The Cosmic Cucumber - a Scratch-build Project

Because the fiscal cliff negotiations included a Social Security Tax increase, my rocket-building budget is lighter this year, so I opted to take a very tired rocket and use it for parts, then buy the components I needed to finish, rather than buy a new kit.

My goal was a rocket that would challenge our club's waiver (9,000') but not go over, that would be fairly good-sized, and fly on a 38mm motor. I have 2 54mm rockets, but 38mm is about 60% the cost of flying. I need most of my rocketry dollars this year to go toward my L3 build.

So I came up with the Cosmic Cucumber. a 4" rocket about 4' tall (haven't finalized the actual length yet) that can support a CTI J530 motor. Since I am playing with mach speeds, it will be built with a Bluetube airframe and the fins will have fiberglass internal fin roots so they stay attached.

The other thing I will do with this is incorporate a Jerry Irvine technique for the tailcone. I will cut six 6" wedges out of the aft portion of the airframe and bring the rest of the frame together to a 58mm wide centering ring at the base of the rocket. That way I save money by not buying a tailcone, reduce aft weight because a plastic tailcone is heavier, custom build the tailcone to the optimum length for altitude, and have a seamless transition from the airframe to the tailcone. The seams will be epoxied together and filled with sealer.

I also get to use calculations involving pi, so my high school algebra teacher will be proud. (Pi x the circumference of the 58mm centering ring)/6 = the distance between the wedges I have to cut out to form the tailcone. I am using 6 wedges because it is a 3-fin rocket. 3 of the cuts will then go farther up the airframe to form slots for the fins. I tried using a razor knife but learned very quickly how dense bluetube is, so I ama waiting for some Dremmel saw attachments to arrive and will use that instead.

This will be a DD combination using an adept 22 altimeter, a beeline transmitter, and at least one onboard video camera. OR sims suggest this should hit 8,500'. My guess is that it will come in somewhere near 8,000.

Here is the design. I am hoping to have the airframe cut this weekend (and will post pictures).

I think the name alone will turn heads.

I like the name and design. Very nice.

I'm thinking a nice metalic or opalenscent green.

First step finished. The new dremmel saw blade worked excellently. The wedges that I needed to cut out of the Blue tube to form the tail cone have been cut. These will not be bent in until the fin assembly goes in, but hopefully you can picture how this works.

Have you tried bending them in? I would think Blue Tube possibly too stiff to do that well, but I haven't tried myself.

I did try. I will need to hold them down with tape and epoxy sections at a time. Thinking I will take shreds of kevlar and fill the seams with them to make sure it all holds together.

Bending the BlueTube will work just fine. I'm currently working on something similiar and I just recently made all my cuts. The airframe is plenty flexible to achieve the desired results.

I've done this before as well, both with BlueTube and LOC tubing. It's a fine technique.
These two rockets are done this way (the black one is BlueTube).



s6

nice idea - subscribed...

oh.. and S6, those are gorgeous!!!

Real nice pics Stealth 6. Keeps me inspired. More photos are coming.

I love the name of the rocket!

I don't have a CNC cutter, but I need to cut my own fins and they all have to be the same size. How do I do this with reasonable accuracy? I stacksaw them. In this case, I have 3 triangular pieces of ply leftover from an earlier project. I traced the fin design on one of them, clamped the 3 pieces together, and with a 90-degree sabre saw, cut all 3 fins at once. All the same size, and all according to spec.

Time to sand the fins. I have found the best way to sand plywood fins is to put my vibrating sander in my vise and clamp it down. With the sander in place I hold the fin over it and move it back and forth. I have also learned that even though it takes more time, use 220 grit sandpaper. The reason is that when you sand this way, you have to be careful not to sand unevenly. The part of the fin underneath your fingers will sand faster than the other parts. If you use a heavier grit sandpaper, even a 150, it gets so out of whack so fast, you can't recover and make it even again.

Got two done, but then it was my son's bedtime. I'll finish the other tomorrow.

Good tips for cutting/sanding identical fins

EeebeeE said:

I don't have a CNC cutter, but I need to cut my own fins and they all have to be the same size. How do I do this with reasonable accuracy? I stacksaw them. In this case, I have 3 triangular pieces of ply leftover from an earlier project. I traced the fin design on one of them, clamped the 3 pieces together, and with a 90-degree sabre saw, cut all 3 fins at once. All the same size, and all according to spec.

View attachment 115450View attachment 115451View attachment 115452

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2 sided carpet tape works great when wanting to stack parts to cut. I like doing it this way so I can use my band saw and chop saw to cut out my fins without the clamps getting in the way.

A couple wood screws through the tang does the trick too.

Had to work late so by the time I came home it was too late to finish sanding the last fin. I was almost done when my wife yelled downstairs that our son was in bed. She puts up with this hobby and I have no right to complain when the noise has to subside. So I did other things.

