Yet "Another" Another Fat Boy XL Build Thread (Fat Boy "XLite")

[KenECoyote]

Hey, hey, HEY!

Seems that Dragon Rocketry's Fat Boy XL is a big hit here!

More recently @Sooner Boomer
had a build thread on building one strong for "as big of a 38mm motor as I can reasonably fit".

Before that, I read @Back_at_it's build thread and I was fascinated with his attempts to lighten the rocket to launch off F and G motors. Additionally, I had been pondering ways to build a heavy 6" X-15 kit lighter and the light Apogee X-15 kit stronger.

So I thought I'd try doing both!

I'll build two bodies... one light -"Notso Fatso" (can I say that?) and one heavy - "Oh-beast Boy"; and I'll use the same nose cone on both!

I'm pretty far along on the lite build, but my posting will be slower and please bear with me as the heavier (or stock?) build has lower priority, so that may not be for quite a while.

 

[KenECoyote]

Back_at_it had great info on his thread where he weighed all the parts and focused on "slimming" it down.

It seemed that the fins were a good area to focus efforts on since reducing the weight in the back would not only make the rocket lighter there, but would also reduce the nose weight to be added! Double points!

However, I wanted to try a balance between the stock strong and heavy plywood and the light, but weaker balsa, so I ordered 1/16" plywood and 3/32" balsa with the plan to sandwich the plywood with the balsa (totals 1/4" matching the stock fin thickness).

 

[KenECoyote]

Today I sandwiched and cut the fins. Here's pics and notes...

The balsa sheets were a tiny bit shy on width, but a wider size was much more expensive, so I sacrificed a small corner (upper left corner of pic).

Same thickness! (Stock fin with laser charred edge on left.)

Side by side comparison.

Weight comparo (grams for precision)...

Stock:

Lightened (cut by more than half!):

I then CA'd the balsa to harden it and that added 2 grams:

I plan to cut a new rear CR like Back_at_it did, but using the same 1/16" plywood.

 

[KenECoyote]

I also tapered the leading edges. I figured this would allow the stronger center plywood to take any hits vs. a soft balsa edge getting dinged up where it's more noticeable.

 

[Back_at_it]

Nice looking build you have going. I'm very interested in the final weight of your lite weight build.

You are giving me ideas on some upcoming builds I have planned.

 

[KenECoyote]

Nice looking build you have going. I'm very interested in the final weight of your lite weight build.

You are giving me ideas on some upcoming builds I have planned.

Thanks! I currently have the rocket mostly done in primer, but need to see how much it weighs after filling and paint. Unfortunately I'd prefer a nice finish on this one, but I think it can take a few extra grams.

 

[Sooner Boomer]

I think there will be a lot of these flying at Airfest this year. Would be great if Scott could be on site!

 

[KenECoyote]

My next focus was on the beefy centering rings. The stock ones are same material as the fins - 1/4" thick high quality plywood.

Given the CR advice at the other thread:

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.

I decided to continue using the 1/16" plywood I had with a double layer for the top cr and single at the bottom.

So I went about tracing and cutting the CRs (I made extra jic). To make things simpler, I first traced them, then cut several matching squares out


A bit of wood glue on each corner and a glob in the middle, then stacked and pressed until dried.

This way 4 are stuck and cut all together and won't come apart until the glued scrap area is cut out. Worked like a charm!

BTW- Excuse the sketchy circle cutting guide... this is a first attempt at a bigger and better planned guide and it worked well enough)


So using 1/16" instead of 1/4" shaves off about 3/4 the weight on the rear CR and about 1/2 on the forward CR.

 

[KenECoyote]

I waffles a bit on the harness mounting point and which glues to use.

Epoxying a kevlar loop to the side (or sides) of the motor mount is plenty strong and light, but I also like the convenience of a bolt eye mount; however the one provided is quite beefy (as it should be).

In the end, I decided to use smaller hardware with a big washer to better distribute the force on the thinner fwd CR.



On the other thread, there was also a good discussion on epoxy vs. wood glue:

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

I ended up deciding to use epoxy on the fwd CR, wood glue for the fins and CA for the rear CR (heavier up front, lighter in the back). When you can't decide, just use all of them!



Remember to close your eye!

