Scratch - Captain Electric's Saturn Screamer {Scratch} Original Design / Scratch Built

Contributed by Geof Givens

Manufacturer: Scratch
(Contributed - by Geof Givens - 11/17/07) (Scratch) Captain Electric's Saturn Screamer

Brief:
Thirty years ago I used to build rockets and pinewood derby racing cars with my Dad. I remember the excitement of shaping space-age designs from a block of fresh-scented pine and sneaking in to the kitchen to melt lead ballast on Mom's stove when she wasn't looking.

Somehow thirty years later I'm now the "Dad" and Dad is "Grandpa". Families and careers have taken us a thousand mile apart. We gather for holidays and visits sometimes as we did last Christmas when Dad was telling me about his new woodturning hobby as I made waffles for the kids. He wondered if there was a way he could help with one of my rocket builds. No, I said, I don't have any projects like that. Besides, I am too busy.

(Scratch) Captain Electric's Saturn Screamer The best ideas come slowly to me so it wasn't until the next day that I realized in excitement that the sexy, retro designs in the 2007 "Design This Rocket" contest might be a perfect match for Dad's woodcraft. We looked online to view the options and selected the "Racer" with a rush of excitement and apprehension. Dad wasn't sure he could make the parts, and I was doubtful that I could make the thing fly, especially with our foolish choice of scale: we envisioned a 4in diameter rocket to be powered by 29mm motors.

This time I wouldn't be running out to Dad's garage with my latest design--we would be shipping the model back and forth across the country via priority post. Email, RockSim, APCP motors...many things have changed in our lifetimes. The essence of this project though, is the same as it was 30 years ago.

When we were nearly done, Dad mailed the model to me one final time for me to add the finishing touches and paint. We had spent nearly nine months working together on a project for the first time since childhood, despite being separated by a thousand miles. Regardless of how the rocket flies, for me this project was about something more important. In Dad's package was a note: "Geof, This was insured for $100, but we both know it is priceless!" Now let me tell you about the build and the flights.

(Scratch) Captain Electric's Saturn Screamer

Construction:
My Dad and I both have a fondness for retro sci-fi style so we were drawn to the Racer. Dad planned to turn the whole body out of pine. After some math, we reckoned that he could be given a budget of 16 ounces for the fuselage, planning for a 29mm motor mount and a rocket about 21 inches tall.

A few weeks later a package from California arrived with sketches and an approximate 50% scale prototype of the fuselage. I adjusted the design for a sleeker look and then modeled the design in RockSim using a series of transition components. This gave us a better idea of the weight and templates for the full-scale version.

The strength of a 5/16-inch thick pine fuselage was totally unknown to us so I did some careful testing on the linoleum-covered cement floor of my bathroom. Broadside impacts for the prototype from drops of 4 and 6 feet yielded no damage. Then I gave it a baseball throw at about 50% effort and it broke convincingly. Sorry Dad, but it was only a prototype, right? Aside from the conclusion that I needed to repair the bathroom floor, I also learned that weight--not strength--would be the limiting factor.

Using the full-scale template I made from RockSim, Dad turned a full-sized prototype. Then after we fine tuned the shape, he turned the final fuselage in two parts (because it is hard to hollow out the inside of a piece longer than six or seven inches). He glued these two parts together, yielding a fuselage of 13.5in. The fuselage was then cut in half at the widest part to enable later placement of the 1/4in ply forward centering ring with an eye bolt for chute retention. He also made a 1/4in ply exterior rear centering ring. The edge of this ring actually forms the final quarter inch of the body length and is shaped to match the curve.

When we first worked with the prototype, we planned to make the rocket entirely out of wood. However, the RockSim modeling showed that this wouldn't work--there would be too much weight aft. Therefore, we decided to use the wood shape as a mandrel around which a fiberglass shell could be fashioned. I used several layers of 6oz S-weave glass and epoxy from US Composites.

