Modification - Fat Boy (2-Stage)

Brief:
I added a booster with gap staging for my stock Fat Boy.

Modifications:
The Fat Boy was built stock.

For the Booster, the engine mount was assembled in the usual manner. The aft centering ring is flush with the tube end. 1/4" vent holes were punched in both centering rings. The coupler extends into the sustainer approximately 1/2" with a pretty snug fit. The fin pattern was upscaled a bit (the root of the fins measures at 3 3/4"). The fins are surface mounted and cut from 1/8" balsa. A separate launch lug was added to the booster to give a little extra guidance.

Construction:
The parts list:

• One Fat Boy kit
• 5" of BT-80
• 1" BT 80 coupler
• 2.75" BT-20
• 2 80-20 centering rings
• 1/8" launch lug
• 18mm engine block
• 2.75" engine hook
• 1/8" balsa stock

There are absolutely no "gotchas" in building the Fat Boy itself. It is a well known kit after all. The booster was no more difficult to build than the original kit. The Booster is pretty stoutly built. I considered using TTW fins, but given the tumble recovery of a booster, I think it is much better to pop a fin on landing than break it.

Finishing:
Finishing and painting went pretty smoothly. I decided against using the supplied decals as I have this thing about stickers. Instead I went with a simple yet dramatic paint scheme.

Flight and Recovery:
First flight was the single stage Fat Boy on a C6-3. Nice straight flight and clean recovery on a 14" nylon chute.

Second flight used the booster with a C6-0 to B6-6 combination. Boost was slow and straight, staging occurred fairly low and the sustainer took of like a shot. It weathercocked a bit but had a very nice high flight. Chute ejection was a bit past apogee but deployed cleanly. The booster popped a fin loose on landing, which was easily repaired. The sustainer made a clean landing.

Third flight used a C6-0 to B6-4 combination which gave an almost identical flight to #2. Ejection occured at apogee and both booster and sustainer returned without damage.

Summary:
The Fat Boy by itself is a fun little rocket, which is suitable to all kinds of modification. I was worried that the addition of a booster would make it to heavy to fly on an 18mm engine, but it works very well. I certainly wouldn't use any booster but the C6-0 though. For a small field staging demo, I think it's just perfect.

PROs: It's a two stage bird that you don't have to chase all over the country side. The gap staging has worked flawlessly so far. Easy to build and modify.

CONs: I would like to have seen 1/8" balsa fins used on the stock Fat Boy. Sticker decals. Blech!

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