Dragon Rocketry AT-3 Sagger Anti-Tank Missile Build

[Back_at_it]

Lately I've been building small, low power rockets and I feel like it's time to build something a little larger. With our clubs flying season over, I'm looking forward to building some larger rockets for our Seed Farm launch next spring.

On the bench is a Dragon Rocketry AT-3 Sagger Anti-Tank Missile. When I first saw this one hit the website I knew I was going to buy it. I love the look of short fat rockets.

https://dragonrocketry.com/products/4-at-3-sagger

Excuse the pun but this thing is built like a tank. I've test fit everything together and it all fits perfect. The quality of the parts is awesome and I have no doubt I could put the biggest motor my certification would allow me to buy in it and it would handle it fine.

- Body tube is 4 inch heavy wall. The fin slots are precut and the tube is marked for the fins and rail buttons.
- Nose cone is 3D printed. These are some of the nicer 3D printed parts I've seen. These have a nice consistent finish that will be easy to sand and finish.
- Fins and 1/4" plywood.
- Tail cone is 3D printed and the same quality as the nose cone.
- Centering Rings are 1/4" plywood wood.
- Motor tube is 38mm heavy wall tubing.
- Includes and 38mm motor retainer.
- 29mm motor adapter and retainer.
- Kit also included a eye bolt, rail buttons and a length of Kevlar as a leader for the shock cord.
- Nose weight is included as well.



Like many of my previous builds, I'm shooting to keep weight to a minimum as I'd really like to be able to fly this on normal club launch days in additional to the few chances we get to fly on the big field. For me that means F and G motors. During this build I'm going to be swapping out certain parts for lighter components in and effort to meet this goal.

First step is building the motor mount. For this I have removed the outer layer of the motor tube where the upper centering ring and fins will attach as well as a smaller section at the rear of the tube where the motor retainer will attach.

First modification. I have replaced the upper centering ring with a thinner 1/8" piece. A small backer has been added for additional strength where the screw eye will attach.



The backer plate was attached to the under side of the upper ring the upper rings was then attached to the motor tubed using T-88 epoxy.



Going to let this cure for a few hours before continuing.

 

[Back_at_it]

Continued the build by drilling a hole in the upper centering ring for the Eye bolt. Normally I'd just tie a knot in the shock cord and feed it through the hole without using the eyebolt to save weight but with this being a 4 inch kit I can get my hand down inside should I ever need to replace it.



The Eyebolt was screwed in place. I'm only using one washer and one nut to cut weight. Loctite was used to keep thing from coming loose.




Here is the underside of the centering ring. The excess length of the eyebolt was cutoff. I wasn't happy with the fillet on this so a small fillet was done on the underside after this pic was taken. The nut was also covered in epoxy for good measure.



Next came inserting the motor mount into the tube. The edges of the centering ring was beveled so it would slide in easily and give the epoxy a place to sit.



The motor mount was located inside the tube and a line was drawn around the inside of the tube so I could tell where the epoxy needed to be applied. It's kind of hard to see but it's there. The inside of the tube was then lightly sanded to open up the fibers.



Using more T-88 epoxy, I inserted the motor mount. The tail cone and motor retainer was then used to locate the depth and to keep the motor tube centered.



Going to let this cure while I prep the fins.

 

[Back_at_it]

Finished the prep work on the fins. I coated each fin with a thin coat of DAP brand wood filler and let that cure before sanding with 320 grit on an orbital sander. Next I rounded the edges of the fins. I kicked around the idea of beveling the leading and trailing edges but lets face it. Nothing is going to make this thing any more aerodynamic

I then began installing the fins. The outer layer of the tube was removed to allow the epoxy to soak into the fibers. A healthy bead of epoxy was applied to the root edges and they were slid into the tube and aligned.

Here you can see where I have two of the four fins attached.




This afternoon I attached the third fin and I'm allowing that to cure.




I should be able to get the last fin attached before bed this evening so tomorrow will be rail buttons and fillets before buttoning up the rear end of this one.

 

[Back_at_it]

Last fin was installed last night and left to cure overnight.



While the fin was curing I decided to make a small modification to the motor retainer and tail cone. As designed, the motor retainer sit completely below the tail cone. With the retainer cap screwed in place there was about a 3/8" gap between the cap and the bottom of the tail cone. I decided that I wanted the retainer cap to sit flush with the tail cone. To do this I enlarged the hole in the bottom of the tail cone to allow the retainer to slide into it.

I made the fit nice and tight so it would stay in place then added a couple of drop of CA to secure it temporarily. Here is what the recessed retainer looks like.



Looking down from the inside. Note that everything was sanded prior to assembly.




Next I added a bead of epoxy around the inside to attach the retainer to the cone.



Here is what the finished product looks like with the cap installed.



Next up was installing the rail buttons. The nose cone has a small hump that sticks out past the body tube approx. 1/8". I decided to add a couple of hand cut standoffs under the rail buttons. Honestly I think there is enough room to get the buttons on the rail without the standoffs but I like to move the rail out a little so it isn't dragging on the body tube on launch. These were hand cut from 1/8" plywood.




The areas where the standoff's will be places had the outer layer removed.




