Twin tube fin rocket

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Lamp

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Location
Victoria, Australia
I had been thinking about stability and decided to make a rocket with 2 tube fins.
First I designed, built and flew a scaled down LPR version.

LPR 2 tube.jpg
This is the LPR version flying on an A8-3

After this flight (which went well), I designed a HPR version with a 2.6" body, 38mm motor mount with two 4" tube fins mounted through the wall. I modeled the rocket in Rocksim which let me check radial stability which was surprisingly uniform. Although I could not figure out how model it with the fins TTW, I designed it to be overstable and figured the difference would not be big enough to invalidate the design. The rocket has so far had two flights and performs well. :)

DSC_4433.jpg
This is the launch shot of the second flight on an H268R.


DSC_4436.jpg
A shot in flight

DSC_4442.jpg
Recovery...

DSC_3329.jpg
A shot of the landed rocket after its first flight showing the TTW fins.
 
What would you like to know...

It is a pretty standard dual deploy build.. the only real difference is the fins. I would post the Rocksim file if I could figure out how to do it.

The only real thing that is different is the fins and how I mounted them TTW. Sorry, but I don't have any photos of this.

I wrapped the bottom of the body tube with a sheet of A4 paper to accurately measure the circumference by marking where the paper started to wrap around itself. I cut measured lengths from a 4" FG tube and evened them up. I assembled the top and middle centering ring on the motor mount, spacing the middle centering ring to match the location where the top of the fins would be. I did not attach the rear centering ring.

When the epoxy was hard, I partially slid the MMT into the body tube, put a fin on the MMT next to the base of the body tube. I marked each side of where the fin intersected the body tube on the paper which I left wraped around the tube. Using this I could measure and accurately mark a fresh sheet of A4 with the center point of each fin and where each fin would go through the body tube. I transferred the marks up the body tube using some angled aluminium. I then glued in the MMT.

I measured and marked 1/2" up from the base of the body tube and the length of the fin. I used a dremel with a plastic cutting disk to cut the fin slots. I also used a dremel to make a vertical cut through each fin and a mini flat file to widen each slot. By distorting the fin tube, I could get one side of the cut into one of the slots and work/roll it around towards the other slot. I could then get the other side of the cut into the other slot. I worked the fin back so the cut was lined up and even onto the MMT. I used a strip of fiberglass to join the fin tube back together inside the body and then epoxied in the rear centering ring.

I drilled small holes into the body tube and injected epoxy using a syringe to form the internal fillets.

Everything else was pretty standard.
 

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  • 2.6 inch 2 tube fin.rkt
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