Boat Tail Question

I need some comments on my idea for a boat tail, from someone with more experience then I have.
I need a boat tail for a scratch built rocket. The main body tube is
a LOC 3.9" tube. I'd like the aft end of the boat tail to be about 3" in diameter and I'm considering using a LOC 3.9 to 3.0 airframe reducer. I'd cut the inserts off each end of the reducer leaving only the outer shell of a 3.9" to 3.0" boat tail. I'd use a 3.9" to 3.0" centering ring in the fore end of the boat tail. On the aft end the ID of the boat tail would fit flush with the OD of a 3.0" internal body tube. I'd then build my 38mm motor mount into the inner 3.0" body tube and mount my fins through the 3.8" tube to the 3.0" inner tube. I would fill the gap between the 3.9" and 3.0" tubes, and the gap between the 3.0" internal tube and motor mount tube, with expanding foam.
On paper it sounds like it will work, but...
Of course if I knew of a source for a ready built boat tail close to this size, that would be the easiest solution. I can't find one on ROL, thus this posting.
Thanks for your comments. Bill Schiller
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How about using a Slimline Tailcone then got built in motor ret.
Dan McCullough

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Easy. Just use a 3.9 nosecone, cut off the base of the shoulder, and the tip to the diameter you need. Then you just need to cut some centering rings to fit.
This is how the Velociraptor is designed.
http://binderdesign.com/velociraptor.html
Note that the aft set of fins actually mount through the boat-tail.
Mike Fisher Binder Design http://binderdesign.com
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tip
to
That is one nice looking rocket, especially the photo of it under power!.
I am at work on 3 different models right now that have through-the-boattail fins. But my boattails are more angular, rather than smooth from the body, like the Velociraptor. My scratch-build cost: $20~$30.
~ Duane Phillips.
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Thanks!
:) You build your whole rocket for about the same price of the nosecone that I cut up.
Mike Fisher
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through-the-boattail
body,
I cut

I wish I would have taken better pictures of the boat-tail construction... I would have referenced them...
In a nutshell:
Mounted fins to MMT with fins overhanging as far aft as I would build my boat tail, but the overhanging root chord edge of fins are precut out to OD of boattail tube. MMT is 43mm OD. Boattail tube/ring piece is 3" OD and 2" tall. Install aft bulkhead. Then rear 5.25" bulkhead has boattail tube/ring epoxied to it. Then epoxy and heavy fiberglass cloth is laid from rear bulkhead to rear of ring... let cure, sand, repeat, but this time add fillets to fins. - Boattail done-. Motor slides up inside boattail to rear bulkhead... which is ok as long as you are not deeper than 1 caliber of ID of boattail... which in my case I am roughly 2/3rds caliber. Plenty of room for bolts to go through aft bulkhead for motor retention.
If further aft motor placement is desired, then:
a) add thin ring to ID of boattail, and epoxy (or your favorite adhesive system) in place another bulkhead on that ridge. -or- b) providing your motor is long enough for stability in the MMT, add wide masking tape thrust ring (or small tube, etc...).
All that having been said... it was a lot of work. Maybe the boattail cost me all of $2 or $3 (tops)... but I think next time will spin one on the lathe with sheet foam just like a nose cone with no tip, and then just cut notches for the fin slots. Less trouble.
~ Duane Phillips.
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There was an excellent discussion on ROL about how to square the aft cut of the tailcone by filling with water and shinning a bright light through the nose (er tail) cone. You might be able to search and find it in the forums area.
Mike's comments are right on. I modeled my Level 2 bird after the BSD Apache which also has a nose->tail cone conversion. I went this route after building the LOC 4" V2 and seeing just how "simple" the tailcone is (notice simple is in quotes). Also, you mentioned the transition, which I also came up with. I bought both the 4->3 and 3->2. They are straight cones (is there a term for those?) and just didn't 'look right'.
If you can get one from any of those vendors, they probably have jigs to crank those out. But then precut isn't "scratchbuilt" so here's what I did. I didn't want anything below the tailcone. No nuts, bolts, screws. So I went with slimline for motor retention. A 4" LOC nosecone was inserted to a length of 4" body tube (TIGHTLY) and from that the aft flat cut was made on a chopsaw. I cut it so the aft outside diameter was just a hair smaller than the outer diameter of the slimline (more later). The finslots are the toughie. Either a jig that holds the cone in position and a bit moves over and into the cone, or a method to hold the cone and slide it into the blade. I went with sliding the cone into a tablesaw blade using the same 4" tube. I marked the tube for the 4 finslots and used those to get the 90 degree angles. The bodytube was held against the fence and the unit was slid into the blade (the blade being 2 blades separated by a spacer, similar a dado, to get the thickness of the ply). Tube was rotated to the next mark and repeat. I believe John Coker has a jig on his website to run a handheld router into the cone or at least for slotting large body tubes. I didn't feel too comfortable with any of the bladed operations. Here's an important tip, which someone might as well get something out of,... Turn the saw blade around. Don't cut the material, burn through it. The chopsaw worked fine going through the tip, as there was a thickness and I had that section of body tube. When I attempted to cut the shoulder of the cone (to remove the 'bottom'), the chopsaw flipped it across the shop, causing some damage on the shoulder. DuMb. There was nothing to really hold on to and I should have just gone the dremel route. Cutting the slots with the blades backwards after that,... Where the heck was I? Oh, I mentioned the slimline, so what I did next was wrap a piece of sandpaper around some 1 1/2 or 2" PVC (whatever was close to the slimline diameter) and sanded out the 'tip' of the cone. I sanded such that the hole was all the way out to the outer wall, instead of any flat on the bottom (I hope this is making sense). So instead of a thin thickness to bond to the slimline, I had 1/2" or so. And here's the cool part, the slimline slid in such that the tailcone slopes right to it's diameter, there's no lip or noth'n down there. So that became my aft centering ring. I made a forward ring which was 3.9" and rested on the top of the cone, not inside. Slimline glued to motortube, then into the tail cone, the forward centering ring {not glued} slid on to center the MT and the cone {not glued} inserted into a section of bodytube to keep everything aligned.. A hole was drilled through the rocket's bodytube into the tailcone and that was used to mount the Tnut for the rail button. Once the glue dried, I removed all the temp structure and glued in the fins. The Tnut was glued inside the tailcone over the hole and covered with tape I then epoxied the centering ring onto the motor tube. In my case it didn't bond to the shoulder because of the saw damage. Holes drilled in the shoulder allowed me to pour in foam. Then epoxy on the shoulder and into the bodytube being careful to align the rail button hole.
Lots of steps to miss so walk through before cutting and gluing. I'll not say the measure twice thingy,... ;)
Joel. phx
The majority of mounting the tailcone tips/understanding came from LOC's instructions for the 4" V2 (pre-production), a nicely done package.

