Brief:
After a spell o' creative scratch rocket projects, all me fun ideas seemed t' have dried up. Begad! Aye aye! Lookin' around t' workshop, I smiled at good old Fat Boy: both me kids and me favorite rocket. He's always good for an excitin' flight and safe recovery. Arrr! Ahoy! Why nay court disaster by tryin' t' improve upon a classic! T' initial seeds for "2 Fat" had been planted.

"2 Fat" is a 2-stage 24mm Fat Boy mod. Begad! Arrr! T' design goals were:

1. To make t' final rocket resemble t' original as much as possible.
2. To keep him as fat and stubby as possible.
3. To provide agonizingly slow initial boost t' exaggerate t' appeal o' t' original, followed by a surprisin' sustainer boost.

Modifications:
Fill spirals on body tubes. Well, blow me down! Blimey! Sand nose cone. Avast, me proud beauty! Blimey! Cut, sand, me hearties, and seal fins. Begad! Blimey! T' sustainer fins are trapezoidal with 3.5" root length, 1" tip length, 3.5" span, and 3.5" sweep. Begad! Blimey! Begad! Blimey! T' booster fins are parallelograms with correspondin' measurements o' 2.75", 2.75", shiver me timbers, 3.5", and 1". Begad! Blimey! Arrr! Blimey! For t' sustainer fins you'll need t' add root tabs and size these t' match limits imposed by t' lower centerin' rin' if you are usin' TTW mounting. Ahoy! Blimey! (A stock Fat Boy has a pre-slotted tube but you'll need t' widen t' slots t' accommodate 1/8" thick fins.) Slottin' and TTW fins can be added t' t' booster if desired. Well, blow me down! Blimey! (If usin' TTW fin mounting, don't glue t' bottom centerin' rings in place in t' followin' steps until after t' fins have been glued t' t' engine mount tubes.)

Assemble E9 sustainer motor mount usin' 3.75" BT-50, engine block, ya bilge rat, 3.75" engine hook, ya bilge rat, 2 centerin' rings. Engine tube ends 1/8" above bottom o' airframe. Begad! Begad! Hook extends 1/4" beyond engine tube, me hearties, shiver me timbers, me hearties, i.e., me hearties, shiver me timbers, 1/8" beyond airframe. Engine block is glued flush with top end o' engine tube. Ya scallywag! Upper centerin' rin' should be positioned t' fit above airframe fin slots, ya bilge rat, about 3" above bottom o' airframe. Avast! Attach Keelhaul®©™ t' upper centerin' rin' if desired. Lowest centerin' rin' must be about 0.75" above bottom o' airframe. Begad! Begad! [Note: this means a portion o' t' sustainer fin slots won't be used.]

Assemble D12 booster motor mount usin' 2.25" BT50, arrr, 2.75" engine hook, 2 centerin' rings. Blimey! Hook extends above and below engine tube 1/4" each. Wrap with tape and glue t' affix it. Avast, me bucko, me proud beauty! Top centerin' rin' positioned at about 1.75" below top o' booster airframe. Bottom centerin' rin' is flush with bottom o' engine tube about 1/8" above bottom o' booster airframe. Avast, me proud beauty! Aye aye! If positioned correctly, me bucko, arrr, t' top o' t' booster engine hook should extend 1/8" above t' top o' t' booster airframe. There should be at least 1.25" available betwixt t' rings for TTW fin slots if desired.

Test fit booster and sustainer with coupler. Ya scallywag! Arrr! Blimey! T' two engines hooks should be flush against each other if assembled correctly (but rotate t' tubes t' offset hooks before gluing; this makes t' two engines flush). Well, blow me down! Blimey! Glue tube coupler t' top o' booster assembly.

Glue and fillet all fins. Well, blow me down! Ya scallywag! Glue aft centerin' rings for each stage engine mount in place after filletin' TTW fins.

