Scratch - Thunder'ceptor {Scratch} Original Design / Scratch Built

Contributed by March Briner

Manufacturer: Scratch
(by March Briner)

Scratch Thunder'CeptorBrief:
Long and skinny, single stage, high-power rocket [though it appears to be multi-stage]. Over 12 feet tall. Recovers under 44" chute.

This project started in my mind's eye in 1991, though construction never started until late 1992. In all, there was 80 inches of 3.1" LOC airframe, 60 inches of 2.26" LOC airframe [not counting a 30 inch long 54mm motor tube], 16 fins, 2 transitions made from epoxy-glassed posterboard, and about 24 feet of 3/4" braided shock cord.

The first pieces of the rocket came from a Caliber ISP kit. The pre-slotted airframe would be used for the booster, and the payload tube and the motor tube would be used for a 3.1-2.26 transition about halfway up the rocket. The centering rings were used to anchor the protruding 2.26 tube to the 3.1 tube. The bulkhead assembly was put in the other end of the 3.1 tube as per usual.

One inch was taken off the root edge of the Caliber fins, and attached to the upper section. The other pieces, more airframe, shock cord, plywood fin stock, etc., came together.

At the time, I was in high school and was enrolled in a wood shop class, so I had easy access to machinery to cut and shape the fins. Cutting and shaping the fins took, in all, almost 25 hours. I chose a fin stock slightly thicker than what came with the Caliber kit [3/32"?].

The first set of fins [from the bottom] were rhomboid, 3 inches wide and 4 inches long. For added effect, I glued an 8 inch length of Estes BT-5 to the tip edge. In retrospect, I should have used solid wood dowels; the thin-walled tubes were bent to hell before I ever painted the rocket. These fins were mounted through the wall through the existing slots.

The next set of fins were 3 inch squares mounted through the wall. A new set of slots were made for them. The NEXT set of fins were elongated square triangles 3 inches wide and 12 inches long. They were surface mounted. Filleting all of these fins took nearly three sets of 15 minute epoxy.

Below the first set of fins was a boattail made from posterboard calculated with the help of Peter Alway's Scale Model Rocketry book. Two of these were made; one for the boattail, and one for the upper transition. Both were given a liberal epoxy glaze almost 1/8 inch thick.

The motor mount tube was 30 inches long and was held in place with 3 centering rings. The shock cord was held in place via a cable anchored to the top centering ring.

Given that there were 12 thick plywood fins at the back end, I'd probably have to put some ballast at the front end. I filled a 2.26 LOC nose cone with plaster of paris [weighed 20 ounces] and epoxied it in place. Doing a swing test on a 12 foot rocket is not as easy as it looks. It turned out that the plaster filled nose cone was making more problems than it solved. I later replaced it with an empty cone.


The paint scheme was simple; white airframe, black nose and fins. Painting it took more cans of Krylon than I anticipated. But eventually, it was finished. I called it Thunder'ceptor [combining the words 'thunder' and 'interceptor', but few people got it].

Scratch Thunder'CeptorFlight:
The first flight was at a Connecticut Tripoli launch in Morris, CT on 28 February 1993. The motor of choice was an AeroTech I210 [single use]. I packed the chutes with plenty of baby powder, shoved in some wadding, and prepped the motor with masking tape [friction fit and thrust ring]. Setting it up on the pad brought a few realities to light. The rocket was almost top-heavy even for a 1/2 inch rod [this was before I replaced the plaster-filled nose cone]. It flew straight and true to a little over 1000 feet.

Future flights were made at Battle Park launches in 1993 and 1994, and at LDRS 1996, all on full or nearly full impulse I motors. 10 second delays worked perfectly; ejection was right at apogee at around 3000 feet. After the first two flights, it started corkscrewing a bit. I stopped flying it when I noticed a kink in the tube just above the forward motor mount centering ring.

The rocket was definitely an attention-getter, which was something in the back of my mind as I built it. It has even appeared in HPR magazine a couple of times; look for launch reports from Battle Park 1993, or the April 1997 issue on p68. I enjoyed building it; it gave me something to do during the cold winter months. The only thing I might have done different was reinforce the tubing more and had it disassemble into smaller pieces [transporting it 7 hours to Battle Park was not easy].

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