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  #1  
Old 09-05-2016, 09:47 PM
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jeffyjeep jeffyjeep is offline
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Default Rock-solid fin mounting technique for new Estes LJII

This will be similar to my technique for internally epoxying the slot-mounted fins on the BT-80 based Estes V2 which I did a thread on over a year ago.

It begins with the LJII's assembled engine mount with it's top CR not attached yet and with a CR5055 epoxied onto the EM tube .25" from the top. (the mount is inverted in this photo.)

"The Vixen" has brought me a Black and Tan! Yum. Good night. TBC
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  #2  
Old 09-06-2016, 09:41 AM
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Bamboo is one of the strongest plant products there is. When I was in Nanjing, China, I observed that it was being used for bridges, dwellings, and even for scaffolding on 20-story building under construction.

I have a butt-load of bamboo chopsticks from Noodles and Company. If I actually used them to eat with I'd starve, so I threw them into the rocket building materials drawer. Using a fresh #11 blade I bevel the already-tapered end of one of the sticks until it inserts and fits nicely into the upper cavity of the fin. I do the same with the remaining piece of stick until it fits in the bottom cavity of the fin. I press the sticks very hard into the fins until it actually distorts the outside surface of the fin and then I back it off until it doesn't. This accomplishes (2) things: 1) it tests the adhesion of the fin halves (I used Tenax 7r) and 2) it allows for maximum insertion of the bamboo into the fin.

I then cut the sticks into a manageable length using the razor saw (using a blade would be futile.)
Holding the fin up to the work light shows how far the bamboo is inserted.

I then use (2) adhesives to secure the bamboo into the fins. First a capillary application of Tenax7r. When that's cured, I use 30 minute epoxy and let it seep down into the fin cavities.

TBC
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__________________
Never trust an atom. They make up everything.

4 out of 3 people struggle with math.

Chemically, alcohol IS a solution.

NAR# 94042
SAM# 0078

Last edited by jeffyjeep : 09-06-2016 at 12:10 PM.
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  #3  
Old 09-06-2016, 12:23 PM
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Now for some techniques.

Each bamboo peg array is just a BCH different on each fin, so I mark the peg placements on each fin base of the ABS wrap. I also mark each base and each fin with A,B,C, and D.

I then use the largest die of my leather punch to punch holes into and through the LJ's body tube and wrap. The holes aren't quite large enough to accept the bamboo pegs so I ream them with a #11 blade until I achieve a slip fit.

The pegs are way too long--so they're heavier than they need to be. Using a razor saw I trim each one to approx. 1/2" extension from the root of the fin. This intense handling also tests how well the pegs are adhered into the cavities of the fins. They didn't budge!

I then mount the fins onto the model. I brush Plastruct Plastic Weld onto the "bosses" of the ABS wrap and then mount the fin--inserting the pegs into the holes. I register and brace the fin until the weld action is complete. It doesn't take long.

I do the same for the remaining fins.

TBC
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__________________
Never trust an atom. They make up everything.

4 out of 3 people struggle with math.

Chemically, alcohol IS a solution.

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  #4  
Old 09-06-2016, 12:41 PM
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Now for even MORE techniques.

The largest scrap piece of BT that I'm willing to sacrifice is BT80 and I cut it into (4) equal pieces. I then mark each piece of BT80 to correspond with the bamboo pegs of each fin and punch the BT80 pieces with the single-hole paper punch.

I then spread the convex side of the BT80 with Titebond Red and place it over the fin pegs on the inside of the model's BT and register it to the ID. Note that bottom edge of the BT80 scrap piece is recessed from the model's BT lower edge to allow for the engine mount.

Now for the "active ingredient"!

One at a time I enrobe each peg set with 15 minute epoxy.

I repeat this for the other (3) fins.

TBC
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__________________
Never trust an atom. They make up everything.

4 out of 3 people struggle with math.

Chemically, alcohol IS a solution.

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SAM# 0078
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  #5  
Old 09-06-2016, 10:47 PM
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Now for the engine mount.

I use the leather punch to punch a hole into the top CR of the engine mount. I then tie a length of 400# braided Kevlar onto the EM tube below the CR5055 that I added in the first post of this thread and enrobe the knot with epoxy. I measure the Kevlar and make a loop in the forward end. I tie a length of 50# line to the loop to use for "fishing".

I apply little dots of Titebond Red label around the edge of the ID of the bottom of the model's BT, insert the engine mount assembly, rotate it to the correct index position, and then register it to the bottom of the model's BT. I then use a long stick of wood to apply 15 minute epoxy to the bottom CR to BT joint. After that hardens I then take the top EM centering ring, "fish" the small line and the 400# Kevlar through the punched hole, and guide the top CR down through the top of the model's BT and onto the CR5055 that I added to the top of the EM tube. I use a scrap piece of BT70 to register the CR onto the EM tube. I then use a long stick to apply epoxy to the joints of the top CR to the model's BT inside wall and to the EM tube. Lastly, I attach an extra-long elastic fabric shock cord to the 400# Kevlar and cut off the "fish" line.

I feel the benefits of these procedures are worth the little bit of added weight!

The LJII's build then continues as per the kit's instructions.

Thanks for watching. Time for a drink. Good night.
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__________________
Never trust an atom. They make up everything.

4 out of 3 people struggle with math.

Chemically, alcohol IS a solution.

NAR# 94042
SAM# 0078
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  #6  
Old 09-07-2016, 08:51 AM
stefanj stefanj is offline
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Thanks for this article.

Pretty radical changes, but looks like it would result in a pretty solid model.

Definitely an E30 / F39 flyer after all that!
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  #7  
Old 09-07-2016, 09:09 AM
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Quote:
Originally Posted by stefanj
Thanks for this article.

Pretty radical changes, but looks like it would result in a pretty solid model.

Definitely an E30 / F39 flyer after all that!


I think so too. Do you think the styrene fins and nozzles can take the heat?
__________________
Never trust an atom. They make up everything.

4 out of 3 people struggle with math.

Chemically, alcohol IS a solution.

NAR# 94042
SAM# 0078

Last edited by jeffyjeep : 09-07-2016 at 09:29 AM.
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