Build Thread: Trivecta 318

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NateB

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I have several builds in the slow stage of watching paint dry and waiting for good painting weather. I decided to start on my first cluster rocket, a Mach 1 Trivecta 318. It uses 3 18mm motors and should fly well on C and D Qjets and maybe even a a cheap pack of Estes C6-5s.

As always, the first step is to wash the fiberglass parts. Here are the parts in the kit load out. Everything was weighed and I built a sim file.
T318-01.jpg


The kit was dry fit with no major issues. There are 3 centering rings that will sit on the outside of the B-60 body tubes and hold the motor mounts in place. Once they are in place, they will need some light sanding to make them flush with the body tubes.

T318-02.jpg


There are no instructions with the kit, but it looks to be a fairly simple build. There is a video at Badass Rockety showing the build of a Trivecta 329. I'm going to try and keep my build lighter so I can fly it on the smallest motors possible in a park.
 

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NateB

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More prep work before getting parts epoxied together. First, I made some alignment marks on the lower body tube and the lower 2 centering rings to ensure the fin slots line up with the valleys in between each motor mount tube. This is shown in the video linked above.

I also made some marks on the motor tubes where each center ring will be. The motor tubes will extend past the aft end of the rocket by 1/4". I marked the placement of the forward centering ring so that 6" of the motor tubes are exposed in between the upper and lower body tubes.

T318-03.jpg


T318-04.jpg


The motor tubes are sanded with 80grit sandpaper in the areas where the fins, centering rings, and recovery harness will attach. Each centering ring was also sanded on one side where they will be epoxied to the body tubes. The fins were etched with a diamond file at the root edge where they will attach in the valleys of the motor mounts. I just ordered a set of fin sanding blocks from North Coast Rocketry and used it to round the edges of each fin. I liked this simple jig and wish I would have know about it sooner.

T318-05.jpg
 

spigalau

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Hey Nate - great to see a build thread on this one, we are looking to do one too.

Questions for you:
a) Motor Retention - What stops the motors from venturing up the tubes ? Is there a smaller thrust ring to glue in each tube ?
b) Recovery harness attachment - to where ?

The 329 build that Ian did had another center ring for the harness and a bolt for motor retention.
 

NateB

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No thrust rings were included with the kit. I was planning on a tight friction fit for each motor. I think a wrap of tape around the motor before it is slid into the motor mount and a wrap of foil tape around the entire cluster will be secure. If I had 2 more 18-20 cases, I would cluster those and make motor retention with a piece of all thread and a nut.

The recovery harness will just be epoxied into the valley of the motor mounts at the top end. The motor mounts are a tight fit inside the BT-60 tube and there won't be room for another centering ring.
 

NateB

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Moving on:

I placed the centering rings and lower body tube in place on the aft section of the motor mounts. I mixed up some Rocketpoxy because I didn't want anything thinner to run where the fins will go during. I used a small amount of Rocketpoxy on the forward centering ring on the lower body tube. Making sure there was enough for a small fillet on the inside of the centering ring and another small bead on the top of the lower body tube. The I used the marks I made yesterday to keep the fin slots aligned to each valley of the motor mount cluster. Any excess epoxy on the outside was cleaned up with alcohol. The aft centering ring is not epoxied into place yet. I will add it after the fins are in place, using enough epoxy to give a good internal fillet on the fins and drip down to make a fillet around the centering ring.

T318-06.jpg


I had extra epoxy mixed up and didn't want it to go to waste. I filled one of the valleys on the forward end with epoxy and pushed one end of the kevlar harness in place, and topped it with a little more epoxy.

T318-07.jpg


With the harness in place, I added more epoxy around the inside edge of the centering ring and slid the top body tube section in place. Once in place, I gave it a twist to even out the epoxy. The body tubes are a tight fit around the motor mounts, but I could see that someone might get the top section on crooked. I rolled the entire rocket on my work space to ensure it is straight, then set aside to cure.

T318-08.jpg
 

NateB

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Before attaching the fins, I marked the body tube and drilled holes for the rail buttons and vent holes for an altimeter.

I have a set of fin guides from Badass Rocketry with built in drill guides. Using that, I marked the location for the aft rail button making sure it was above the motor because it will extend into the motor mounts.

T318-09-1.jpg


I decided to mark the hole close to the top of the fin, then used a piece of angle aluminum to extend the line to the nosecone.

T318-10.jpg

T318-11.jpg

I used the fin guide again to mark each location for vent holes. The kit comes with mini rail buttons, a 3/32" but was just right for the hardware to thread into the fiberglass. The same bit was used for the vent holes.

T318-12.jpg


The bit walked a little bit when I drilled the holes for the rail buttons. I screwed the rail buttons in place and verified that they will slide with ease on my rail. The buttons are then removed and will be attached with a drip of epoxy on the bolt after painting.

