Original Design / Scratch Built
Contributed by David Sindel
Assemble the top end--nose cone, shock cord, and streamer--as normal or put in whatever recovery system you want.
Glue the engine block 2.5" from one end of the BT-20. Glue one of the T-20+ sections over the BT-20 so it is 1" from the end nearer to the engine block and 0.5" from the other end. It is advisable to insert a motor into the BT-20 to prevent it from being crushed. Slide the 1" free end of the BT-20 into the non-nose cone end of the body tube until the 2.5" piece of T-20+ butts up against the body tube.
From the balsa, cut three medium-sized fins. I made mine trapezoidal with 2-inch-long roots, 1" tips, 2.5" spans, and flat bottoms. Space them evenly towards the bottom of the blue (2.5" long) tube. On the trailing edge of one fin, add a 0.5" x 2.5" spin tab canted at 45 degrees. Add a launch lug in the root joint of one fin and you're done. I recommend filleting all three fins and the spin tab.
This design can be easily up/downscaled or shortened or lengthened. I used a Totally Tubular kit simply because it was cheap. Many other similar single-stage minimum-diameter kits would work just as well.
Extra parts, not used, will be five 2.5" pieces of T20+ (fits over BT-20) and one 2.75" piece of BT-20.
I used three sheets of wadding and masking tape to retain the motor.
Because of the spin, it flies fast and straight and barely weathercocks at all. You can launch it on a smaller field than you could normally. It also stays fairly low--about 10% lower than an Alpha III--because of the drag from spinning.
If you use a larger motor, particularly the C6-5 (or a composite D if you have a very large field), then it spins very fats and makes a nice spiral smoke trail and whirring noise. It's a very cool effect and not what you expect from such a small rocket.
The first flight was on a C6-5. It roared off the pad to about 900 feet and made the spiral smoke trails and noise. Ejection was out of sight and it came back about 400 feet away because of the wind.
Second flight was on a B6-4. Not quite as spectacular, but it landed very close. Third flight was on an A8-3 to about 300 feet. It was not very interesting--the spin had almost no audio/visual effect at those low speeds.
PROs: Cheap and easy. Neat audio and visual effects. Doesn't weathercock or go too high.
CONs: Fairly fast descent on stock streamer.
What You Can Do