Brief:
I built FISH (Fiesta Island Special Hotrod) as an experiment with 4 objectives in mind:

1. Creation of a truly saltwater-proof rocket; reflecting the island launch site the San Diego DART club uses
2. Exploring the potential of PVC sprinkler conduit as low-cost rocket-building material
3. Creating a very low-cost, "minimal" rocket suitable for E-F-G-H sport flying, possibly up to L1 certification and with performance about that of the LOC Legacy (see photo).
4. Trying to find something that the tube-fin design is especially suitable for

I think I have succeeded at of these with FISH. On its maiden flight with a AT F50-6, I misjudged the wind and managed to put FISH in the drink after a flawless 1200' boost. It floated beautifully like a giant upright fishing bobber until it was rescued by a nearby ocean kayaker. No damage and ready to fly again after just wiping off. It flew. It didn't melt. It cost only $2.65 to build the airframe, less store-bought chute and cone--no fins, no rings, no motor mount, no glassing, just 7 quick saw cuts and some epoxy. Done in about an hour from scratch, including painting.

Construction:
FISH is constructed from Schedule 40 ASTM D2241 PVC sprinkler conduit, 200 psi-rated (I assume all those numbers mean something) available in any home improvement store. Neat stuff. Strong, cheap, but a little heavy. Internal diameter is 30.5 mm, perfect for 29mm motors plus a bit of friction-fit wrap. It works best for long, thin designs that can serve as its own nose ballast. Tube fin designs make the best use of this stuff, I think. I designed a 4FNC version of FISH with light ply fins, but it is a lot less versatile since it would need wide-open spaces for G-power and an H-motor would produce Mach-speed that would rip those unglassed fins right off. I have flown FISH on E30, F50, G35, and G80 motors giving it 500' to 1800' boosts according to the MicroAlt, making it suitable for smaller fields. Sims say an H128 should give only about a 2200' climb, so there is easy L1 potential there.

Nothing more complicated than a smaller Estes kit here. The build process is as follows: Buy one 10ft piece of 30.5 mm ID thin-wall sprinkler conduit at Home Depot (enough to build 2 FISH). Use a fine-blade hacksaw, cut off 2 24" pieces and 6 x 2" pieces with a 22.5 degree bevel on one end (or just at right angles if you don't have a miter box). Attach the 6 tube fins to the bottom of a 24" main tube with 5 minute epoxy . Attach the recovery system, 24" or 28" nylon chute on 8' of nylon USMC boot lace, through a drilled hole 3" above and 3" below the payload joint and then covering the knot with a plastic fairing made from the end of a cigar tube (see photo). Fashion a tube coupler with 3 epoxied layers of 5" pieces of nested BT55 Estes tubing having 1/8" sections removed to reduce diameter and inserted halfway into the bottom of the payload section and epoxy in place. Plug this end of payload compartment with wads of epoxy-soaked paper towel to make a gas-tight seal, or if you want, use a commercially available 29mm payload section and bulkhead adapter for this part. You can use the eye screw they provide for the upper recovery attachment. There is no motor mount--motors are friction fit with tape in the usual way, however, you MUST install an infrared thermal barrier to protect the low-melting PVC from radiant motor heat! This is easily done by slitting (lengthwise) and inserting flush with the walls a 14" length of Estes BT55 cardboard tubing with epoxy smeared on the outside only into the aft end of the body tube. Add a 5" length of 3/8" launch lug tube centered 15" above the tube-fin assembly and in line with the center of one of the tube fins. Fit a proper nose cone as there are few choices on this size. The Quest Mirage and Quest Lightning kits have nose cones that fits with a little tape and the superb but pricey PML solid polyurethane 29mm cone. I recommend the latter though. (Hey, why not? The rest of the rocket only cost you $2.65!!).

Finishing:
Paint rocket to taste. No need to prime the PVC but be sure to wipe clean with alcohol to remove manufacturing residue. I used Testors' Copper, which makes FISH look like its made from copper pipe.

Flight:
Motor is retained by friction fit with masking tape. In addition, leave 3/4" of motor protruding, wrap with enough tape for motor stop purposes, and then further secure the motor as follows: run a 6" piece of tape from the upper inside surface of each tube fin in turn and wrap twice around the exposed motor end. This method of motor retention for tube fin rockets is bulletproof; I have never had a motor spit on ejection with this approach.

No ballast needed for flying E through G motors. CG should be 12" above the tube-fin array for 2 caliber stability. Don't forget to tape or otherwise tightly affix the nose cone to the payload compartment or you will lose it (along with any contents in the payload compartment) when the chute pops. I am embarrassed to say I lost a MicroAlt this way. From my flights on E30-7's, F50-9's, G35-7's and a G80-10 (the last one was a frozen rope shot to 1860'), I have calculated a Cd of 2.18--about what you expect for a tube fin design. The new Ellis Mountain I69-15 single use should be cool in FISH too. Sims say 5000' at "only" 500 mph and a heck of a long walk to get it back without dual deployment. I am sure you cannot "over speed" the FISH airframe with any motor on the market. For really small fields, FISH can be flown on E30-7's with an adapter, which gives pleasant climbs to 450-500'.

Summary:
PROs:

• Finally, something practical a tube fin does better than any other: long, thin PVC rockets
• Ultra-cheap and ultra-fast to scratch build. This PVC grade is available everywhere
• Waterproof! Good rocket for launch sites with liquid surroundings
• Possible low cost L1 rocket but should use some payload ballast (6-10 oz) with an H128 or H238 to limit altitude, so there will be less risk of loss
• Excellent range of motor sizes, no glassing needed for high speed flight...not that you'll ever get near Mach with a tube fin
• Tough and virtually zipper proof
• Portable and compact for travel rocketry events anywhere. When broken down, 52" FISH will fit in a suitcase without risk of crushing or appearing scary to airport X-ray people

CONs:

• Heavy for its size, needs somewhat more powerful motors than usual
• PVC can crack upon landing on very hard surfaces (rocky or playa), although hasn't happened yet
• I wouldn't want one to hit me. Be careful flying rockets made of tough stuff like PVC conduit (although NAR legal)
• Some folks think tube fins are ugly