Dragon Rocketry Fat Boy XL Build

[Back_at_it]

With the LOC Star Fighter finally completed, I want to get started on a new build. I read a post about this rocket and picked it up right away. The kit is top notch like everything from Dragon. I did a test fit of the parts and everything fits perfectly.

I have a couple of ideas on how I want to built this kit. My club flies at a soccer field and we're limited to F motors so I'm I'm going to try to drop some weight on this kit to get it down where I can comfortably fly it on F67's. Advertised weight is 39oz. I'm hoping to get it down in the 30-32oz range.



Mock up.

 

[DragonRocketry]

With that motor and 1oz of nose weight puts you at a 1.44 margin and altitude of around 460'

 

[Back_at_it]

With that motor and 1oz of nose weight puts you at a 1.44 margin and altitude of around 460'

Is that at the the advertised weight of 39oz?

 

[KenECoyote]

With that motor and 1oz of nose weight puts you at a 1.44 margin and altitude of around 460'

That sounds appropriate for a Fat Boy rocket. Big, low and slow! Should be quite a sight.

 

[DragonRocketry]

Is that at the the advertised weight of 39oz?

No, the advertised weight is with 6 oz nose weight, plus it is rounded up to allow for glue and paint. Mine ended up 29oz without recovery items and has 5oz nose weight as I am looking at doing a H220T in this thing. I should be at 1.55 cal. I could have used less weight but want to be on the safe side. Does not look good for a kit manufacture chasing people with an unstable rocket...

 

[Back_at_it]

No, the advertised weight is with 6 oz nose weight, plus it is rounded up to allow for glue and paint. Mine ended up 29oz without recovery items and has 5oz nose weight as I am looking at doing a H220T in this thing. I should be at 1.55 cal. I could have used less weight but want to be on the safe side. Does not look good for a kit manufacture chasing people with an unstable rocket...

Completely understand. I think my first flight will be up at Bong when no one is around

My initial plan is to fly on F67's for the club launches and G74 and G80's when I'm on my own. Of course all of this depend on final weight.

I like that you made the base plate of the nose cone removable so I can adjust weight as needed.

 

[DragonRocketry]

Completely understand. I think my first flight will be up at Bong when no one is around

My initial plan is to fly on F67's for the club launches and G74 and G80's when I'm on my own. Of course all of this depend on final weight and.

I like that you made the base plate of the nose cone removable so I can adjust weight as needed.

G80 will give you a nice 1400' flight

 

[Back_at_it]

Time to get started on this build. Since I'm trying to build this rocket to fly on an F67 I'm going to need to keep the weight down where possible so I can't do my normal process which is to over build everything Figured a good place to start is with weighing all of the components included in the kit stock and seeing where I can reduce weight.

Nose Cone Complete with all hardware in place. 12.0oz
Body Tube 3.7oz
Fins (all three) 6.3oz
Motor Tube .40oz
Motor Retainer .40oz
Upper Ring 1.0oz
Lower Ring 1.1oz
Eye Bolt .90oz
Rail Buttons .20oz
Total Weight of items included in the kit 26.0oz

I should mention the kit also 6.3oz of Nose Weight. I don't think this will be needed for my build as I plan to stick with relatively small motors but I am glad that it is included for those that want to fly larger motors. Looking at some early SIM's I've run, it looks like I will be able to remove some mass from the nose cone and still be stable but it's too early to talk about that right now.

During this build, please remember that I'm attempting to build this rocket to fly on a relatively small F67 motor so changes in construction will be made accomplish two things. The first is to reduce over all weight and the second will be to move weight forward toward the nose cone in the attempt to avoid or minimize the amount of nose weight required to be stable.

I began the build with the motor mount. The outside layer of the motor tube was removed then lightly sanded to open up the fibers to allow adhesive to bond. The upper centering ring was sanded slightly on the flat surfaces and all of the edges beveled to remove any sharp corners including the hole for the eyebolt. The upper ring was attached 1/16" down from the top of the motor tube. This was first tacked in place with a couple of drop of CA then fillets of BSI 30 min. was applied and let cure.



Next it was time to attach the shock cord. The kit includes an eyebolt for this but to reduce weight I have decided to leave the eyebolt out and attach a LOC 15" Kevlar shock cord around the motor tube under the upper ring. This was looped around the motor mount then pushed through the hole in the upper ring. The weight of the shock cord is .70oz.



Here we see the underside of the upper ring. The shock cord was attached with BSI 30 Min. epoxy.



After letting the epoxy cure. the motor mount was inserted into the body tube. The inside of the tube was marked where the upper ring would sit then lightly sanded to open fibers up. The motor mount was inserted until the rear of the motor tube sat 3/8" up into the body tube. This will hide the motor retainer and move the CG forward. This was attached using more BSI 30 min.



A view down the tube from the top shows a nice fillet around the top of the upper ring. Also note that epoxy was spread around the top 2 inches inside the body tube to add some strength to help prevent zippering.



Today I'm working on the fins and will be attaching them over the next couple of days.