Here we have the standoff's epoxied in place. A backer piece will be added inside the tube as well should I ever need to replace the buttons or use longer screws. First external fillets were also added at this point.

 

[Back_at_it]

It's been a while but I'm back with a pretty good update. Internal and external fillets have been completed using T-88 epoxy. The fins are extremely rigid at this point so it's time for some more assembly.

Starting with the tail cone. The edges of the tubes and the tail cone have been beveled to ease in assembly. All contact areas have been sanded with an 80 grit drum roll then wiped clean.



Next came the epoxy. I did this in two steps. The first was a thin layer that was spread over the cardboard attachment points. The inside of the body tube as well as the outside of the motor tube. The epoxy was then heated with a heat gun and given a few minutes to soak into the materials.

I then added epoxy to outside of the tail cone as well as the inner lip of the motor retainer. Additional epoxy was spread on the motor tube and inside the body tube and the pieces were slid together.

Excess epoxy was wiped away the rocket body was stood upside down with a 5lb flat weight on the tail to keep everything together.



Time for the nose cone. Normally the nose cone would simply be prep, filler, primer, paint but in this case one of my dogs decided he had a new play toy. More like a new chew toy. Thankfully he started at the base and not the tip. I promise this is not one of my weight saving ideas as I will need to add nose weight to make this stable.

After looking over the issue I decided to try to remove the base from the nose cone. Thankfully this cone was printed with the base separate and glued together. After about 10 minutes of fiddling around I was able to get the base loose from the cone without damaging the cone. A little sanding to clean up the glue and the nose cone was good as new.

To my surprise, a LOC 4" coupler is an exact fit inside the nosecone which makes sense if I really stopped and though about it. I cut a 6 inch section of LOC coupler and epoxied that into the base of the nose cone. The outside of the coupler was then coated in epoxy and heated to allow it to soak in. I'll still need to do a bulkhead inside the coupler but that will need to weight until after I figure out the CP and CG and add the require nose weight.

Should have some assembled pics in a couple of days.

 

[Back_at_it]

Back with the fun stuff. Started sanding. Used a 100 grit disc on an orbital sander to knock down the bulk of the ridges. From there I used a fair amount of thinned Spot filler and coated the entire nose cone. Here we are after about 20 mins of sanding. Looking pretty good but it still needs more.




Next up I applied three rather heavy coats for SEM high build and let that cure over night. This morning I knocked down the primer with some 220 grit but there were still some tiny imperfections so a very light coat of DAP brand wood filler was applied. This is currently drying so I should have this in final primer by the end of the day.

If I had to do it again, I'd skip the spot filler and go straight to filler primer then wood filler on top of that. Much easier to apply and sand.




The main body is getting primed a bit later today as well.

 

[Back_at_it]

Managed to get primer on last night. Shot on three coats of SEM high build and let that cure for a few hours before sanding. Once sanded I realized there were a few more imperfections that I cared for so out came the wood filler. I spread on a super thin coat over the entire rocket before sanding that back with 320 grit.

There were still some stubborn areas where that needed something a touch stronger so spot putty was applied to these areas and sanded back with 400 and 600 grit.




After sanding, one top coat of SEM high build was sprayed on to make everything even. Going to let this cure today and knock it down with 600 grit tomorrow before painting.

 

[Back_at_it]

First two colors have been applied. Still need to paint a section of the body tube semi-gloss black as well as the tail cone. The main body is Krylon Satin Hunter Green. It's little shinier than I expected but it's still curing so it might dull up a little. Thinking I'm going to let this cure then hit it with one more coat.




More to come this weekend weather permitting.

 

[Back_at_it]

With the paint dry, I loaded up the rocket with recovery gear and the largest motor I plan to fly in this rocket. In this case that is the H219.

After running simulations in OR I decided that I wanted the CG to be right at 15 inches from the tip. I Added a total of 12oz of BB's and epoxy to the tip of the nose. If this seems excessive, keep in mind that I removed the printed shoulder which was several oz. so I had to add that back in. You would not need to add anywhere near as much if you use the stock parts.



Next came the base. I sanded the inside of the coupler and inserted the bulkhead.



After allowing the bulkhead to cure overnight, I added a ring under the coupler to help prevent it from pulling out then added a generous epoxy fillet.



Once this is cured, I'm going to drill a 1/2" hole and fill the cone with expanding foam to keep the nose weight from moving. Currently the CG is sitting at 14.50 with a stability of 1.23 with the H219 in the tail and all of the recovery gear on board. The foam will probably add another once so I'm very confident that this one is going to be stable.

Laying out some vinyl decals next. My next update will probably be after the holidays.

 

[Back_at_it]

Got the nose cone filled with foam and let it cure overnight. Overall it added and additional 1.2oz of weight bringing the final CG to 14.25".



Knowing that foam is somewhat flammable, I hollowed out a small bit of the foam and covered it with a decent drop of wood glue.

 

[Back_at_it]

It’s been a little while since I have worked on this one but I’m happy to say that it is finally done.

After looking at numerous photos online I decided to cut some vinyl. I found more than 50 different patterns and colors online so this is kind of a mashup of a few of them combined.

Here is my finished anti-tank missile.