Brief:
Tea-Bird 4.0 is the 4" upscale version of the tubefin "Tea-Bird" model (made from Crystal-Lite(tm) ice tea containers) I presented earlier on EMRR. I originally designed it as "Tea-Bird 38", but after a few test flights, I decided it could handle much more power, so I extended it by 2", and installed a 54mm motortube to allow up to a CTI J210. Not your basic tubefin, it features epoxy composite tube fins for 500+ mph boosts and a hollowed nose cone to carry the chute. Design follows computer predictions for maximum tubefin performance - why its so stubby. It's still fast, cheap and easy to scratch-build. I also built a 5.5" version for K445 power.

Construction:
Built of 4" LOC tube stock, 13.5" body tube and six 3.5" tube fins. Motor tube is 10.5" of 54mm LOC tube. Two 1/4" 4"x54mm ply centering rings with 1/8" eye-bolt and EZ-link holding the 8' para cord recovery attachment to a 36" Top-Flite chute. Nose cone is a 4" LOC with all but 1 1/4" of the base sawed off (nose cone is only 1" shorter than the body tube!). 2 1/2" of 3/8" brass tube used for launch log. 5-minute epoxy was used throughout. Ratio of length above tube fins divided by diameter gives 5.56, which predicts minimum drag coefficient (Cd) of 0.83. Observed best Cd = 0.88, on CTI I285-8.

The two unique features of this tubefin are the composite tube fins (needed to prevent flutter) and the hollow nose cone to accommodate the recovery system in an "extreme stubby" airframe designed to give maximum performance. The result is a very fun, low-cost HPR rocket that goes together quickly and is easy to transport. Parts are cut out with a 32 teeth/inch fine hacksaw. Tube fins are glued on two at a time on a flat surface to assure perfect alighment - first 2 opposites, then the 2 top, then flipover and glue on last two. Neighboring tubes are securely glued to each other as well as to the body tube. "Securely" means plenty of epoxy, but without slopping it into the triangular spaces between tube fins. Attachment is 1/4" from the bottom of the budy tube. I use soup cans to hold the parts in place for gluing. Next, the laminated doublers are attached to the tube fins. Half doublers are cut by bisecting the 3 extra tube fins you need to cut out. Also needed are six 4" strips of 3" fiberglass tape. Mix a quarter-sized gob of epoxy right on the outside surface of one of the doublers, smear uniformly, and apply doubler to the inside of the outer surface of a tube fin. Now, pull it off and apply a piece of glass tape to the glue surface of the doubler, and reapply aasebly to the glue surface of the tube fin, creating a cardstock-epoxy-fiberglass-epoxy-cardstock sandwich. Clamp in place with heavy paper clips or the like. Repeat with the other tube fins. When this sets, it creates a bullet-proof, light-weight composite tubefincan that resists 500+ mph speeds and landing dings. For good measure, I sealed the leading edges of the tube fins with a strip of ordinary masking tape to prevent delamination at high speed (you can just paint over this)..

The 54mm motor tube was glued into the centering rings in the usual way, leaving 1 1/4" protruding on each end. A 1/8" steel eye bolt was attached to the upper centering ring to hold the recovery system (1/8" EZ-link with 8' of heavy para cord - attach now unless you have little hands). Motor tube assemble was epoxied into the body tube in the usual way, with plenty of epoxy; while the epoxy was still wet, I slid down on top of the upper centering ring a 1" section of 4" tube stock to better lock it in place.

The stock LOC 4"cone is modified by sawing off all but 1 1/4" of the base "hip". An anchor for the recovery system is created by securely epoxying in place a piece of 1/2" hardwood dowel rod inside the cone just above the base "hip". "Securely" here means with lots of epoxy, with bits of wood scrap epoxied to surround the attachment points, and then more epoxy on top of that. See photo. Honestly, I feel this is a superior way to use big LOC cones, even if you don't need the 4" of extra space for the recovery system that doing this creates.

The brass tube launch lug was epoxied in place with its center 1 1/4" above the tubefincan. CG with motor should not be aft of this point. Finish was with lime and orange Tamiya rattle-can. The 36" LOC chute was attached to the shock cord so that the top of the canopy just touched the dowel rod attachment in the cone - this is to insure that the momentum of the cone being ejected will pull the chute out of the cone base. No need to worry about the chute being stuffed past the dowel rod and up into the hollow cone by the ejection charge - it just doesn't happen.

Flight:
Tea-Bird 4.0 was tested in this version and also with a lighter weight version and a shorter version (by 2" - made no difference I could measure). Motors tested ranged from AT F50-4 to CTI I285-8 (I never got around to flying it on 54mm motors, I used 38-54mm adaptors, sometimes with a 29mm adaptor inside). The heavy version gave 567' on a new model AT G80-7 and 967' on an AT H180-6 (using Perfectflite Alt15k in an external pod), so Tea-Bird 4.0 is an easy L1 cert ship. CTI H143SS-6 gave 1218' and I212SS-7 gave 1626'; I285-8 (stuck out aft 3") gave 2037' at 505 mph with Cd calculated from the data of 0.88, not too bad for a tubefin. Performance on a 54mm CTI J210 is calculated at 3400' at only 430 mph and J280SS-8 at 2485' and 495mph, so within test limits. I'll get around to flying these motors eventually. Cd vs. speed plot summarizes allmotors tested.

Summary:
PRO - very easy and inexpensive to scratch build, especially for an H-I ship. Durable and doesn't go too high, easy L1 cert rocket. Very compact for transport, just throw in the trunk, fits behind the seat of my Miata. Unique looks as an "extreme stubby".
CON - Unique looks as an "extreme stubby". Speed should not exceed 500 mph or so on your SIM. Estimate Cd from graph I attach, SIM program estimate won't suffice, because Cd depends on speed for all tunbefins.

Other:
Think about the technique I use here as a better way to use plastic nose cones. I am sold on it.