Building a Bohica's Dead Ringer

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Rich Holmes

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Last October I came across this YORF thread and this (archived) TRF thread having to do with rockets that use centering rings for drag stabilization instead of fins. Bohica’s two “Dead Ringers” — not to be confused with the FlisKits rocket kit of the same name — were similar but not identical designs using different motor mounts; there was a 13 mm one and an 18 mm one. Both used BT-50 tubing but the rings were different. Bohica provided a drawing with some measurements of the 13 mm version and a written description of the 18 mm. Solomoriah independently came up with a very similar idea, which he called the Spike, using BT-20 and three or four CR2060s. I haven’t found any posts indicating whether or not he ever built it.
Of course OpenRocket hasn’t a prayer of simming this design, although CPMcGraw put in an interesting effort which however may or may not have any relationship to reality. Bohica said his 13 mm version was “super stable” and I think he meant to imply the 18 mm one was too, with about 0.5 oz weight in the nose cone. Wbblair built an 18 mm one and reported it was stable during boost but flipped, once, to fly tail first after burnout; he had less weight (7 grams) in the nose, though. Micromeister built a T-3 based downscale. With 5.4 g nose weight he said it was “dead on stable… arrow straight boost…”
So with proper weight, it’ll work. And it is, to my eye, cool as all heck, so I’m building one.
I like Bohica’s 13 mm design best, so I’m working from that; but given how draggy the rings are, I’m going with an 18 mm motor mount.
View attachment 152521
 
That is awesome looking! The rings remind me of the effect when something breaking the sound barrier. How would the launch rod go through the rings though? Would there be holes in the rings?
 
Holes in the two big rings. The two smaller ones aren't wide enough to put holes through, so the launch lug is on a standoff and the rod clears the two smaller rings. At least I think that's how it's going to go.
 
Or maybe holes in three rings and offset to clear the first. It's a mystery! We'll see.
 
I set this up for a replaceable Kevlar shock cord leader. Never done this with an 18 mm motor mount in a BT-50 tube, and it needs a modified procedure; the centering rings are too thin to punch a hole through for the guide straw.
Instead I cut a piece out of one centering ring wide enough for the guide straw (which will be made from the cotton swab).
View attachment 152670
From the other ring I cut a piece wide enough for both the straw and the motor hook.I glued them to the motor mount tube, with one edge of each gap lined up.
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I also glued in the thrust ring and cut a slot for the hook, which I secured for now with a piece of Scotch tape. I glued the straw to the tube and to the lined-up edges of the two gaps.
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After applying glue fillets to the rings I wrapped a strip of duct tape around the tube to secure the straw and hook, and trimmed the straw flush with the centering rings.
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I've noticed a problem with cutting pieces out of centering rings like this, which is that when you do that the rings tend to open out a bit, and they don't really get squeezed back down to their original diameter when you glue them to the motor mount tube. So I had to sand them some to get them to the point where I could dry fit the mount into the body tube without deforming something. Wonder if it'd make any sense to cut the sections out after gluing them to the tube? You'd have to be careful to avoid gluing down the section to be removed. Or... maybe it'd be feasible to use a short piece of body tube as a circular clamp when gluing the rings down? Needs some thought. Anyway, sanding works.

Next I took a piece of blue tape long enough to go through the body tube, folded most of it in half, and stuck the end over the guide straw and forward centering ring.
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I applied epoxy inside the body tube and on the outside of the rear centering ring, pushed the motor mount into place, and pulled the tape off. I was able to see light through the guide straw; it's not clogged! This should work.
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The one other thing I did was to attach the launch lug, since I want to use it to align the holes in the rings. (Two of them.) First a standoff, a hair under 3/8" wide; then the launch lug.
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And we're done for the night.
 
Quite possibly one of the coolest rockets I've seen. KUTGW!
 
So I finally figured out how to make an OpenRocket simulation of the Dead Ringer that may not be completely useless for flight simulation purposes.

