Brief:
Originally, I built Da Bomb to only test a way of preventing tubefin flutter in HPR tubefins (Yes, Virginia, there is tubefin flutter). The experiment worked great, and the byproduct is a neat, cheap, and very small sport rocket for 38mm motors good for F-I power. It would make a good scratchbuilt L1 cert rocket for a younger flier on a budget (and who isn't these days?) Da Bomb is also one of the highest performance tubefins ever built--flown at 650 mph!

Construction:
Da Bomb is a tubefin built from 2.6" (65mm) PML phenolic tubing. Don't bother starting this project if you're not going to glass the outside of the fins per the instructions--phenolic shatters like fine china without glassing. Rest of the parts include an Estes Fat Boy nose cone, a piece of 38mm motor tube, and foamboard for the centering rings to test another idea I had. Da Bomb is as small and cheap to build as an Estes Big Bertha (see photo), but it flies on I motors vs a C6-5!

Stuffing big APCP motors in Estes rockets and the like is a time honored part of our sport, but it usually involves complicated modifications to hold these things together. Da Bomb is not complicated at all. In fact, it's very conventional. Body tube consists of 15" of 2.6" PML phenolic, six tube fins are 2" long pieces cut from the same tubing. The 6 tube fins are glued in the usual self-jugging fashion (for tubefins) to the bottom of the body tube in pairs set 1/4" up from the bottom. I use cans of soup to hold the fins in place while the glue dries (I used 5 minute epoxy throughout). First two fins glued on left and right, then two more on top, followed by turning over the assembly two glue the last two in place. Airframe is now basically done (why I like tubefins). An 8" length of LOC motor tube is cut and centering rings are "hogged" out of 1/4" foam board. "Hogged" means crudely cut (with an X-Acto blade in this case) after drawing the pattern right on the material. I made 4 of these centering rings, doubled them with epoxy, and then glued a 1/32" ply duplicate on top of each stack for stiffness. (I like 1/32" ply as it cuts with a plain scissors like paper.) The resulting 1/2" thick centering rings were glued 3/4" from each end of the top and bottom of the 8" motor tube. The motor tube assembly was then slathered with epoxy and glued into the body tube. Recovery system attachment is a 4" knotted loop of para cord passed through an 1/8" hole drilled 2 1/2" from the top of the body tube with the external knot then covered with a simple fairing I cut from a Cheerios box and covered with epoxy. I usually use a piece of plastic cigar tube for the fairing, but I was out and the liquor store was closed. Recovery system consists of 8' of para cord attached to the loop and to the Estes Fat Boy cone with a loop 1' from the top for a 28" nylon parachute. A second hole was cut in the Estes cone and the para cord looped through these holes to avoid the useless molded in ring on the cone. For HPR flights (H-I), 70 grams of lead fishing sinkers were added to the cone and held in place with lots of wadded up toilet paper. For F-G flights, no ballast was used. The launch lug was a 1 1/4" piece of a ballpoint pen body big enough for a 3/8" launch rod glued with its center 4" above the fincan and exactly in line with one of the triangular holes between two adjacent tubefins (the launch rod passes through this triangular hole). I attached the launch lug on the side opposite the recovery anchor fairing. Lastly, the outer surface of each tubefin was reinforced by glassing on a small rectangle of glass cloth cut to fit and any excess fiberglass was trimmed off.

Flight:
All flights were made with a small Nomex® heat shield, but Da Bomb is small enough to use dog barf (insulation) if you prefer. Motor retention was with friction fit and "lots of masking tape". Maiden flight was on a G80-7 (with a LOC 29-38mm adapter) and was perfect, reaching a modest 905'.

I used an external altimeter pod with PerfectFlight Alt15K. A G77-7R next gave 819' on the Alt15K. Calculation of speed and drag coefficient gave Cd=1.6 at 236 mph, which was about what I expected. Removing the adapter and switching to 38mm power, I started working my way up the Cesaroni product line: flight on a G69 with 5 sec delay gave a straight boost to 1140'. A CTI H153-8 in Da Bomb gave 1837'(note from above that 70gm ballast added for HPR launches). A perfect flight and would make a good combination for an L1 Cert flight since it wasn't too high. Launch in a CTI I212SS-9 gave 1998' and a slight weathercock. Calculation of speed and drag coefficient from this flight gave 549 mph and Cd=1.37. This is as fast as I have ever gone with a tubefin rocket. Since we were doing "research here", I decided to chance a flight on a CTI I285 (10 sec delay), even though the motor case stuck out about 3" below the rocket. I missed the photo because I really wanted to see the boost--and it did not disappoint! Momentary hesitation as the motor lit then an monster trail of flame straight up to 2688' as recorded by the Alt15K. I was glad to have extra eyes as a 19" rocket is hard to see a half-mile up. On getting it back, I noticed a bit of delamination damage on the leading edge of one tube fin, which I crudely fixed with tape otherwise everything held together. Calculation of speed worked out to 648 mph. This verified glassing the outside of the tube fins as a good means to prevent high-speed tubefin flutter. Plus, there aren't many rockets this size that fly in I power! Note the group photo of the huge range of motors that Da Bomb can fly on--amazing considering its small size.

Summary:
PROs: This is an easy-to-scratch-build HPR rocket, which can also be flown enjoyably on low power, too. Only 19" long and weights a bit over a pound, so it's easy to transport and just throw in the trunk since with the tubefins there is nothing to break off. Easy to make foamboard centering rings worked out just fine, although it has not been considered a suitable HPR building material. Rocket would make a good sport rocket for a kid who wants to transition to L1 under the new rules.

CONs: The unglassed portions of the phenolic airframe are subject to cracking. Small size to spot at 2000+ ft if flown on I power. Not certain if Estes will continue making the Fat Boy plastic cone, probably an ogive one could be substituted.