T318-13.jpg
 

NateB

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Time for the fins. This is an easy task on this rocket and due to the valleys in between the motor tubes, the fins practically align themselves.

T318-14.jpg


I mixed up some US Composites epoxy thickened with milled fiber. I really need to get some cabosil as well, but this will work. The thickened epoxy is buttered on the fin root and a little around the sides of the root edge too. Each fin is inserted into the slot and held in place with the fin guide and a binder clip. You can see the small fillet forming around the fin and motor tubes.

T318-15.jpg


With the fins in place. I added a little more thickened epoxy around each fin and smoothed it out with a bamboo skewer. More epoxy is added to the centering ring and it is moved into place.

T318-16.jpg


Finally, I used alcohol and a paper towel to clean any excess epoxy left where I didn't want it and let it cure. The only steps left are the external fillets and determining if any weight is needed in the nosecone.

T318-17.jpg
 

NateB

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I tried a few different nose weights with my sim file. It should be stable with no additional weight, but I added a little for an extra margin of comfort. I wanted to offset the weight of the motor cases if I ever get 2 more 18/20 cases. One case weighs just under 12 grams, so I added 30 grams of BBs, a small amount of epoxy, and some foam to lock it all in. The recovery attachment is just a loop of Kevlar.

T318-18.jpg


Edit: That didn't end up quite as I had planned. When I cut off the extra foam and cleaned up the nosecone, I noticed that it blistered the thin plastic about halfway down. I should have used a deeper cup of water as a heatsink.

I have a LOC nosecone with a sleeve to adapt it to the BT-60 body tube. It is heavy enough that I won't add any extra weight.
 
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NateB

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Time for the external fillets. First, mark the area for the fillet and mask off places you don't want epoxy. I used a fondant tool and pencil / paper to mark the lines.

T318-19.jpg


T318-20.jpg


I mixed up some Rocketpoxy and let it sit for 10 minutes. After resting, I applied the epoxy for the fillet and waited another 10 minutes.

T318-21.jpg

I didn't notice any major bubbles, so I pulled the extra epoxy off with a fondant tool dipped in a little alcohol. 1 pass to remove the extra epoxy and pull it around the top of the fin and a second pass to make it nice and smooth.

T318-22.jpg

Pull the tape and clean up any spots you need to with a finger dipped in alcohol, a razor blade, or another tool you like.

T318-23.jpg
 

NateB

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All of the fillets are now on and the last pair is curing. No pictures, because the process and results were the same as the first pair. I also epoxied the sleeve and kevlar cord for recovery attachment on my Loc nosecone.

Once cured this rocket will be flyable, but there are a few things I want to tidy up before painting. For the color scheme, I'm thinking a metallic blue for the body tubes, nosecone, and fins and metallic silver for the motor mount tubes.
 

NateB

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I had some time off this morning while mechanics performed some scheduled maintenance. As I mentioned earlier the centering rings are just slightly oversized from the body tubes. This most likely is to prevent them from splitting when the holes for the motor mounts are drilled.

T318-24.jpg


It didn't take much effort with a sanding block to get them flush with the body tubes.
T318-25.jpg

Weather permitting, I'll start the painting tomorrow after giving it a good cleaning with alcohol or acetone.
 

Donnager

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Are the motor tubes a snug fit in the "fin" and body tubes?
 

Donnager

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Yes, the fit is very snug. Even the dry fit took a little force to get the motor tubes in the body tube sections.
I've just built a Rocketarium Trident (29mm). The fit for the three tubes in the main tube is tight enough it kinds of eggs out the front in a triangle(ish) circle. It sounds like Mach1 is doing the same kind of thing. It should be very strong.

If I can get this cluster working like I want, I may get the Mach 1 version next.

I saw on the Mach 1 site they had a fiberglass retainer for the larger motors. You might call him to see if he can make one for this rocket.
 

NateB

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The fit for the three tubes in the main tube is tight enough it kinds of eggs out the front in a triangle(ish) circle. It sounds like Mach1 is doing the same kind of thing. It should be very strong.
The thin fiberglass body tube is slightly deformed by the three tubes as well. It is subtle, but they are tight.
 

NateB

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First layer of primer went on tonight. Once it is cured, I'll sand it down and fill and spots that need it. I can already see spots on the nosecone that will need some Bondo and sanding. I will probably use 3 layers of primer total, then a layer of gloss black before spraying the metallic colors.

T318-26.jpg
 

NateB

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Two layers of primer ended up being enough. After sanding the 2nd layer, I was happy with the coverage and moved on to gloss black which will serve as a base for the metallic colors I am using.

T318-27.jpg
 
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