Dimetrodon - A square rocket that doesn't quite fit in

Nytrunner said:

That's a crazy vortex story. I wonder if it'd work for a guy on an inner tube.....

Any luck working out how to fabricate the couplers?

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It'd probably work for an inner tube if you're OK with a week in 35-40 degree salt water. I have two suggestions for couplers. The easy way would be to epoxy in some fiberglass tube and then use standard couplers. Since I have to figure out how to build a nose cone anyway, I think I'll probably build it up out of some parts shaped on a router plus some plywood pieces.

Kurt, that is an excellent story. This one ought to hold together fine since it's all fiberglass, but I love the idea of a shred followed by chute "teleporting" to altitude and then descending.

boatgeek said:

It'd probably work for an inner tube if you're OK with a week in 35-40 degree salt water. I have two suggestions for couplers. The easy way would be to epoxy in some fiberglass tube and then use standard couplers. Since I have to figure out how to build a nose cone anyway, I think I'll probably build it up out of some parts shaped on a router plus some plywood pieces.

Kurt, that is an excellent story. This one ought to hold together fine since it's all fiberglass, but I love the idea of a shred followed by chute "teleporting" to altitude and then descending.

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Yeah, You'll do fine by glassing it. I couldn't have made that story up about the triangular rocket if I wanted to. I was "too dumb" at the time!
Then there was the one with the short, butt heavy rocket with a research motor. Did three loops, hit the ground flat, flopped around like it was in an epileptic seizure and thank God took off at a 25 degree angle to the southwest "away" from us.

It was a small launch with maybe a dozen fliers in attendance and the rocket did go away from us. Surprisingly, the rocket was found even though I thought it was gone after disappearing towards the horizon. I was told later there were no fins left on the tube. Lord knows when they were torn off. Rocket behaved as if the CG was too far aft (it was). This ranked up there with several scary flights I've seen.

Kurt

OK, Here's some pics from layup day at the end of last month.

Everything ready to go, two layers of wax paper on the mandrel, glass wound on to a piece of ABS pipe, rocket retrieving pole as a turning bar, and my daughter ready to help wherever needed. Note the wax paper isn't super tight on the mandrel. This came back to bite me later.


Now we've got a wrap or two on the mandrel, and I'm adding epoxy from the cup, to be pushed flat with the roller.


This is one end of the tensioning system. The glass is wound up on the black ABS pipe, then a PVC pipe is clamped over the top (material doesn't really matter, it's what I had on hand). This puts weight on the material without allowing the glass to spool off of the ABS pipe. I think they were both 1.5" pipes.


Here's another view of the tensioning rig. The sledgehammer is there to keep the sawhorse from tipping over.


All done! Finished piece with a layer of wax paper over the top.


Dammit! The wax paper was loose enough that the glass on the bottom was falling away from the mandrel. Stand it up on end, bust out some scrap butcher paper from the "packing stuff into boxes" closet, and tape it all together to provide gentle pressure to hold the flats on to the mandrel. I didn't just tape it down because my first test piece had major wrinkles in the corner when I tried to hold it tight with tape. The paper here is carefully aligned so it only pushes on the flats, not on the corners.


I let that stand for not quite long enough, pulled it off the mandrel, pulled one layer of wax paper out of the middle to give some clearance to slide the tube off the mandrel later. At about that point, I discovered the epoxy was too soft and the whole tube was deforming under its own weight. Through some magic (and I'm not sure how it happened, really), we got it back on the tube to harden up a lot more. My advice now is to check it regularly and don't pull it off the mandrel until it's hard to slide the tube off the mandrel. That way, you're less likely to pull it too soon. After it had hardened for a day or so, I pulled the tube off the mandrel again and set it aside to cure for another couple of days.

Looks good, here are some things that should relieve you of some grief.

Parchment paper. Does not stick to the epoxy and also will slip off metal real easily.

Next up use peel ply for that smooth hasle free finish.

Hope this helps.


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Parchment paper probably would have been a good idea. I got wax because we already had lots of parchment (in little sheets) and we were nearly out of wax paper. Peel ply would also be a good thing, again for next time I roll a tube.

We rolled the upper body tube today, and I got some miscellaneous bits and pieces done. Somehow, when I put together the CAD file for my coworker to cut fins on the NC router, I only put in three fins. Now I have to cut the last myself. Not really looking forward to that, but it could be worse.

Turns out I really shoulda used the parchment paper. Long story short: the tube stuck halfway off the mandrel because I let it cure a little too long. The surface texture of the mandrel was tearing the wax paper on the corners, which bunched up and jammed, peeling back more wax paper, jamming more... On Sunday, we tried the brute force approach, which moved the tube about 6", during which time my wife threw out her back helping me. We'll file that in the very big folder of things I owe her. After that, I knew I had to get smart, which led to the Rube Goldberg rig here:



Here's how it went. A 2x4 over the porch railing tied to 2 vise grips clamped to the top of the fiberglass tube. They bit into the tube, but I was going to trim that off anyway. The vise grip handles were taped shut to avoid any unpleasant surprises. The mandrel is hanging down. I clamped a 2x4 to the bottom of the tube to make a step so I could put my weight on it. It started extended much more than in the picture so that the mandrel couldn't fall very far if something went badly wrong. I adjusted the height halfway through pulling out the tube. The tube slipped out pretty easily under the influence of gravity. Right after this picture was taken, I could go up the porch steps and pull the tube off the top. As Eeyore said, thinking first and then hard work. Unfortunately, I did the hard work part first and then the thinking.

