More Half-Baked Designs Thread

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neil_w

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What is the difference?
I believe the nearest thing to an "official" definition of a ring fin would be something that is coaxial with the airframe.

Additionally, the aspect ratio (length/diameter) of ring fins would typically be low, often less than 1. Tube fins would typically have a much higher aspect ratio, greater than 2. I believe there is actually an ideal (from an aerodynamic perspective) aspect ratio for tube fins, although I don't recall the number.

Putting aside all those "typically"s, there are all manner of variations that live in a gray area between the two.

Definitely ring fin:
1629905138233.png


Definitely tube fins:
1629905175429.png


You decide:
1629905256885.png

1629905407899.png
 

jqavins

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The top one would probably work for that, the bottom one probably not.
That pretty much coincides with my own gut feel.
  • The first one, no problem.
  • The second one, probably no problem either, but I'm not so sure.
  • The last one, dicey. I hadn't considered the boundary layer effect, so even dicier than I thought at first, but I still wouldn't count it out.
Does the boundary layer depth depend on the tube diameter? I wouldn't think so. If not, then it could be overcome just by scaling the whole thing up.

The open round things don't encircle the body, so tube fins, despite the small L:D.
The (mostly) open round things do encircle the body, so ringtails. But something of a grey area, since each one, as interrupted by the others, does not encircle the body, just like mine don't. But unlike mine, that interruption is not by the body. So it's grey, but I'd have to say light grey if ringtails are white; clearly (to me) more ringtail than tube fin. Maybe these intersecting ring designs are just a thing unto themselves.
 

Dotini

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I think this might work quite well, and I'd like to build it. If somebody would provide me with the calculated length of one of these ring segments relative to a BT-50, 55 or 60 tube, I will build this right away and fly it.
1629896936108.png
 

jqavins

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I can do the calculations for you soon. As show, the central circle is 2 inches in diameter, and the truncated circles are 1¾, or an 8:7 ratio. So if you use a BT-55 (33.7 mm OD) and BT-52 (29.9 mm OD for Apogee's version) fins you'd have a 1.127:1 ratio, which is almost perfect. Then I or someone can compute the part of the fin tubes to remove. If you want a different size, just name the tubes and someone will generate the numbers. (BT-80 and BT-70, for example, gives you 1.173:1

But I can't do it right now.

Incidentally, I was thinking of a bias cut on the leading edge of each fin tube. But of course you can do what you like with it.
 

Dotini

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I can do the calculations for you soon. As show, the central circle is 2 inches in diameter, and the truncated circles are 1¾, or an 8:7 ratio. So if you use a BT-55 (33.7 mm OD) and BT-52 (29.9 mm OD for Apogee's version) fins you'd have a 1.127:1 ratio, which is almost perfect. Then I or someone can compute the part of the fin tubes to remove. If you want a different size, just name the tubes and someone will generate the numbers. (BT-80 and BT-70, for example, gives you 1.173:1

But I can't do it right now.

Incidentally, I was thinking of a bias cut on the leading edge of each fin tube. But of course you can do what you like with it.
Thanks! I'm in the process of finishing up one model and starting another while some critical parts are on the way for a third. For this current fin tube build, I'd like to work with a BT-55 tube. But I'd like to make the ring segments from 0.020" PETG flat stock, so I'll need the cut length that when formed describes the desired arc.
 

jqavins

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Can do.

EDIT: 1.987 inches (so call it 2) along the outside of the curve.

How do you plan to bend the sheet into a circular arc and attach it in just that shape?

RE-EDIT: You may wish to know, the angle between the body tube and the fin where they meet is 112°.
 
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Dotini

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Can do.

EDIT: 1.987 inches (so call it 2) along the outside of the curve.

How do you plan to bend the sheet into a circular arc and attach it in just that shape?
Here is a model I'm completing now, shown in its infancy. I cut a strip of plastic, joined the ends and it formed a circle. It is very flexible.
DSC00369.jpg


What I plan to do is locate three polystyrene T-sections around the perimeter of the tube, then locate the arc segments in the angles thus provided. They should naturally slot right into place, then be secured with CA. If I make them the correct length, they will assume the proper shape.
 

jqavins

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I'm less confident that they'll become circular arcs, rather than, say, parabolae, cycloids, or some such other thing. Maybe I'm all wet and worrying about nothing.

In the example above, you've got the angle where the ends meet (zero) constrained, and my concern is that that's crucial for my design. I know it's crucial in the case above. Imagine bringing the two ends together with no overlap; they'd come to a point. My thinking is that you'd get circular arcs if you can constrain the angles at the ends, but something else of you don't.

On the other other hand, parabolae, cycloids, or some such other thing might be just fine anyhow. (The the first other hand is the one where all wet.)
 

