Scratch Thunderbolt Original Design / Scratch Built

Scratch - Thunderbolt {Scratch}

Contributed by George Pike

Manufacturer: Scratch
(Contributed - by George Pike - 11/21/03)

(Scratch) Thunder RoadBrief:
Until now I've been strictly a low power BAR. In the year or so since returning to rocketry (courtesy of my kids, their favorite PBS show "Zoom", and an Alpha kit from the local hobby store), I've had fun building progressively more interesting model rockets including a few two and three engine clusters and multi-stage rockets, but staying in the A to D motor range and using pretty standard LPR components. Thunder Road is a scratch-built design that punches through a couple of barriers in my rocketry resume with a design that can fly on either a cluster of seven C & D motors or on single mid-power G motors.

Construction:
I'm a pretty frugal rocketeer, and the two mailing tubes that came into the office one day became the inspiration and base for what, after several design evolutions, became Thunder Road. Essentially it uses two 2.2 inch mailing tubes totaling about 34 inches in length for the main airframe. This transitions to an 18 inch BT-60 tube, which in turn transitions to a 12 x 1.25 inch aluminum foil core at the top. All in all the rocket is about 6 feet tall. The mailing tube main airframe houses a central 29mm x 19 inch motor mount/stuffer tube. It was designed to fly on a single G motor, or I can use an adapter with an Estes D or E motor as the center of the seven motor cluster. For cluster flights the rocket uses six variable length BT20 tubes in pairs adjacent to the fins for the outboard cluster motors. Fins are 1/8" basswood mounted through the wall. It uses a BNC55 central nose cone (with the shoulder sanded to fit in the aluminum foil tube), two balsa transitions that were in the discount bin at a hobby shop in Portland, Oregon (six BT70-BT60 transitions for $5 and five BT60-BT55 transitions for $3!), and six hand turned balsa nose cones for the outboards. Kevlar® cord and half-inch elastic were used for the shock cord and 1/4 inch (inner diameter) fiberglass tubing was used for the dual launch lugs.

The design took some doing to work through a few problems. First was how to join the two mailing tubes, which were pretty well matched, but not a good size for any stock coupler. (They were about 2 inches ID, 2.2 inches OD.) What I ending up doing was turning a 4 inch long block of 2 x 2 inch balsa (available from BMS) for a coupler. I then drilled a hole large enough to hold the 29mm motor-mount/stuffer tube. I cut two centering rings out of 1/4 inch plywood (standard stuff from Home Depot, actually got a 18 inch by 4 foot piece from the scrap lumber bin for 50 cents!) for the motor mount. The assembly uses the centering rings around the motor mount at the base of the rocket, then the coupler to center the upper part of the stuffer tube and provide extra strength for the mailing tube joint. At this point, however, I simply glued the balsa coupler to the lower mailing tube

(Scratch) Thunder Road I then marked and installed the upper centering ring on the motor-mount/stuffer tube and installed a engine block and two engine hooks (made out of piano wire from the hobby shop) for positive motor retention. Not knowing exactly what motor I might use, I made the hooks 125mm long, or about the length of the longest Aerotech G motor--spacers are used for shorter motors. The motor mount (without the lower centering ring yet installed) was then installed in the lower mailing tube and threaded through the balsa coupler, and glued using Titebond yellow glue (with two centering rings and about 4 inches of glued contact to the balsa coupler, I felt I didn't need to use epoxy.) I then glued the upper mailing tube to the coupler, rolling it on my garage floor in order to insure a straight line connection.

The three fins are a basic swept triangle design, cut to measure 4 inches out from the 7 inch root edge with a tab for through the wall installation. In order to correctly place the three fins (since my Estes fin guide wouldn't work on the non-standard tubes), I wrapped a piece of scrap paper around the airframe and marked the circumference as accurately as possible. I then used my metric ruler to measure the circumference in millimeters, divided that by three and marked the paper as indicated. I then re-wrapped the paper around the airframe and marked the tubes. I hand cut the slots for the tubes using the frame for our front screen door (a nice clean aluminum straight edge!), and noted that it was much easier than I thought, plus surprisingly accurate. The top of the slots was even with the upper centering ring. The fins were then glued to the motor mount and the lower centering ring installed right against the fin tabs for a secure connection. Fillets of Titebond yellow glue were also used at the fin/airframe joint to increase the security of the connection.

(Scratch) Thunder Road The outboard motor tubes were basic lengths of BT20 tube mounted in pairs right next to the fins. I decided, strictly for funky aesthetic reasons, to stagger the pairs in 22 inch, 18 inch and 14 inch lengths. Mounting them against the fins also allowed for two points of contact to ensure that they stayed put. Generous fillets along the root edge of the tubes also helped. One problem that I wrestled with was how to exhaust the ejection gasses from the outboards. I decided to try routing the gasses from the two longest tubes through the main airframe and into the upper body tube to join with the ejection gas from the central motor to ensure enough oomph to eject the chute. (I was concerned that some of the ejection gasses from single G engine flights might travel out these vents and result in not enough oomph to eject the chute, but that proved not to be the case.) For the shorter outboard tubes I drilled three .25 inch exhaust ports on each tube. The nose cones for the outboards were hand turned from 2.5 inch blocks of 1 x 1 inch balsa (also available from BMS.) I used my variable speed hand drill to turn the cones, it takes some practice but isn't that hard and the cost comes to about 10 cents a cone.