In the assembly of this with three centering rings, it is a challenge to ensure all three are glued in place. So in this instance, I will glue the forward centering ring in place. Then I will assemble the fin/motor tube section. That way I can carefully glue the fins to the motor tube, using the fin slots as guides so I can pull the assembly back out and use fiberglass cloth to secure the fins to the motor tube. I will get a much stronger bond. I will replace the inside fillets with strips of balsa glued to the fins and covered with a generous amount of epoxy. Fillets on the outside will be more conventional. Those fins shouldn't go anywhere.



Next comes the thing I dread most in building rockets...cutting the airframe in two. I am going without using a switchband to improve aerodynamics. Instead I drilled a 5/8" hole for the switch in the airframe, then cut the airframe in two through the center of the hole. I have found the best way to cut an airframe in half is with a miter saw. When you think about it, paper and epoxy have about the same density as a hardwood. A miter saw has a cross-cut tooth pattern so it doesn't gouge the frame, and you end up with a decent cut that is as close to 90 degrees as you can get. If you end up slightly off kilter, slide a coupler tube into the tube and sand the areas that need to be leveled.

Then I couldn't help myself. I had to dry fit everything to see how it will look. It's one thing to see it on paper. It's another to see it come to life. What do you think?

I think that tail cone is going to need some major fuller :wink:

Looking good!

It will be drawn tighter than what you see in the photo. I just wrapped some masking tape around it to get an idea as to how it looked. Thinking of using a boatbuilding technique where I drill 1/32" holes along the lines that need to be drawn in, twist tie wire to hold it together while the epoxy dries, then cut the wire outside the airframe flush to it and file off any nubs. It's an element of the "stitch and tape" process except that I will not use glasscloth to cover everything. Instead, I have some worn out kevlar shock cord from a crashed rocket that I will drench in epoxy and use to seal any open seams.

Time to build the fin assembly. As you may recall. the forward centering ring is already glued in. What I want to do now is glue the fins in place but ensure that they can still be taken out at this point because I want to fiberglass the roots to the motor tube to make sure they are as secure as I can get them.

After determining where the fins meet the forward ring and sliding the middle ring in place I put epoxy where the fin root would meet the tube but not com in contact with any centering ring, then slide the motor tube with the middle ring in place into the airframe making sure that the epoxy bead is visible through the fin slot. Then I glued the fins to the motor tube through the fin slots which acted as guides to ensure that the fins were properly aligned. I did this one by one, and after the epoxy hardened, slid the motor tube back out so it could be buttered.

Once all the fins were tacked on, I cut small pieces of glass cloth and epoxied it to the motor tube and the fin roots. At this point I also glued the middle centering ring onto the aft end of the fin roots. The bottom centering ring is positioned where it should be but I will not glue that into place until the fin assembly is glued into the airframe.

Tomorrow I will cut strips of 1/8" balsa to the upper end of the fin root. These will be coated in epoxy and will act as the internal fillet at the airframe. At that point I will bend the tailcone sections into place.

I snicker, every time I see this thread -- I absolutely love the name of this rocket!

-Kevin

With the tailcone, 3-1 ogive nose cone, and a 4" airframe it looks like the "huge monster pickle" described by Arlo Guthrie in "The Motorcycle song - the Significance of the Pickle."

EeebeeE said:

With the tailcone, 3-1 ogive nose cone, and a 4" airframe it looks like the "huge monster pickle" described by Arlo Guthrie in "The Motorcycle song - the Significance of the Pickle."

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+1 point for this reference.

Really starting to take shape now. To form the tailcone I used automotive epoxy blended with some wood flour (very fine sawdust). I figured the wood flour would help to strengthen the bond between the cuts on the tailcone. Plus I could also use the mix for fillets.5-minute epoxy was used to hold the base of the tailcone to the lower centering ring.

EeebeeE said:

With the tailcone, 3-1 ogive nose cone, and a 4" airframe it looks like the "huge monster pickle" described by Arlo Guthrie in "The Motorcycle song - the Significance of the Pickle."

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Close, but I don't think it's quite the size of "four pregnant watermelons".

I should add this photo. There are 1/4" x 1/4" ash gunwhales glued to the uppor portion of the fin root. These slid under the airframe when I put the fina assembly in place. I epoxied them very yeavily then slid assembly in place. My thinking is that this will work as well if not better than internal fillets. The external fillets are epoxy mixed with wood flour. I will add a layer or two of liquid nails which will smooth things out, then finish with bondo to get everything nice and smooth.

Lots and lots of sanding to get the excess epoxy off. With wood flour as a thickening agent, it was even harder to sand it off. But I eventually finished. Had to get all the crap off so that I had a better understanding of where the filler needs to go. I will pick up some bondo tomorrow and fill the gaps. I will aslo use it on top of the fillets to smooth them out. The nose cone will have the old paint sanded off. That takes a while because I use a reciprocating sander with fine sandpaper. If I use coarse it gouges the plastic with deep grooves.