 

[David_Stack]

You've already committed to epoxy as your adhesive, but it would have been interesting to know how much weight could have been shaved from your build had you used an aliphatic glue (e.g. Titebond) instead...

 

[Back_at_it]

You've already committed to epoxy as your adhesive, but it would have been interesting to know how much weight could have been shaved from your build had you used an aliphatic glue (e.g. Titebond) instead...

I kicked that around a lot for this build and I almost went with TBII. I use it on pretty much all of my smaller builds and it has worked out great. We all know that the bond is only as strong as the materials that are being bonded and the cardboard will fail long before either the epoxy or wood glue.

That said I read a number of articles from wood workers that restore furniture and ironically, lite weight plane builders. Each talked about bond vs weight and which is easier to work with. The results were pretty much the same on everything I read. They all pretty much said that the materials will fail before the bonds in almost every case but for weight it was pretty much a wash. While wood glue is lighter, you need to use more of it to get the same bond which kind of offsets any weight savings.

If I can control myself and not over use the epoxy It shouldn't be much of a difference in over all weight. Maybe I'll just need to buy a second one and build it with wood glue

 

[ep29030]

With TTW fins, the flight stresses are taken by the fin/MMT bonds. The aft centering ring is just along for the ride. You could save weight by using either a very thin wooden aft centering ring, or a foamcore one. For that matter, you don't even need an aft centering ring once the fin tabs are bonded to the MMT.

 

[Back_at_it]

With TTW fins, the flight stresses are taken by the fin/MMT bonds. The aft centering ring is just along for the ride. You could save weight by using either a very thin wooden aft centering ring, or a foamcore one. For that matter, you don't even need an aft centering ring once the fin tabs are bonded to the MMT.

You read my mind. Didn't want to get too far ahead but I've already cut this from poster board.

Will drop a full ounce off the tail of the rocket.

 

[KenECoyote]

So you're putting your Fat Boy on a diet?!?

Actually I think what you're doing is very cool and I may copy some of it for some future builds.

 

[Back_at_it]

With the motor mount cured it was time to start on the fins. Looking at the fin tabs I decided there was an opportunity to drop some weight here. I didn't want to get to carried away but I drilled out three 3/4" holes in the tabs. At the same time, I marked the leading edge of the fin 1/4" down as a reference for the bevel.



The leading edge was then sanded with an orbital with 120 grit then cleaned up by hand with 320 on a sanding block.

The good news is that I dropped .20oz from each fin for a total of .60oz off the rear of the rocket.



Time to install the first fin. The body tube had the outer layer removed around the fin slot. The root edge was sanded and epoxy was applied directly to the motor tube as well as spread onto the root edge of the fin. The fin was then inserted, aligned and left to cure.



After a couple of hours I repeated the process for the second fin and left that to cure over night. Third fin will go on as soon as I get a break in work and can head down to shop for 5 mins.

 

[Back_at_it]

Third fin was attached. This has been a really simple straight forward build. Everything fits absolutely perfect.



With the fins attached and cured it was time to do the internal fillets. To help the epoxy flow a little better, I warmed the inside of the rocket with a heat gun then mixed up some BSI 30 min. and placed it in the joints. This is never a clean job for me but I did keep the fillets to a minimum and wiped up any excess that got on the tube.



Next up was the rear centering ring. Since this rocket has TTW fins, the rear ring doesn't offer anything structural to the build so I went with a lighter alternative. This was cut from a piece of post board I picked up at hobby lobby for $1.99 for a 24'x48" piece. The included 1/4" ring weighs 1.1oz. This ring weighs. .10oz. so we knocked a full ounce off the rear. You will be seeing more of these rings in future builds. After a little sanding around the inside of the tube, I attached the ring using TBII.



Next came the motor retainer. I scuffed the inside of the motor retainer with 100 grit to give the surface some texture then attached with more epoxy.



I decided to use a 1/4" launch lug for this build as I typically launch off a rod and don't take the rail to normal launch weekends. This build will be light enough that the 1/4" rod will be more than enough. One thing not shown is that I did epoxy two small squares of 1/8" plywood inside the body tube in the correct locations should I want to add rail buttons later.

 

[Back_at_it]

With everything on the rocket curing I decided to turn my attention to the nose cone. This is a really nice piece with a removable bulkhead so nose weight can be added as needed. The bulkhead is held in place with four screws. A screw eye with two washers and two nuts is also included.

As a completed assembly the nose cone weights in a exactly 12oz or 340g. At 12oz I feel like there is some weight savings to be had here but I can't make that decision until I get a final weight and CG on the rocket. The fit in the body tube is perfect.



Here is a look inside the cone with the bulkhead in place.



Bulk head removed.



And the bulkhead and hardware out of the cone. This is all really well done and nicely finished.



To prep the nose cone I first washed it with hot soapy water and air dried with a heat gun on low. Next it was time to sand. I hit the outside of the cone with 100 grit on an orbital to knock down the major ridges. I then hand sanded with 240 and 320 grits. All told I had about 15 mins. in the sanding and it was looking really good.