Dotini

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I'm less confident that they'll become circular arcs, rather than, say, parabolae, cycloids, or some such other thing. Maybe I'm all wet and worrying about nothing.

In the example above, you've got the angle where the ends meet (zero) constrained, and my concern is that that's crucial for my design. I know it's crucial in the case above. Imagine bringing the two ends together with no overlap; they'd come to a point. My thinking is that you'd get circular arcs if you can constrain the angles at the ends, but something else of you don't.

On the other other hand, parabolae, cycloids, or some such other thing might be just fine anyhow. (The the first other hand is the one where all wet.)
Yup, you are right. I made a quick and dirty test article, and the results weren't the best. My methods and materials are not up to this task, so it's back to kit bashing for me.
 
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jqavins

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What you can do is use BT-52 or BT-50 size clear payload tubes. Cut away the unwanted section, and what you've got is preformed in the circular arc that you want.
 

neil_w

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What you can do is use BT-52 or BT-50 size clear payload tubes. Cut away the unwanted section, and what you've got is preformed in the circular arc that you want.
I haven't touched a clear payload tube in... oh, 40+ years. I'm suddenly having an idea that maybe I built an X-ray back in the day, but can't be sure.

Anyway, the point is: are the clear payload tubes rigid enough to hold shape when cut?
 

jqavins

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Obviously I'm not certain, but I rather expect so. That is, I expect they'll hold their shape when cut if they're not loaded. And once the remaining section is glued in place so the cut edges' locations and alignments are fixed, they'd be locked in, assuming they kept the circular arc shape when first cut.

But then, if I do it myself, I will be using regular cardboard tubes.
 

jqavins

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Epicycloids:
1630007912739.png

They're what you get with Spirograph when you put the little wheel outside the big one, and the pen all the way at the little wheel's edge. Forming those fins would be tricky. Maybe use (probably 3D printed) forms, both convex and concave. Press some ammonia softened balsa between them while it dries. Or something similar to that, e.g. hold some FG cloth and epoxy in said forms, card stock moistened with wood glue.
 

jqavins

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Yes, I expect that'd probably work, if my mind sim is any good.
 

jqavins

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Nope. And if I do it, I'll stick to the original notion of circular arcs, which give a much sleeker look, imo. Or, if I go with the epicycloid version (not likely, but who knows) then Spirgraph.
 

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i believe everyone (except Neil) is welcome to share new ideas??

heres 3 ideas I drew up this week,
one is actually a copy from an old design of the month. I believe it was called the Newfoundland space tanker? I added the stylised components in red. If anyone has the print it would be nice to see how close I am to the original

the next two are two engine clusters.
1D1452C9-F9CC-41F7-BBF7-F194870CC658.png

973E1CF6-28CB-45E4-B4FA-B4033EA472BF.png

357D85F7-F960-4F69-BDA7-EAB7C4E409E9.png
 

BABAR

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Inspired by the above, and many other intersecting ring fins such as this, I wonder about truncated tube fins. Sort of like "What if ring fins actually intersected the body tube?" Here are some way less than half baked possibilities for aft views of what I'm talking about. I like the last one best, but I'm not sure if there's enough open space in the partial rings to make it stable. (Well, stable without an ungainly amount of nose weight; I'm confident the CP will be shifted aft somewhat.) RS allows for tube fins, but with the tubes partially buried in the body I doubt it will calculate and simulate correctly. I'd have to build and test.
View attachment 479178
View attachment 479179
View attachment 479180
You haven’t added the third dimension, length.

that said, I think 1 and 2 would be fin, as long as you keep the length to diameter at saaaaay 2 or less . Looks like the Estes Super Neon may be 4 to 1, but it also has fins and a lot of base drag.

I am having a hard time finding articles which specify maximum effective tube fin length To diameter rationa (I can find articles on NUMBER of Tube fins, per Larry Brand 7, although again how he picked optimal length/diameter ration is unclear), and apogee articles on optimal ROCKET length to tube fin length and number of tube fins.

in any case, while intuition doesn’t ALWAYS work, I think of any fin mounting where there is a circumferential (even if not ROUND) containment if air as a tube fin or tube fin variant. Therefore, regarding @Dotini ’s comment, ring fins, especially if the are at or above the tail end of the rocket (you could have severely rear swept spokes where most or all of the ring is tailward of the nozzle), ring fins, especially if you use complete full length fin ”spokes” divide the ring into individual “chambers” So to me qualify as a type of tube fins. A specific characteristic of tube fins is the potential to RESTRICT air flow through the lumen. The maximum “worst possible case” is that there is no effective flow and it act as closed cylinder. Moreover, ring fins (the more spokes the WORSE) and Your “truncated” fins add some extra “corners” which I expect will cause both increased drag but will be even worse in constricting flow. I suspect model 3 is gonna be pushing it for airflow, again a lot depends on the chord length (nice fin terminology thread attached).

something @Dotini has considered, and would be an interesting modification to your current truncated style, would be what happens when the end of the ring or tube is caudal to the motor nozzle? Is there an “anti-Krushnic “ effect where the jet blast actual accentuates flow through the ring or tube, thus making it MORE effective than it otherwise would be?

again, the kind of thing it would be nice to have your own personal wind tunnel and all the associated stress gauges for testing.



fin terminology post 3.









