New Techniques - Fat Boy Kit Mods

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Jan 17, 2009
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I have recently been working on learning new finishing and building technigues, some of which I have discovered from being a member of this forum, by incorporating them into some of my recent projects. I would appreciate any comments or suggestions on my latest projected - an upgraded classic Estes Fat Boy kit.

Here's a list of the modifications (made and in progress):

  1. Card stock reinforced fins.
  2. 24mm D/E Engine Mount.
  3. Card stock reinforced Centering Rings.
  4. Kevlar / Elastic Shock Cord.
  5. Enhanced Shock Cord Mount.
  6. 18" Mylar Chute w/ 6" Spill Hole and Kevlar Shroud Lines.
  7. Ejection Baffle System.
  8. Protected Parachute Compartment.
  9. Additional Nose Weight.

    I have attached a picture of the project with modifications for your review. Let me know what you think.

    (NOTE: Updated to correct discovered baffle weight problem.)

Here's the actual Rocksim file:

(NOTE: Updated to correct discovered baffle weight problem.)
Here is a 3D shot of the modified nose cone w/ protected parachute compartment sleeve. (i.e. cover)
*Whistle* 11oz!! Now thats a Fat Boy...can I suggest a 29mm Mount??;)

Looks good to me, very interesting deployment scheme, wadless? I'd sugges AT SU's and Reloads only for this one...D15T at the bare minimum, an E30 would be great, as well as an F21!

Yeah, it's definately on the hefty side, but I needed to add a good bit of nose weight to compensate for the Estes SU E9-4 engine. The original kit (less modifications) weighs approximately 4.5oz. The mods more than doubled the weight, but it still sims to 490+ feet consistently in Rocksim.

Yes, it is a wadding less baffle design which has the hot gases making two 90 degree angles with the chute and shroud lines and elastic portion of shock mount protected in the parachute compartment.

Thanks for taking a look and sharing your thoughts!

29mm hmm! :D

Last year I built a FatBoy to fly on 24mm motors. I made some strenghtening mods to take the stress since I plan on sticking an F21 in it soon. It weighed in at 8.0 ounces w/o engine or chute (this includes nose weight I added.). It flies great on an E9-4.

Here's a thread I did on the build, Click here.

Someone named it the Morbidly Obese Boy, hence the nickname M.O.B.:D

I've attached a t/o pic to wet your whistle. This is on an E-9.

You're going to have a blast!!!:D

I found another t/o pic. One of my best... Again, an E9. Check out the flame! Way cool...:D


They are very cool Fat Boy launch pics! The flame on the second one is awesome. I can't wait to get mine completed and out to the field. Wow, if your's was coined the M.O.B. at 8oz. I think I need to work on reducing the weight of my project.


I really like your design regarding dealing with the ejection charge. How do you deal with residue build up in the nose cone? I wasn't sure about the sleeve tube placed inside the nose cone, but after seeing your pics, I'm going to give it a try. This mod came about after reading numerous reviews on EMMR regarding burnt/melted chutes and shroud lines. Lot's of good info and worth reviewing.

You have certainly come up with a winner! If you have a big enough field to fly that thing on E's and F's, then it might last forever with all your clever mods.
Does the weight (11 and 1/4 ounces) include the motor?
Keep in mind that there are a few little tricks you can apply to help out your situation, if you so choose:
You can add a little fin area aft and also add a little durability at the same time by adding a strip of 1/8 x 1/4 spruce to the trailing edges of the fins. (You can find spruce at any good hobby shop, usually in the section with balsa) It is a LOT tougher than balsa. If you keep the added piece small and blend it in to the basic fin with a good sanding job, it pretty much becomes invisible.
Using big motors in this design can cause a big shift (aft) in the c.g. You can help this out by positioning the motor mount a bit farther forward. As big as the FB body tube is, you can safely position the motor nozzle as far as 3/4 to 1 inch inside the rear end and still avoid the dreaded Krushnik effect.
Last, with fat-n-squatty rockets (that is, "low fine-ness ratio" shapes) some weird stuff begins to happen with stability, and you don't need the full one-and-a-half-diameters of distance between the c.g. and c.p., as is used for a rule of thumb on more conventional designs. I don't remember the exact number, but for fat rockets, the stability margin is something like half of the usual. This will allow you to reduce your ballast weight, but do some more checking on this before you take my word for it!
Really like your work, and thanks for posting the RocSim file.

Last month I picked up about 10 Fat Boys at Meier's, the "Launchables" version, for 80 cents each (all their "last-year's-models" were $0.80, no clearance sign or anything, I didn't ask any questions, go figure).