Ooookay, back to work now that I have the cert to fly this thing on a J or K. Thanks to Nytrunner for the poke to ask about progress.

I have parts cut for all of the couplers and the nose cone. The main coupler/AV bay is also largely assembled. Here's how that went down:



The coupler is built out of 1/2" plywood epoxied together. I cut the parts on a table saw to get reasonably accurate dimensions. This shows the parts clamped together with glue curing. Using plywood is kinda heavy, but I had the material on hand and this rocket isn't built for top performance anyway.



Here's a look at it with the corners rounded to match the corner radius of the tubes. I used a plane instead of my original router plan. It didn't take too long and was pretty satisfying. I am considering glassing the surface with a thing layer just for dimensional stability, but haven't made a final decision on that yet. Regardless, it will need sanding and a few voids filled before glassing. I'll glass both this one and the nose cone coupler at the same time if I do.

I haven't made a lot of progress on this for a while, but then kicked back into gear over the last week or so. In that time, I've:

Glued up the nose cone and the other coupler
Glassed the couplers and the nose cone
Glued the nose cone coupler to the cone itself
Added a couple more layers of glass to the nose cone to make it match the outside dimensions of the tube
Did external fillets
Dry fit!



More pictures below, including the raw glued up nose cone, coupler with switch band, glassed nose cone, and fin can with fillets.

I'm working on the drogue and main chutes in between other projects. A 66" x-form uses a truly ridiculous amount of fabric. Fortunately, I have some 1.1oz silicone coated fabric that packs like a dream. 5 yards rolls into a package a foot long and 2" in diameter.

Here's the finished drogue, 12" square in the middle. The main is #$%&@! enormous, 66" square in the middle. For both main and drogue, I built them out of three pieces: one long one in the middle and two squares on the sides. You'll notice I pulled an overlapping seam in the long piece to give the sides a little more structure to hold on to at the corners. All seams are 1/2" wide, and the hems are double rolled. On the main, the inside corners will also be reinforced with 1/2" tubular nylon in both directions to make sure they don't tear.

All of this is sewn on my mom's early 1960's Kenmore sewing machine. All steel except for the dials, and enough power to go all the way through your finger. Ask me how I know.

That nose cone is something else entirely. Was it done by hand like the coupler?

Amazing

Nytrunner said:

That nose cone is something else entirely. Was it done by hand like the coupler?

Amazing

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Thank you! I cut the plywood sort of freehand on a table saw, beveled the end, and epoxied it up. The corners were rounded with a low angle block plane.

Here's the finished baby, picture taken at the office's "Bring your hobby to work" day. My Eggtimer Quantum is resoldered so the drogue channel works, all flight electronics are mounted in the AV bay, and everything is ready for flight late Saturday or early Sunday. The motor will be a K740 C-star. Dry weight is just shy of 18 lbs. With the motor will be around 21 lbs. The main chute is orange down the center with navy blue squares on each side.



The paint is a blue hammered finish, which explains some of the roughness you see in the picture. I hoped that would cover some of the small pits and imperfections in the finish, but it really just brought them into sharp focus. Since the picture was taken, my older child painted yellow spines on the fins so it looks a little more like a dinosaur sail. I hope to get video and stills from the launch. I will definitely report back.

Resisting the urge to call it a flying fence post.

Nytrunner said:

Resisting the urge to call it a flying fence post.

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I wouldn't (resist, that is).

Came out pretty cool. If I were a vampire I'd keep clear of that one.

Nytrunner said:

Resisting the urge to call it a flying fence post.

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I resemble that remark...

neil_w said:

I wouldn't (resist, that is).

Came out pretty cool. If I were a vampire I'd keep clear of that one.

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Thank you!

Successful flight! Dimetrodon flew at Fire in the Sky in central Washington Sunday morning. I had to take it off the pad because I was getting a flaky signal from one of the two altimeters. It turned out to be a loose power connection. Back to the pad on the next rack and it went up on a K740 C-star. It was a beautiful straight boost to about 4350 feet (~200 over the sim) and then a long ... long ... long descent. The main came out at apogee, and it was way large. At one point, it caught a thermal and starting rising. I took a bearing as it dropped behind a rise then walked out with my recovery team to pick it up. 3/4 of a mile down the bearing line, there Dimetrodon was, laying across a shallow puddle.

Some pics:


I have video of the boost, but it's presently on Facebook and TRF wants to link to it instead of embedding. If anyone has suggestions for embedding that video, let me know.

Thanks to WAC for putting on the launch and to all the people who reinforced the decision not to fly with a flaky altimeter. I believe that the main came out at apogee because there wasn't enough friction between the nose cone and upper body and inertia took care of the rest. That didn't happen in ground testing because the nose cone landed enough point down that the dirt stopped it. Something to think about for future ground tests.

I apparently was not paying attention because I had not realized it was so big... impressive. Congrats on the successful flight!