Tube Inside a Tube Fin

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jqavins

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I don't see another subforum that looks to be better for this question, so here we are.

Would a tube fin function if it had a solid cylinder suspended on its axis? Picture a good sized tube, say BT-60, with internal pylons supporting a BT-5, which is closed off. Maybe closed off with a nose cone. And maybe it has an A3-4 in it. Or maybe no engine but a boat tail or completely pointed tail cone?
 
Without knowing the answer, I would guess that it would. The sides of the tube fin could still have an angle of attack and provide a corrective force. My guess.
 
I have an untested BT-20 sitting inside a BT-80(?) that I'm waiting to test when the honey-do list subsides. Not certain that a full length tube will adequately function as a "fin" though the inner body is a little shorter in hopes of pulling more air through the interior of the BT-80.
 
It very much depends on how long the tubefin is, ditto the inset portion.

If the outer tube is, say, about 2/3-3/4 as tall as it's diameter ; and the inset bit is about half that, tapered on nose and tail -- you'll notice some increased drag.

If the outer tube is about 1.6 times as tall as it's diameter, flow will choke up and it will drag like a closed cylinder -- put whatever you want inside.

Whatever you do, don't try to get into the transonic region
 
If you're under 1.5 L/D mentioned above, wouldn't the inside vanes act as fins?

My NASA Pegasus upscale has 24mm mounts inside 3" tubes - but the tubes are way longer than 1.5 diameters. It's not stagnant, as they whistle, but I'd accept the notion that thru-flow speed is greatly reduced compared to over-flow.
 
I took another, closer and slower look at this:
If the outer tube is, say, about 2/3-3/4 as tall as it's diameter ; and the inset bit is about half that, tapered on nose and tail -- you'll notice some increased drag.

If the outer tube is about 1.6 times as tall as it's diameter, flow will choke up and it will drag like a closed cylinder -- put whatever you want inside.
What the above does not cover is any case where 3/4 < L/D < 8/5, or where the inner tube is longer than the outer one. In my rough concept, shown in my equally rough sketch, 3/4 ≤ L/D ≤ 3/2, shown with L/D = 3/4. And the inner tube, in concept and sketch, is always longer than the outer. So I'm right out of dhbarr's ballpark. Thanks all the same for the answer.
Whatever you do, don't try to get into the transonic region
Oh, heck no! I don't think I'd ever try to take any sort of funky fins much past 0.5 mach. For performance go with plane old boring finny fins; for style points go slow.

If you're under 1.5 L/D mentioned above, wouldn't the inside vanes act as fins?
I've often wondered how much stabilization with ring tails comes from the pylons acting as plain plane fins even if the ring were left out. And what I've got here could be looked at as four ring tails attached around the central airframe. The fact that I have no idea what else to say in response is proof that I have a lot to learn about tube fins and ring tails.

But I knew that, which is why I tried to ask a simple "Would this screw things up?" question. I guess it's never really that simple.
 
If you're under 1.5 L/D mentioned above, wouldn't the inside vanes act as fins?

My NASA Pegasus upscale has 24mm mounts inside 3" tubes - but the tubes are way longer than 1.5 diameters. It's not stagnant, as they whistle, but I'd accept the notion that thru-flow speed is greatly reduced compared to over-flow.

I've been looking for a whistling rocket design for a while. I searched the rocketry forum for your Pegasus models. Was this the upscale or downscale that whistled?
 
Also, were trying for a whistler and got this to work? Or did it surprise you by whistling? (From what I've read, the latter seems to be far more common.)
 
I've been looking for a whistling rocket design for a while. I searched the rocketry forum for your Pegasus models. Was this the upscale or downscale that whistled?

3X Upscale. 4" body, 3" side tubes. Built from MDRM and Leviathan parts.
Not a whistler on purpose.

To my knowledge, the main requirement for a rocket to whistle is to have two leading fin edges with some separation. I think all the Wildman Darkstars I've seen have whistled.
 
3X Upscale. 4" body, 3" side tubes. Built from MDRM and Leviathan parts.
Not a whistler on purpose.

To my knowledge, the main requirement for a rocket to whistle is to have two leading fin edges with some separation. I think all the Wildman Darkstars I've seen have whistled.
Thanks. I need a new project!
 
Squared fins instead of rounded lead to some good whistling as well. I've got an upscaled Cougar and a scratch Warlock-ish rocket that whistle pretty good without split fins.
 
Squared fins instead of rounded lead to some good whistling as well. I've got an upscaled Cougar and a scratch Warlock-ish rocket that whistle pretty good without split fins.
Thanks. I have found that real plastic whistles and "whizzers" like from Estes MiMi don't seem to do much. I even tried putting in an "air funnel" to the plastic whistle. No joy.
 
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