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[snip]
Bill,
I used a LOC AR-3.00-2.14 airframe reducer to make a tail cone for one of my scratch built 3" diameter rockets. It took a bit of work but it really turned out great.
I removed the 2.14" mating section by cutting through the angled transition section at a point 2" down from the 3" diameter end. This resulted in a cone 2" long with an inside diameter of 2.25" and an outside diameter of 2.56". I cut the mating section on the larger (3") end down to 1" in length. This gave the tail cone plenty of mating surface to the 3" body tube and allowed me to place a 3.0-1.5 centering ring within an inch of the end of the body tube. The 1.5" (38mm) motor tube is cut short enough so that a Slimline (snap-ring type) motor retainer glued onto the motor tube fits flush with the end of the tail cone. The rocket has a set of long G-10 strake fins forward and a set of smaller G-10 fins aft that follow the angles of the body tube and the tail cone. All of the fins have thru-the-wall tabs that glue into slotted wood pieces glued to the motor tube. It's built very strong and the rocket looks really sharp. I made a set of very detailed drawings in Corel Designer (CorelDraw) and I can email you the files if you would like to see them. I can export the drawings into some other format too if you would prefer that.
You can also check out Performance Rocketry at www.performancerocketry.com. They sell various types of boat tails, fin cans, nose cones, etc.. I believe they're all made from fiberglass but I'm not sure. I also don't know if they make them in the size you need. But it may be worth it to have a look. Good luck.
Regards, Michael Newton
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Bill Schiller wrote:

Here's one very similar:
http://www.flash.net/~samily/Midget600-20-3-2.jpg
It's a 6" airframe with a 54mm MMT. The fins are mounted to the MMT. The aft ring is 4.5". I drilled holes in the ring through which I was able to pour foam.
I simply used masking tape to form the tail cone, then poured in the foam. The foam bulged the tape, but that's OK. After it had set, I removed the tape, then used a rasp to cut it down. After a couple layers of 'glass and some FNF, it looks purty good. BTW, the foam is not completely covered by the glass cloth; only at the fin roots. In the middles, it's just FNF over the foam.
HTH.
Doug
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Very nice idea... I didn't have any 2-part foam when I did mine. So I went with what I had on hand.
~ Duane Phillips.
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On Sat, 06 Dec 2003 23:03:52 GMT, Bill Schiller

Leon Kemp described an interesting method in an article published recently in the UKRA magazine/newsletter 10...9...8...
http://www.ukra.org.uk/newsletter/volume7issue3/34.html http://www.ukra.org.uk/newsletter/volume7issue3/35.html http://www.ukra.org.uk/newsletter/volume7issue3/36.html
Looks like a lot of work, but gives you a lot of control over the final shape.
--
Darren J Longhorn http://www.geocities.com/darrenlonghorn /
NSRG #005 http://www.northstarrocketry.org.uk /
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I did this on a LOC NORAD. If you go to http://www.canadianrocketry.org /, look in "Association Information" for the "Earthrise Newsletter", and download Vol 2 No3, I wrote a fairly extensive article on the process, which starts on page 10.
The photo at the top at the start of the article is from the NORAD, as are a few other shots. Most of them, though, are from a second project that I did specifically for the article, as a number of people requested that I write the article on how I had done this, and I hadn't taken enough pictures.
Leon Kemp
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