I'm planning a 24mm version, 29mm version, 2 stage version, and possibly a 3 stage version.

I just read a great MODS article on EMRR about a glassed 29mm version with a beautiful paintjob at:

That article included a reference to an article about short rocket stability by Robert Galej titled "What Barrowman Left Out", which explains why Fat Boys don't need a full 1 caliber stability margin. Most of the math in the article was way over my head but the general concept, why short rockets have very little angle of attack stability variation, made lots of sense.

You can download a PDF version of that article at:

Thanks again for the great post.

Even without all the modification that you gentlemen feel you need on the Estes Fat Boy...

Mine still wound up caught on a power line!

Glad I didn't do a lot to it...lost a perfectly good $8.88 rocket!

I'll just go to Wal-Mart and get another one!:D

I really like the slow takeoff on an 18mm C. If I wanted a neck-snapper, I would just buy a Quark and be done with it. That's why I'll never over-motor my Fat Boy.
The Fat Boy is a very sturdy kit, stock, due to the fin can design. My first one, was a 24mm version. Stock fins, stock rings cut out for a 24mm mount, Kevlar shock cord anchor, elastic cord, and 2.5 oz of BB's in the nose. She weighs 5 ounces empty. I've logged 2 D12 flights, one E9 flight, one E11 flight, and four F21 flights. Each flight was recovered with a 12 inch nylon chute. The ONLY repair I've had to make to it was to re-glue the nose weight in.

But, I'm currently building one to handle up to a 29mm H, hehehe :)
I think I've found a problem with the Rocksim design. I couldn't figure out why these simple mods would add so much weight. I reviewed the parts list and found that the 1.5in. baffle tube (equivalent of a BT-80 tube coupler) showed a weight of over 5 ounces!! I'm working on correcting it now. I'll repost as soon as it's completed.

Also, has anyone tried a PNC-80K (?) on the Fat Boy? I had an extra one sitting on the bench from an unfinished Maxi Alpha 3 and it looks great on the Fat Boy! Just curious if anyone else has changed to this cone.

I just updated my earlier post with updated Rocksim and graphic images of my modified Fat Boy project. Apparently, Rocksim defaults to a 0 inside diameter for a custom tube coupler, that is one that is not picked from the database. This in affect made the tube a solid paper bulkhead! Once I selected the Estes JT-80C and put in the correct length, weight problem was resolved. I was also able to dramatically reduce necessary nose weight!

I reworked a second version of the Fat Boy mod project with the PNC-80K nosecone. Here's a photo of my Fat Boy during a dry fitting of the fins with the PNC-80K. I think I may just have to build it this way! :D
Here's the Rocksim file (with PNC-80K):

(Wish you could post multiple files to one message!)
Hi rkt2K1 !

Don`t worry about the stability of the FatBoy!
I made enough, when I was building and prepping mine for the F20W ! ;) :)
At the beginning of building I hoped for 1,5 Cal. stability, but practically (ok, I made slightly overkill 4mm Centering Rings... etc.) I hoped for 1 Cal. ... :rolleyes:
It had actual only around 0,7Cal. stability, but flew very straight and it was a little windy, though. Shouldn`t make problems...

I wouldn`t change the Nose Cone... but - Ok, it looks very cool with the PNC-80K- Cone... :cool:



I just got two Fat Boys at my local Meijer's...marked down on clearance to...$.80 each!!

$.85 with tax!

That's better than Hobby Lobby!

sandman, I would say it is definitely time for a road trip.
Time to go harvest as many of these marked-down Estes kits as possible, list them on ROL or ebay, and make a couple extra bucks to pay for other hobby needs.
Wow! $0.85 for a Fat Boy! How do all keep finding all these great deals?? I wander the local K-Mart's, Walmart's, Michael's, A.C. Moore, Toy's R Us, etc. and never find any deals like these! I hardly find any rockets at all anymore! :(

Hey, does anyone else think that the Fat Boy with the PNC-80K cone looks like a fat Der Red Max?? What do you think? A Fat Max!

Anyone have any suggestions on making the holes in the baffle plate without a drill press?? I have both a Dremel and regular drills. Or should I just try to cut them by hand?

I punch holes in cardboard with a hand-held hole punch. (The pliers-style doohickey that you use to punch single holes in notebook paper.) This gives me a quick and clean hole about 1/4 inch in diam. I can also get it inside the front inch or so of a motor mount tube to punch holes there.

After punching holes in the cardboard, I soak the remaining material in CA or thin epoxy for strength and fire-proofing.