Monday, March 21, 2011

Review: Tarkus II

The motivation for this 29mm, sci-fi-inspired rocket began with a container from a bottle of Highland Park single-malt scotch. The truncated oval tube screamed spaceship. From there, the rocket design proceeded in ‘real-time’. The name arose part way through the build and came from Emerson, Lake and Palmer's 1971 album.  The design has undergone one major revision for aesthetic purposes.


Construction:
Motor Mount - A 29mm motor tube is mounted in a section of BT-60 tubing. The front of the motor tube is centered by a Giant Leap 29mm-38mm phenolic ring. It's a bit small so I shimmed it with some scrap tubing. The rear is attached to a ply centering ring that matches the lower diameter of a cardboard cone. I forget where I got that cone. The top of the cone was trimmed to fit the BT-60. T-nuts are installed in the aft ring for motor retention. Later in the design, as it grew in size and weight, I reinforced the BT-60 with some heavier scrap tube.



Main Wings - I guess these reflect a Star Trek influence. The curved main wings were cut from 8" diameter Sonotube using a RotoZip cutting wheel. They are inserted through slits on the aft body. The top section of the wing is one piece whereas the lower tier is two-part. The nacelle tubes are 24mm LOC. The front of the nacelles are bomblets from the Estes BLU-97B Cluster Bomb. The back of each nacelle has a plastic nozzle centered in a piece of BT-5 tubing. I also forget where these nozzles came from.



Top stabilizer – This part looks more like it came out a Roadrunner cartoon than a Star Trek film. I'm referring to the green plastic doohickey (a lime juice bottle). For some reason, I really like that shape and when another made itself available, I decided to add it on. It's mounted on a piece of Lexan, which is sandwiched between two dowels and epoxied to the main wing where it passed through the body. As it turned out, this part set the color scheme for the entire rocket.

Aft body - I cut holes in the top and bottom of the scotch container to fit the BT60. I also used tiny bolts to attach an oval piece of plywood to the container's plastic top. As mentioned earlier, the body is slotted for the curved wings and for the top stabilizer fin. A 3/16” Kevlar® shock tether is attached to the main wing internally.

Fore body – This is made from 3” mailing tube. The tube is offset with the main body with the BT-60 epoxied to the lower tube wall. I also cut a section off of the back end of the tube. This gives a smoother transition and proved useful when attaching the two body sections. I added a Gatorflex® cover for the gap. This worked great! The thin Gatorflex® has a tough (but brittle) plastic layer and over a sheet of dense foam. It is slightly flexible, which made it perfect for this application. That is, the front of the plate is fairly flat but the back needed to conform to the curve on the aft body. I based the rough cut on the section of tube that was cut off and added a little slop to account for changes as I bent the back end to fit. I also trimmed the foam to fit the hole but allowed some overlap of the plastic layer over the tube. It was glued in place with 5-minute epoxy and was held in place with tight wraps of blue masking tape. Once the epoxy was set, I ground the overlap off.

Nose cone - I wanted a non-standard nose cone design and started carving pink foam. I came up with something, but wasn’t satisfied. The 'eureka' moment was when I realized that if I flipped my cockpit-style nose cone over and tilted it down, it looked like what I wanted! I slipped it over some 3" coupler stock and twisted the coupler so it dug into the foam. I filled it, sealed it, added a plywood bulkhead and an eyebolt - I had my nose cone. This reminds me of the cone on the Star Wars Lambda-Class Shuttle.

Misc.:
* I wasn’t sure there was enough horizontal fin area so I screwed on a bug chunk of Lexan® at the base of the horizontal stabilizer fin.
* I also didn’t like the fore body offset and thought I’d add a smaller tube below the main tube. As I was test fitting various tubes, I ran across some half-oval balsa stock and decided it was just what I was looking for.
* The final issue was stability. I didn’t want to add weight to the cone. Being unglassed foam, it is pretty soft. (It will come down on it’s own ‘chute.) So, I added two spent 24mm cases underneath the front of the fore body adjacent to the balsa runner. If needed, I’ll could add lead shot and cap them off. As it turned out, no extra weight was added.

Finishing:
To follow the color scheme set by the lime juice bottle, I painted the nose cone and nacelles with Model Masters Lime Green®. I used yellow trim Monokote®, and some other miscellaneous stickers to form a snake-like cockpit design. Well, the results look more like Puff the Magic Dragon or Cecil (or was the Beanie?) than the front end of an alien space ship. If it flies, I may re-work the design. I also accented the lime green nacelles by painting the ‘cones’ yellow. I finished the rest of the rocket with Rustoleum Hammered Green® paint.

Flight:
I flew the Tarkus II on a G71 drilled down to a 5 second delay. I used a large Kevlar® pad and 42" chute for the body section. The nose came down on its own 36" chute to give the foam a nice soft landing. As expected, I had to invert the nose cone to clear the rail. The boost was a little wobbly and it arced towards the direction of the upper stabilizer. It was stable! Ejection was a little late, but it recovered undamaged.

The Rebuild:
So, this odd-sci-fi-roc was a amalgam of Star Trek, Star Wars, Looney Tunes, Beanie and Cecil, and Highland Park scotch. Everyone who saw it seemed to like it and it received a fair amount of chuckles also.  Overall, I was satisfied with the result.  I like the oversize nozzle cone, the truncated-oval rear body and the way it transitions to an offset tube, and the curved wings with their warp-pods.

However, a couple of things didn't work out quite so well.  These were the plastic lime juice container on the rear stabilizer and the nose cone.   I used one of those juice containers as a replacement cone for my Q-Modeling Stiletto. I liked the shape and it provides a stark, obvious contrast to the nice military-styled rocket.  Others seem to like it a lot but, in the end, it's clear it's a recycled plastic container.  So even though the general shape is great, this quality adds too much of a 'junk-factor' to what I was envisioning to be an interesting sci-fi ship.  The nose is a little of a mixed bag.  I wanted an offset style that had somewhat of a 'biologic' look to it.  The foam was a pain to form.  Easy to work with but tough to form planes with symmetry.  The end result was not too bad but then I went nuts and added facial features which clearly looks like a poorly drawn cartoon.  Hardly the feel that I wanted.

One day my buddy Warthog gave me a empty Perrier body and I had my replacement nose.   I found that the had a molded seam right at a point that would mesh perfectly with a 3" mailing tube.  Marking bottles for surgery is always a problem but this seam made it easy.  While the result mates perfectly with the tube, a piece of the inner telescoping tube required some build-up to fit snugly.  I thought that Krylon H2O paint would work well on the bottle, which is transparent green.  I decided I'd try painting it from the inside.  This kept the outside nice and shiny and resulted in a deep emerald green color.  I then filled it with 2-part foam.  To top it off, literally, I added a plastic tip from a DIY termite trap. 


I also replaced the plastic lime bottle with a tube fin.  Much better.  Finally, I added nose cobes to the short tubes on the front.  All painted to match.

The Return to Flight:
The boost on the original was a little wobbly.  I thought a bit more thrust would help so I tried one of the new (HP)G138-14A motors.  I drilled the -14 to a -6 using one of the new deal adjustment tools.  The boost was fast and arrow straight.  Ejection was several seconds late and, unfortunately, the 'chute tangled and it came down hard.  Amazingly, all I need to do is reattach the stabilizer fin and re-seat the tube fin.