View attachment 152744

OR can't simulate this rocket as-is because it can't handle drag stabilization or lack of fins, so it puts the CP up in the nose and says it's completely unstable. You can override the CG and put it in front of the CP, but that seems kind of silly: the CG as calculated is presumably pretty accurate, while the CP is completely fictitious. But I've now learned about the trick of overriding the CP by adding a massless conical transition on the back of the rocket. I even made it vaguely flame colored:

View attachment 152745
I adjusted the transition to put the CP in what seemed a reasonable place. Bohica said his 18 mm Dead Ringer was stable with 0.5 oz mass in the nose but unstable without; so is this model.

With that stabilization we can try some simulations. OR says this gets a whopping 64 feet off the ground on an A8-3. Draggy! The whole flight lasts only 6 seconds, and the recovery device (I put a chute in) deploys barely over 20 feet off the ground. For comparison, if you delete the rings (but keep the CO override) it sims to 183 feet.


In fact this is almost certainly too draggy. Bohica had a 13 mm Dead Ringer and he said it flew on a 1/2A. So OR is probably overestimating the drag.


Anyway, flights on a B6-2 and a C6-3 sim pretty well, with altitudes of 110 and 214 feet and deployment near apogee. We can probably regard these as worst case scenarios.
 
If there's one thing I learned from
View attachment 153707
Norm Abram

it is that you don't measure two things against a ruler if you can measure one against the other. So I didn't take a ruler, and measure the exact height of the launch lug from the body tube, and then transfer that measurement to the tail rings... no, I just slid a ring up against the launch lug, and marked it.
View attachment 153701

Then I used a rotary punch to punch the holes in the two largest centering rings.
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Gluing the first two rings in place — after first marking the ring positions on the tube, and making sure the launch lug line was extended down to the bottom of the tube. Elmer's yellow glue here.
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And the last two rings. A dowel through the launch lug and ring holes makes sure everything's aligned.
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Fillets. Titebond No-Drip No-Run.
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And that's about all I can do until painting weather returns. We have snow piling up tonight, so that won't be soon.
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Any chance you'll fly it nekkid, so I can tell my wife I'm done drooling over something she'll never understand??

Hmmm?

Hmmmmmmm?

Pretty please?

g-train
 
Probably not. Not at all sure I'd fly it before Spring even if it were painted.

But what's stopping you from flying one?
 
Oh, wait, I realized there was one more thing I could and should do before painting: Prep the nose cone.

Bohica said it needs 0.5 oz, or 14 grams, added weight. I went to rough up the inside of the Estes PNC-50YR nose cone only to find it was pretty rough already.
View attachment 153816

So I dampened the inside of the nose cone a bit, dropped some lead BBs into water, then fished them out and added them along with some Gorilla Glue until the total added weight was 14 g.
View attachment 153817
Okay, 13.96. EXCUSE ME.

Then I glued the shoulder piece in place with Testor's plastic cement. And yes, that's the sixth adhesive I've used on this rocket (Testor's plastic: Nose cone; Gorilla glue: Nose weight; Titebond No-Drip No-Run: Fillets; CA: Kevlar guide straw; 30 min epoxy: Motor mount installation; Elmer's wood glue: Everything else.)

Now we wait for painting weather.


 
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Flying Ring-shank Nail of Death.jpg
This is the Flying Ring Shank Nail of Death. Built her a few years back. Flights are dead straight, although once thrust stops, it decelerates pretty quickly. That's to be expected, though, with such a draggy model.
 
I can see applications for clear rings being used on a scale model of something with vectored thrust.. like a Falcon 9... :)
 
I really like the ring shank design. Don't know how you'd sim it, though.
 
Ultra short transitions I think and the base drag trick ought to get pretty close.
 
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I'm assuming you meant ultra "short" transitions. But what is the "base drag trick"?
 
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