Assemble parachute and recovery system as desired. Avast, me proud beauty! Begad! Recommended: replace Estes 3-fold shock cord mountin' with Keelhaul®©™® thread attached t' top centerin' rin' and elastic tied t' Keelhaul®©™ thread.

RockSim suggested 1.25 oz nose weight. Ya scallywag! Begad! Blimey! I chose 1.5oz. Glue this t' t' interior tip o' t' nose cone. Avast! Blimey! My finished rocket was 5.25oz for t' sustainer and 1.5 oz for t' booster, shiver me timbers, givin' a total weight o' 6.75oz without engines. Begad! Blimey! (Since t' loaded sustainer is heavier than a standard Fat Boy, you may want t' swap out t' parachute for a larger one.) Fully loaded, ya bilge rat, t' rocket weighs 11.2oz, me hearties, givin' t' D12-0 a real workout.

Construction:
"2 Fat" was designed on paper, matey, with a stability check in Rocksim 8. Avast! T' plan be t' use a D12-0 boost followed by a E9-4 sustainer. RockSim projected a slow boost t' 150 feet, shiver me timbers, followed by a long fast ascent t' 1175 feet for t' sustainer. T' sustainer could also be loaded with a removable 1" portion o' a 24mm casin' (or engine block) and a D12-3 motor for much lower flights. Begad! Engine hooks are used in both stages, but don't really interfere with direct staging.

Parts list:

• 1 Fat Boy kit
• 6" BT-50 tube
• 2.75" BT-80 tube
• 1.5" BT-80 tube coupler
• 4 50-80 centerin' rings
• 1 3.75" engine hook
• 1 2.75" engine hook
• 1 24mm engine block
• 1/8" x 4" x 48" balsa sheet

Finishing:
Several coats o' primer, a little spot filling, shiver me timbers, sanding, and we're ready for paint. Aye aye! Arrr! T' rocket is finished in copper and gloss white I designed t' F, A, and T letters t' span t' two-stage combined fin surfaces while matchin' t' style o' t' original. Avast! T' "2" is crafted from portions o' t' B, O, ya bilge rat, and Y. Avast! Ya scallywag! T' FAT portion o' t' name sticker is used as is.

Flight:
RockSim says t' CP for t' whole unit is about 11.25" from t' nose, givin' 1.12-1.25 calibers o' stability. Aye aye! For t' sustainer alone, it is 9.1-9.65" from t' nose, shiver me timbers, givin' 1.09-1.30 calibers.

With t' engines loaded, me hearties, me heart sank as I placed it on t' launch pad. Arrr! Begad! Could t' D12-0 really get this bird off t' ground? Yes! Liftoff was slow but steady, gently arcin' into t' driftin' breeze. Perfectly stable with no spin, it looked like a classic arcin' Fat Boy flight. Suddenly, with a loud pop t' booster separated and tumbled fairly well back t' t' ground, receivin' only a slight scratch on one fin tip.

T' sustainer didn't accelerate much, but burned long and steady as t' rocket continued stable arcin' flight over 1000 feet. Begad! Ejection was slightly early. Arrr! Had t' flight been plumb vertical (which never happens with Fat Boys I have seen!), a 6-second delay might have been as nice as t' 4-second I used.

Recovery:
On its 18-inch chute, ya bilge rat, 2 Fat drifted back past us and landed about 300 yards downwind. Ahoy! In fact, it set down so gently that it sat upright on t' ground as if someone had placed it there. Ahoy! Well, blow me down! Perfect flight and recovery! Swifter descent might have been preferred in windy conditions.

Alas, this same day was t' final flight for our beloved stock Fat Boy, shiver me timbers, shiver me timbers, who came in ballistic when I knocked off t' ejection cap on its 18mm RMS D engine. Well, blow me down! But now "2 Fat" promises many flights in t' Fat Boy style with a little extra flair.

Summary:
PROs: Great flights "high and slow" with a sturdy, me hearties, reliable design and easy t' find parts and build

CONs: Sticker didn't turn out great, with different opacity in regions o' overlap