Notice that I left off the topside hardware and just sank the eyebolt into the wood covered in epoxy.

 

[Back_at_it]

Nice work. This one is coming along.

 

[DigBaddy]

Will be interesting to see your final weight. I have no doubt it'll still be quite sturdy.

 

[KenECoyote]

Nice work. This one is coming along.

Thanks and look what you started!

Actually I really appreciated your thread. This year I'm finding new thrills in learning new methods and techniques and your thread was at the perfect time since I was at a bit of an impasse on how to proceed with lightening two other kits.

 

[KenECoyote]

Nice looking build you have going. I'm very interested in the final weight of your lite weight build.

You are giving me ideas on some upcoming builds I have planned.

Will be interesting to see your final weight. I have no doubt it'll still be quite sturdy.

Here's an early peek!

Notes:

• Unfinished (obviously), so it still needs filler, sanding and paints.
• I've added a regular 36" chute (what I had on hand) and a large nomex blanket (4.2oz)
• No nose weight added except for my epoxied eyebolt and cross-rods (details and pics coming), which may be 1-2oz.
• Cg is currently 10" from the nose tip with the chute and blanket in the body, but I forgot that the kevlar cord was stuffed in the motor tube.

So when all is done, this may very well be close to what I end up with, but we'll see.

 

[KenECoyote]

Nose cone!

Here's where I really strayed from the kit design.

The original nose cone comes as two hefty pieces... a nose cone and a coupler with eyebolt for the base.

I figured that I can ditch the coupler and eyebolt hardware and apply my own anchoring in the nose tip. This way the added weight is max forward and less is needed. Additionally I use all-thread hardware, so I can add more weight as needed, which allows me to switch the same cone to a heavier build and just add weight as needed.

The parts:

• 1/4-20 hex head cap screws
• 1/4-20 coupler (with cap screw inserted at one end as an epoxy anchor)
• 1/4-20 eyebolt
• 4oz Lead disk weight included as example (drill hole in center and retain with additional washer)

I use a length of all-thread temporarily to hold the coupler in place while I add enough epoxy to get just under the halfway point of the coupler.


Then I used masking tape and a spare centering ring to center the assembly.
When that is set, I want to be extra sure that it doesn't come out, so I will run wire hangar rods through on top of that epoxy layer.

To figure out wire placement, I make an L bend on the end of a hangar wire, heat it up under a flame, insert it inside the cone and melt 4 holes through.



Then I run straight hangar wire through each set of holes and snip off. Tape over the outside holes so epoxy doesn't drip out and top it with a thin layer of epoxy (just enough to cover and secure).




When the epoxy has set, I add high strength thread lock and screw in the eyebolt with pliers.

 

[KenECoyote]

To have the nose cone attach to the body, I used a 4"×48" shipping tube I got from Staples on stupid sale for $2.66!


I cut it to the right length, removed a sliver (about 20mm), glued it back and then epoxied on a fiberglass strip to strengthen the joint.





Coupler was then CA'd into the nose.

 

[KenECoyote]

Fat Boy Bottom...

I epoxied in the forward CR & motor tube assembly, using the rear cr and fins in place to center the motor tube. Tape was used so I could take the rear CR back off when the epoxy had set on the forward CR.


I then applied an epoxy fillet from the top.


Next, I used good old wood glue to glue in the fins, followed by internal fillets using the same glue.

I used an Estes tube marker's fin alignment tool since I felt it was plenty good enough here.


Then rear CR goes in!

I originally thought of using CA here, but decided to just continue with the wood glue. I dunno... something just feels right about using wood glue to seal this baby up.

I followed with a small glue fillet and then I installed the motor retainer using CA.

 

[KenECoyote]

The other day I primed it using Rusto Filler Primer.

(For a "light" build, it actually feels pretty sturdy!)

Today I started sanding the nose cone a bit, but it needs filler along with the body.

So that's where this stands now. I'll update again once I have a chance to fill, sand, repeat and paint.

Thanks for following along!

 

[KenECoyote]

Quick note... yesterday I had left the little guy on the floor by my desk after weighing and forgot about it. Later when walking by, I accidentally kicked it walking over to my desk!

Luckily no damage at all, so it has passed the "punt test"!