Next I applied a thin coat of Bondo brand spot putty and let that cure over night. After sanding with 100, 240 ad 320 grit paper this morning, here is where we are at. I feel like a couple of coats of high build primer and this should be ready for paint.




Next up will be External fillets, sealing the fins and spirals then primer.

 

[DragonRocketry]

Nice build. Looking forward to the final weight.

 

[KenECoyote]

I got my FB kit today and the nose cone is different. It doesn't matter so much to me since I like to epoxy all thread to the tip, but just figured I'd note the change.


(The glue was already in place when I got it.)

 

[DragonRocketry]

I got my FB kit today and the nose cone is different. It doesn't matter so much to me since I like to epoxy all thread to the tip, but just figured I'd note the change.

View attachment 573397
(The glue was already in place when I got it.)

The first batch was like the one in this kit and was changed do to the amount of time and cost it takes to make them. The first cone took 13 hrs to make. The new cone can be done in 9 hrs.

 

[jmasterj]

The first batch was like the one in this kit and was changed do to the amount of time and cost it takes to make them. The first cone took 13 hrs to make. The new cone can be done in 9 hrs.

The new one is the black one being used by the @KenECoyote? Too bad, I like the one in this build thread better.

 

[Back_at_it]

I got my FB kit today and the nose cone is different. It doesn't matter so much to me since I like to epoxy all thread to the tip, but just figured I'd note the change.

View attachment 573397
(The glue was already in place when I got it.)

Out of curiosity, What does the new one weigh ?

 

[DragonRocketry]

The new one is the black one being used by the @KenECoyote? Too bad, I like the one in this build thread better.

I can put the other back in, but the price off the kit will go up.

 

[DragonRocketry]

Out of curiosity, What does the new one weigh ?

11.6 oz

 

[KenECoyote]

The new one is the black one being used by the @KenECoyote? Too bad, I like the one in this build thread better.

OOC Why would you prefer the old style?

The original looks cool, but for me it doesn't really make a difference since I apply my own build system to the nose cones.

 

[jmasterj]

OOC Why would you prefer the old style?

The original looks cool, but for me it doesn't really make a difference since I apply my own build system to the nose cones.

I prefer the non-permanent bulkhead mounting.

 

[KenECoyote]

I prefer the non-permanent bulkhead mounting.

Ah, ok. I was wondering if it was to add nose weight or altimeter, etc.

 

[jmasterj]

Ah, ok. I was wondering if it was to add nose weight or altimeter, etc.

I like variable nose weight setups and have made a couple similar to that orange nose cone.

 

[Back_at_it]

I managed to get the fins sealed last night using DAP brand wood filler. I’ve then sanded everything down to 600 grit.



Next I shot on two coats of SEM high build primer then filled the spirals with Bondo brand spot filler. Everything was allowed to cure then sanded down to 600 grit.

Here is the project just before I shot on the last coat of primer in white.

I’m going to let the primer dry for a few hours and hopefully get some paint on the main body this evening.

 

[Back_at_it]

The weekend was really nice here in Chicago and I managed to get a little time between work projects to do some work on the rocket. First I shot on two coats of Ace Brand Safety Red on the main body.



With the body drying I painted the nose cone. This was shot with 4 light coats of Krylon Metallic Black Stainless. This color is amazing and I'm looking forward to building a rocket where I can use this as the main color.



With the paint dry, it was time to load the rocket up with all of the recovery gear and check weights and CG. My only addition to this build going forward will be some vinyl so that won't change the weight or CG.

Complete with all recovery gear installed the finished rocket came it at 27.9oz. and the CG without motor was at the 9.8" mark with a stability of .758. Not bad but I thought there was room for improvement.

When I first got this kit, the length of the nose cone shoulder kind of surprised me. I felt it was a bit long but figured it was done for balance. The more I looked at it the more I felt like I could remove some weight and move the CG of the cone forward since the additional weight was at the back side of the cone. I broke out the Dremel Tool and cut the shoulder down to 1.25". This dropped the weight of the finished, painted cone from 12.3oz down to 9.9oz. This was a reduction of 2.4oz. but it did push the CG back to the 11" mark. I decided to add 1oz of nose weight to the very tip of the cone. This moved my CG back to " from the tip.



The finished rocket is currently sitting at 26.4oz. The stability empty in OR without the stability hack is .734. When I insert the largest motor I plan to fly which is the G80-7 the Stability falls to .546. Before getting into the stability hack I decided that I wanted to compare this to a proven rocket so I pulled out my old Estes Fat Boy (BT80) that was upgraded with a 24MM mount and has flown on that motor dozens of time.

I loaded up the Estes Fat Boy with all the recovery gear that I normally use and shoved a D12-5 in the tail and took some CG measurements then plugged those into OR. The Estes rocket has a stability of .438. The Dragon upscale is currently sitting at .546 so on paper the upscale is more stable than the original.

While I'm feeling pretty confident this this will fly just fine, the first flight will be away from people. More than likely up at Bong.

 

[crossfire]

Nice paint job. But good luck with the now short nose cone shoulder. It can now slid or twist out of the tube durning motor burn.