Modification Two-Stage Fat Boy Modification

Modification - Two-Stage Fat Boy

Contributed by Kevin Johnson

Manufacturer: Modification

Oy! Fat BoyBrief:
Kit bashed two Fat Boys into one two-stage rocket.

Modifications:
After building the sustainer per the instructions, I started the booster by building the motor mount/center rings. Using the completed mount as a guide, I marked the aft end of the second BT and cut it down just in front of the fin slots. I then glued the motor mount in place and attached the fins just like on the sustainer. I made a tube coupler using a 1 inch section of the remaining BT, slit and overlapped to fit inside the BT-80. This was glued to the top of the booster.

Construction:
Sustainer is a regular Fat Boy built to the plans, minus the engine hook. The booster is the fin can and motor mount from the second kit.

This is a fairly simple mod, just takes some care in making sure your cuts are straight, and the booster's fins are securely glued. You want to make sure that the sustainer's motor sticks out at least 1/2 inch if you use an engine block for good contact with the booster motor for staging. I wanted a cool name for the two stager without having to cut up the decals too much. OY! look at that rocket! seemed like the sentiment I was looking for. All I had to fabricate was the exclamation point, and that was pretty easy. I didn't have any light blue paint for the fins, but the fluorescent yellow makes a nice contrast to the stock fin decals.

Flight:
I probably should have thought a little more about the staging and used a 24mm mount for D12-0 grunt off the pad, but the boost looked ok (if a little off vertical) with a C6-0. I prepped by inserting the sustainer motor, securing it down with a couple of turns of masking tape. I then taped the booster motor to it with scotch tape. Then I slid the booster section over the motors and taped the overhanging motor to the booster's motor tube. I have started to use the biodegradable packing peanuts made of corn starch for recovery wadding (cause ran out of the normal stuff! it worked very well on my test flights). They don't burn, are light and fill BT's from 13mm on up and melt almost instantly when water hits them. I put about 7-8 in the tube, folded the chute on top of them and set the nose cone in place. The shock cord was the standard Estes paper mount and elastic that came with the kit. I cut the spill hole out of the supplied chute.. I didn't want too much drift. The rocket left the pad much like a standard Fat Boy, then arced over up wind slightly. Staging was clean and the sustainer kept on heading up and up wind. Ejection was good and the rocket drifted back down close to the pad.

Summary:
This is a pretty easy mod. I guess the con is that I had to use 2 kits to get 1 rocket, but what the heck.. it was fun. Might consider making the booster a 24 mm mount for more thrust on lift off.

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