For glassing, both fuselage sections were cut in half lengthwise. In the gaps removed by the saw blade, cardboard spacers were placed. Each section was taped back together with the spacers retained snugly in the gaps. All molds were sprayed with light adhesive and sheathed in mylar. The aft centering ring was left exposed so that it would bond to the fiberglass. The plan was--after glassing--that the mandrel could be removed by pulling out the cardboard spacers and then hoping that the mylar separated from the fiberglass or (as a backup) that the mylar separated from the wood. Neither plan worked. I had to bust out the wood with a hammer, chisel, and a lot of blood, splinters, and elbow grease.

At this point, the forward centering ring and a standard 29mm motor tube were installed.

Next up were the fins. I'm a scientist and let me just say that there's a lot of geometry involved in figuring out the fin dimensions from a 2-D photo in perspective with the rocket slanted every which way from the camera! The vertical fin is somewhat oversized compared to the target for greater stability. All fins were cut from 1/4in balsa using RockSim templates and a little creative fudging to compensate for the body curves. Airfoils were beveled on each side. The fins were then tacked on the fiberglass body and given two layers of tip-to-tip fiberglass for attachment and strength.

Dad turned the fin pods on the lathe out of a hard wood. He mailed several choices so I could attach the ones that looked best.

(Scratch) Captain Electric's Saturn Screamer Early on, Dad and I had decided to deviate slightly from the design photo. The contest picture suggests that the cockpit might be partially inset into the fuselage. When we had planned on a solid wood body, we knew we couldn't meet the simultaneous goals of thin light wooden walls and an inset cockpit. When the strategy changed to use the wood as a mold, we stuck with our original cockpit plans for simplicity. Thus, we planned to overlay the cockpit on the fuselage. I made a freehand sketch of the shape and cut a block from 1 inch thick balsa. This I cut and sanded to fit smoothly over the body with the least possible extra thickness.

From the beginning, I had nightmares about the pilot's head. Being a clever guy, Dad knew right away to trick me into accepting this part of the build by noting that only a person with real talent would dare attempt such a thing. I sketched the head on a 2.5 inch cube of balsa. It took only a few minutes to learn that this was utterly useless because as soon as I started carving with my Dremel, I removed the sketch lines. Finally, I decided to simply carve away anything that didn't look like a head. Near the end, I exaggerated the nose with a dollop of wood filler, then went through several cycles of sealing, sanding, and priming. The eyeballs were added at the very end by using a toothpick to gently set in drops of goop mixed from wood filler, white glue, and water. The photo shows how the head progressed.

The windshield was cut from a plastic peanut butter jar. Tacking it on to the cockpit with CA proved nearly impossible because it was bent and under torsion when fit correctly. Eventually I got it tacked on using a lot of straight pins to hold it in place. Then I used two-part epoxy putty to build up strong "caulking" on both sides.

By late summer, the body was mostly complete. I flew to my Dad's house and brought the rocket along. First, I re-emphasized to the kids how important it would be to refer to the model as a "spaceship" and not a "rocket" when passing through airport security. After a pleasant (albeit somewhat extended) conversation with TSA, we made it through. At Dad's house we crafted a hardwood button mount for the aft rail button, to compensate for the football shape of the body. This and the forward button were JB Welded on. I left the rocket with Dad for him to make a wood nose cone.

Pic w/ Dad Dad ended up turning the nose cone by eye because he felt the Rocksim template was a bit too stubby. For months, we had also ignored the needle contraption at the nose (affectionately dubbed the "golf tee") because we had no idea how to make it strong enough. As revenge for the head, I assigned this task to Dad. He turned one from solid oak, and inset it in the nose. To his credit, the golf tee turned out to be absolutely bulletproof despite three hard landings.

With his Dremel, Dad carved the "racing stripe" that appears as a relief feature in the contest photo. The forward portion of this sheath/stripe is shaped with a fancy S-curve so that it looks round when viewed from above or from the side. If we had actually made it round, it would have looked distorted from most angles. Dad shipped all parts back to Colorado for the last time.

Knowing the approximate final weight, I adjusted the RockSim model and realized that we needed to move up a motor class. This meant adding 16 5/8oz of nose weight in the form of melted lead. Over this I put a bulkhead with threaded rod to enable shock cord attachment and the addition of up to 15.75 oz of extra weight if needed for larger H motors.

