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Madcow HV Arcas 2.6" 38mm fiberglass with homemade optional avionics bay

By Richard Evans

Madcow Rocketry - Arcas {Kit}

This is the newer 38mm version of Madcow's Arcas. I will be building the kit with a homemade avionics bay, and making the kit stronger by using carbon fiber tailings and West Systems epoxy.

The final paint job will be using the basic paint job as shown but using Duplicolor candy apple red, and Duplicolor white.

Aeropack 38mm motor retention.


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Kit as received (2012-03-25 00:00:00)

Inspection of the kit, all parts present. The fiberglass nosecone had some "dips" near the shoulder that will need to be filled in some.


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Wash fiberglass parts. Epoxy syringes (2012-03-25 00:00:00)

Not in the instructions, but I know to wash the fiberglass parts in warm soapy water to remove any mold release that may be present. 

I use these syringes for the West Systems epoxy so you don't need a full pump and have a lot of wasted epoxy. I use .5 mL for hardener and 2.5 mL for resin. You can use more for larger parts, just use the 5:1 ratio (resin/hardner).




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Homemade centering rings for added strength. (2012-03-25 00:00:00)

I want this to be a strong kit. I decided to sandwich the thin fiberglass (G10) centering rings between ply centering rings. I made these using an adjustable circle cutter and a forstner bit (1-5/8).


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Homemade bulkheads for avionics bay. (2012-03-25 00:00:00)

Once again using he circle cutter, I made both inner and outer av bay bulkheads.


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Final fitting of avionics bay bulkheads. (2012-03-25 00:00:00)

Fine tuned bulkheads for proper fit. I slightly tapered the parts where they will slide into the tube easier. The eye bolt is temporary for epoxying.


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Perfect fit (2012-03-25 00:00:00)

Bulklheads on avionics bay.


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Fiberglass centering rings (2012-03-25 00:00:00)

Sandwiched between the ply rings. I used JB Weld to "glue" together. "Glue" one ring at a time and let cure, be sure to drill hole for eye bolt before gluing the next ring.


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Eyebolt in centering ring. (2012-03-25 00:00:00)

I used 2 nuts, 1 washer and 1 lock washer. Use a dremel to trim down the washers and nuts (if needed). Fine tune the ring for proper fit on the motor tube and body tube.


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Centering ring epoxied to motor tube. (2012-03-25 00:00:00)


Following the kit instructions, I epoxied the centering 1" behind the forward part of the motor tube. I used West Systems epoxy and carbon fiber tailings. I mixed the carbon fiber into the mixing cup with the epoxy and thoroughly wetted the fibers. I then took my time to try and evenly spread the mixture around the centering and motor tube. It takes some time to do this and it does not look too pretty,  but functional. The West Systems epoxy gives you plenty of time to work with.

I am going to leave the aft centering ring off for now. I will epoxy in place after mounting fins and internal fillets.


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Fiberglass fin prep. (2012-03-26 00:00:00)

Using a dremel tool with a cut-off wheel (or some kind of sharp saw), cut notches in root of fin. This will increase the surface area for epoxy.



Lester Anderson (March 27, 2012)

Notching the root edge of your fiberglass fins for added epoxy surface area, slick idea! I wonder if it would work equally as well for Plywood fins.

Richard Evans (March 31, 2012)

Never thought about that Lester, but I don't see why it would not work just as well. I would not recommend doing the notches on "thin" ply tho.

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Avionics bay hardwre (2012-03-31 00:00:00)


With limited space for hardware for the avionics bay, I will only be using a single 1/4" allthread and modified eyebolts for the shock cord attachment.

I found some short 1/4" rod couplers at OSH. using a Dremel tool with cutoff wheel, cut off the threads on 2 1/4" eyebolts to 1/2 the length of the coupler. I used JB weld on the eyebolt threads and threaded them into the couplers. Be careful not to get JB weld on the other half of the coupler threads.







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Avionics bay hardware cont. (2012-03-31 00:00:00)

 Cut a piece of allthread about 1" longer than the avionics bay length. Thread a nut onto the allthread and place 1 flat washer on top, place one of the bulkheads on the allthread, place another flat washer on then the eye bolt/coupler. Tighten with wrenches (I did not want to use split washers as they would make the bulkhead crooked). Feed this assy. down the forward body tube to the forward side of the coupler (this should slide down fairly easily sand the outside of the bulkhead if needed). Install the other bulkhead onto the aft side of the allthread making sure the bulkhead is seated properly in the coupler. Mark and cut the allthread a little shorter than needed so it can be tightened without bottoming out before the assy. is completely tight (allow for a flat washer). I did not use a nut on the opposite side of the aft portion.



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Forward body tube/coupler (2012-03-31 00:00:00)

SAND the coupler with 80 grit. I use a 1 1/4 wooden dowel with 80 grit taped to it to get into the body tube to sand. Follow the directions for proper placement of the coupler into the forward body tube.

I used West Systems epoxy (with no carbon fibers this time). BE CAREFUL! Some of the epoxy migrated out of the coupler into the forward part of the body tube. Normally this would not be a big issue but in making this kit dual deploy, the forward bulkhead would not properly fit into the forward part of the coupler. I used a Dremel tool with flex shaft and a cutoff wheel to carefully remove excess epoxy.


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Mount motor tube assy into aft body tube (2012-04-01 00:00:00)


Take measurements on where the forward centering rings will be. In my case, I'm using a Aeropack motor retainer and I had to allow for the length of the retainer body. I made sure the retainer cap extended far enough out of the aft body tube to allow access. If you are using the built up centering rings, be sure there is enough room between the fin slits and centering ring.

 Dry fit the motor tube assy to ensure proper fit. It should slide fairly easily into the body tube, sand if needed. Using the wooden dowel and 80 grit sand paper, sand the inside of the aft body tube where the forward centering ring will live. I used West Systems epoxy only (no carbon fibers, i will use epoxy and fibers for fillets later). Using a stick with the measurements transferred from above on the stick, apply a fairly heavy bead of epoxy to the inside of the body tube using measurements. Slide the motor tube assy into the AFT end of the body tube, double check measurements. I then put 2 pieces of masking tape over the aft centering ring (allows for removal for later steps, sand as needed so it slips in and out easily). slide the aft centering ring over the aft end of the motor tube assy, just enough so it keeps alignment. apply some masking tape inside the motor tube then secure to the body tube (to keep it from sliding). Set the aft body tube in a vertical position to allow excess epoxy to make a fillet around the forward end of the forward centering ring. ALLOW TO CURE 24 HRS!



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