The shock cord was a length of Kevlar® cord tied around the motor mount and connected to an 8 foot length of one-half inch elastic. This was attached with a heavy duty snap swivel to a large screw-eye epoxied to the transition at the top of the mailing tube airframe. This transition was a basic 7060 balsa transition, with the shoulder sanded down to match the mailing tubes' inner diameter. I used an 18 inch length of BT60 tube and 6055 transition to the aforementioned aluminum foil tube and a balsa BNC55 nose cone to provide the overall 6 foot length and ensure stability. Two 1/4 inch launch lugs cut from a fiberglass tube were epoxied into place about 24 inches apart on the main airframe.

In total the rocket weighs in at about 17 ounces unloaded--a bit heavier than I thought due to the heavier mailing tubes, large basswood fins and probably heavier glue fillets than were necessary.

Finishing:
Finishing mailing tubes is a lot of work. I have found that the easiest thing to do is to carefully peel of the outer paper layer, then paint a diluted mix of Elmer's Fill & Finish (diluted to about the consistency of thin finger-paint) over the entire tube. Next, give it a good sanding using 220 grit sandpaper first, followed by 320 grit sandpaper. Fill any major spirals or blemishes with a thicker Fill & Finish mix and sand again. This gives a pretty solid and smooth base for the paint. Diluted Fill & Finish was also used for the balsa nose cones and transitions. Thunder Road is named after the Bruce Springsteen song, so I painted it with predominantly black paint scheme (the Boss's favorite stage color--the black paint also hides the exhaust ports in the outboards) with red fins in honor of redhead Patti Scialfa (Mrs. Springsteen) and a touch of purple for Little Steven Van Zandt. I used Rustoleum white primer, Rustoleum high-gloss black, Painter's Touch (a discount Home Depot brand made by Rustoleum) purple paint, and a Walmart brand cherry red paint. I like the Rustoleum white primer better than any other primer I've tried, but I've found that the spray can runs out of gas before it runs out of paint and I end up having to toss out half a can of paint.

(Scratch) Thunder RoadFlight:
First flight occurred on October 11, 2003 at Tripoli Pittsburgh's Dragons Fire 31. Try as I might, although the rocket was completed, it was not painted, so she went up "naked" on her first couple of flights. For the first flight I decided to try a cluster of six C6-5 motors in the outboards and a D12-5 with an adapter in the center. Prepping took quite a bit of time. I decided to use two parachutes, a 22 inch mylar for the base, and a separate 16 inch mylar chute for the upper part. I used several sheets of Estes wadding (remember, I'm still a BAR and that's all I had) and wrapped a couple of wadding sheets around the chutes as a bit of a heat shield. Loading the six outboard motors was a bit of a chore as they all were friction fitted and each one needed a different length of tape to provide the friction. The central D12-5 was loaded into the adapter, with the adapter then loaded into the central motor mount.

Out to the pad we go--my first trip out to the high-power area of the range. I had a pair of triple connection clip whips to connect all the igniter wires together but it was a chore. Generally I twisted the outboard pairs together to make three sets of connectors, but I need to carefully join the ignitor leads from the center engine to two of the outboard pairs, taking care to make sure they would be on different polarities. That's a lot of dangling wire and I had to take care that there was no metal-on metal contact which might cause a short. As it turned out, one of the pad's alligator clips was loose and we lost continuity on the first try. One the second try however, all went as planned and I was thrilled as all seven motors lit. Thunder Road flew perfectly straight to exactly 850 feet (as measured by an onboard PerfectFlite Alt15K altimeter). Recovery was very smooth, although I was surprised by how much it floated considering the weight and small chute. I'll admit after the fact that I thought the weight might be too much even for the cluster--and heaven forbid had not all of the motors lit, but it worked very well.

The next day I decided to cross the other barrier and loaded up my first composite, mid-power motor, an Aerotech G35-7 Econojet. This time I only loaded one 22 inch mylar chute with a bit more wadding (concerned about the faster, hotter ejection charge from the G motor). Back out to the high power pad and 5...4...3...2...1... Liftoff and another flawless flight, much higher (estimated about 1300 feet, no altimeter on board this time) and still a lot of drift notwithstanding the relatively small chute. In fact it drifted past the tree line but very fortunately threaded past the taller trees getting hung up in a small tree only about 6 feet off the ground. Needless to say, I'm now building more mid-power rockets and looking at Level 1 certification by next summer.

Summary:
I really like this rocket. It's tall, lean, and flies straight and high. It wasn't easy to build but ultimately went together very well. It's a keeper

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