Fiddling with my first ever high-power casing, I realized that the motor retention clips we had inset in the aft centering ring wouldn't work for the H case as they would for the 29/40-120 case. I cut them off and used brass threaded insets in the aft end with bolts and washers for retention.

(Scratch) Captain Electric's Saturn Screamer We used 11 feet of 3mm nylon cord plus 1 foot of 6mm climbers' rope (inside the body) as a shock cord. These were bought from REI. The chute was protected with a 12in Nomex® cloth.

With the increase in motor size and nose weight, we needed a new chute. I made a 120cm diameter 12-gore semi-ellipsoid rip stop nylon chute using Richard Nakka's pattern and 46 feet of 2mm climber's cord from REI.

Here are the final stats: 21.25in tall including 3.25in spire, 4in maximum diameter, 46.5 ounces with standard nose weight, and a 29mm motor mount.

Finishing:
The target photo shows a very difficult, weathered look. We imagined that our rocket was fresh off the showroom floor, not yet scarred and blemished by years on the racetrack. Aside from that, we aimed to match the EMRR photo exactly. Paints were by Krylon and Wal-Mart. We improvised for the stickers. When the paint was all dry, I sanded away a spot in the cockpit and firmly JB Welded the head into position. Some nine months after inception, the project was done!

(Scratch) Captain Electric's Saturn Screamer

Flight and Recovery:
The flights were planned for Northern Colorado Rocketry's 2007 Oktoberfest. Simulations showed weight and stability both to be concerns. After hours on the computer, we planned the following launches: (1) H165R with 13.5oz extra weight for a stability margin of 0.83 for 45fps off the rail to 652 feet in my L1 certification attempt; (2) go for broke with H180W with 13.5oz extra weight for 57fps off the rail to 979 feet; and (3) for the multi-motor bonus, G71R-4 with 4oz extra weight and 41fps off the rail to 500 feet.

It was a sunny and calm day on the high Colorado prairie. Dad had flown out from California for the big event. I had lost my nerve and certified L1 the day before (on a sure-thing PML X-Calibur), so today was all about getting Captain Electric airborne. The first flight was on the H165R. After endless photos, we were ready to go. Boost was stable, but the nose cone drag-separated before apogee. Unluckily, the nose hit the vertical fin and nearly broke it off. The captain landed under partial chute. Despite a crowd of 100 or more, nobody (including us) had any fiberglass for fin repair so Dad and I fixed it up as best we could with masking tape and 15-minute epoxy.

(Scratch) Captain Electric's Saturn Screamer Somehow, we managed to cram in the 29/180 H180W motor and the huge chute in the repaired rocket. We taped the nose cone shoulder severely, preferring to experience a new failure mode instead of a repeat of the first. On the H180, the rocket absolutely ripped off the pad, straight and stable until boost finished. During the coast phase, the rocket turned unstable and spun into the prairie before ejection. At the moment of impact, the ejection charge deployed, neatly spreading the chute over nearby cactus. We broke a wing, two wing pods, the spire, and half of Captain Electric's head. We were done for the day.

Over the next month, I stubbornly repaired the rocket with fiberglass and a sense of humor. The pieces were put back together as best I could, the windshield was removed, and I added a white flag with a red cross, some red crosses on the wings, and a band-aid superglued to what remained of the Captain's skull. The rocket was renamed Captain Electric's Medevac.

On November 3, 2007, the rocket went for its final flight on the G71R. Boost was slow but straight to 600-700 feet. Finally, a perfect boost! At ejection, the nose popped and spectators with binoculars tell me the chute came out but failed to fully unfurl. It was another hard landing for Captain Electric.

(Scratch) Captain Electric's Saturn Screamer

Summary:
The end of this saga comes 11 months after it began. Tomorrow I fly to Dad's house for Thanksgiving. I'm carrying Captain Electric in my suitcase so he can enjoy a well-earned retirement with Dad in the California sun.

More photos of the build and launches can be found here. I hope you enjoyed reading about the project as much as we enjoyed working on it.

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