with my finger, and let it dry. Afterwards, a pass with 400 grit smoothed it out much better, and so it was time for another coat of primer (high-build this time). While I was at it, I masked off the mating surfaces on the nose cone and primed that as well:



As you can see, those fillets are starting to look a lot better:



What I haven't done yet is prime the middle section of body tube and the avionics bay: unfortunately, I'm still trying to come up with a good way to protect the coupler and static holes. Once I figure that out, I'll probably start trying to prime the entire model in one shot –*I'll need to do that for paint, anyways.

 

[mpitfield]

Looking very nice!

 

[rocketgeek101]

You know I'm curious; why did you choose to substitute the plastic nose cone for a fibreglass one? Seem like overkill for a cardboard rocket.

Good build so far! Your fillets look great!


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