 

[KenECoyote]

Did some filling and sanding followed by another round of priming today...


Normally for a lightweight build, there shouldn't be as much paint and priming, but I want to give it a good paint job since it'll be on the nearby pads. I think I can spare a few grams here!

Unfortunately it looks like I'll need to do another round of filling, sanding and priming.

 

[KenECoyote]

Ok, I just made a few swaps since I have to keep focus on the lite aspect.

I swapped out the 36" chute for a 52" lightweight from a G-Force and I exchanged the nomex blanket for a bunch of wadding (or dog barf). Should be just under 20oz painted.


Currently 1lb 3oz (19oz) before final paint.

My rocket buddy is making a 3" rocket at the same time with tubes I gave him and he told me it's 20oz rtf, so we may end up having a drag race using F52 motors! Stay tuned!

 

[KenECoyote]

Update: Due to a crummy week for painting and tacky paint, the rocket wasn't quite finished with paint yet, but I still got to launch it today (in a black base coat) and drag race my buddy!

At the last minute I decided to switch to an F44-4 (24mm) since the F52 seemed a bit much for a first flight of an untested rocket which I had heavily modded. Plus I thought lower was better initially.

Here's the video where my "Fat Boy" beats my buddy's rocket he named "BULLET"!!!


View attachment 20230506_124318_1.mp4

 

[KenECoyote]

FYI - I've always been confused over how rocket drag race winners are determined, so beforehand I told my buddy:

• 1 point for first off the pads (advantage his?)
• 1 point for first one down (advantage mine?)
• 1 point for closest to the pads (even advantage?)

I got all three points and even the veteran announcer was surprised by my Fat Boy's performance!

 

[KenECoyote]

Update:
I had been waffling a LOT on the paint scheme for this. At one point I was going to launch it again in black, but with a red vinyl lightning bolt I had cut the night before, but I didn't get to launch it that Sunday (too many things came up including having to dig out a 4" lawndart )

With the red lighting bolt I was going to call it "Insane Bolt" (play on Usain Bolt ) since it had won a drag race and surprised some (including myself) with how fast it took off.

Well, given I had more time, I was going to go with the Estes original FB scheme since I think it's really cool; however, with metallic red paint for a bit of style.

While looking for metallic red paint, I found a can of Red Duplicolor Metalcast for an anodized look over chrome and I wanted to try it out.

Initial tests on a piece of body tube with a Rusto silver was pretty good, but a bit blotchy.

I then tried some Spaz Stix Mirror Chrome, which is a beautiful chrome paint, but it doesn't take any clear coat sprays without turning dull (that I've seen). It came out much nicer and more even. From a distance or across the room, it looks like real anodized metal!

 

[Back_at_it]

Metalcast colors really pop over silver or gold. I did blue metal cast over a gold flake base on one of my kits and the effect was awesome. I've been meaning to rebuild that kit as it crashed about three years ago. I've been putting that rebuild off for way too long

 

[KenECoyote]

So the dilemma here is that the mirror chrome is so nice that it needs a nice base of black and any imperfections will show up.

I did a test on the nose cone and one side looked great, but the other side had sanding marks as well as a prominent drip.

 

[KenECoyote]

To further complicate things, the black base coat I had on the body was not ideal. There was some orange peel and a bit of unevenness.

So I tried to spray on another coat of gloss black... disaster! The paint started splattering! So I grabbed another used can of gloss black and sprayed... DISASTER! The paint started to wrinkle badly!

I then thought of something... maybe the wrinkling is caused by incompatible paint even though it was the same brand (could be years apart in manufacturing).

I then VERY quickly grabbed a new can of gloss black, sprayed it over the wrinkled areas and they smoothed down!

Now this likely worked because the wrinkled paint was still wet and I had diluted the stuff that was reacting badly, but what a save!

Still, the rest of the black was not as smooth as I would like, but I figured I'd leave well enough alone now.

 

[KenECoyote]

I was planning on perhaps wet sanding the paint to smooth it out, but I didn't have the proper grit wet sanding paper and I decided it was good enough!

Today I sprayed on the mirror chrome and then a few coats of Metalcast and it looks really great from a few feet away!


I still have to decide on decals ("FAB TOY"? "FAsT BOY"?), but for now it's in pretty good shape.