960E4496-0750-413A-9D62-3D729E9F19BA.jpeg
 

BABAR

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Inspired by the above, and many other intersecting ring fins such as this, I wonder about truncated tube fins. Sort of like "What if ring fins actually intersected the body tube?" Here are some way less than half baked possibilities for aft views of what I'm talking about. I like the last one best, but I'm not sure if there's enough open space in the partial rings to make it stable. (Well, stable without an ungainly amount of nose weight; I'm confident the CP will be shifted aft somewhat.) RS allows for tube fins, but with the tubes partially buried in the body I doubt it will calculate and simulate correctly. I'd have to build and test.
View attachment 479178
View attachment 479179
View attachment 479180
You haven’t added the third dimension, length.

that said, I think 1 and 2 would be fin, as long as you keep the length to diameter at saaaaay 2 or less . Looks like the Estes Super Neon may be 4 to 1, but it also has fins and a lot of base drag.

I am having a hard time finding articles which specify maximum effective tube fin length To diameter rationa (I can find articles on NUMBER of Tube fins, per Larry Brand 7, although again how he picked optimal length/diameter ration is unclear), and apogee articles on optimal ROCKET length to tube fin length and number of tube fins.

in any case, while intuition doesn’t ALWAYS work, I think of any fin mounting where there is a circumferential (even if not ROUND) containment if air as a tube fin or tube fin variant. Therefore, regarding @Dotini ’s comment, ring fins, especially if the are at or above the tail end of the rocket (you could have severely rear swept spokes where most or all of the ring is tailward of the nozzle), ring fins, especially if you use complete full length fin ”spokes” divide the ring into individual “chambers” So to me qualify as a type of tube fins. A specific characteristic of tube fins is the potential to RESTRICT air flow through the lumen. The maximum “worst possible case” is that there is no effective flow and it act as closed cylinder. Moreover, ring fins (the more spokes the WORSE) and Your “truncated” fins add some extra “corners” which I expect will cause both increased drag but will be even worse in constricting flow. I suspect model 3 is gonna be pushing it for airflow, again a lot depends on now long front to back the




Larry shows data related to optimal number of fins and length of rocket to fin ratio, but I haven’t succeeded in finding how he determine the optimal tube fin itself length the diameter ratio.

Apogee has a couple of articles at least

which discusses drag

and


which talks about configurations, but again nothing on optimal ratios




View attachment 479757
 

jqavins

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You haven’t added the third dimension, length.
Indeed not. I guess "half baked" is putting it generously in this case; I just thought this would look sleek and cools and I wonder if it could work.

For l:I was indeed thinking of about 3:1, knowing that there is a maximum length for tube fins to work, so if 2:1 is more effective then so be it. I was also thinking of beveling the leading edge at something in the ballpark of 30° from the body tube wall, and the trailing edge in the ballpark of 75° degrees. My gut says the critical length for the L:D would be almost the outboard length where the tube is fully closed, since the flow restriction may be somewhat effective a little bit before that. Perhaps Kraken's designer can shed some light on that.

As I think more, of the leading face of the tube is beveled to such an angle, the root edges would end up really long with outer measure being three times the diameter, so more like 2:1 might look batter after all.
1630331563646.png

1630331592717.png

Of course, these plane trapezoids are rough stand ins for the actual shapes.

i believe everyone (except Neil) is welcome to share new ideas??
Absolutely. And, lest anyone forget, I was teasing Neil a bit, and he's totally welcome to post ideas here if he wants to.

I really like that third one. Funky.
 

Dotini

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Indeed not. I guess "half baked" is putting it generously in this case; I just thought this would look sleek and cools and I wonder if it could work.

View attachment 479825
View attachment 479826
Van Milligan has said tube fins are no more draggy than standard fins of the same area, but are less efficient.
So, considering your beveled tube fin, you may be making it even less efficient by moving the bulk of the area closer to the body tube and reducing the area farther out away from the body tube. Remember, the tip of the conventional fin is more more efficient than the root. So now, why not consider reversing your bevel, putting the longer edge outboard and the shorter edge closer to the body tube? Too uncool, not sleek enough?
 

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Indeed not. I guess "half baked" is putting it generously in this case; I just thought this would look sleek and cools and I wonder if it could work.

For l:I was indeed thinking of about 3:1, knowing that there is a maximum length for tube fins to work, so if 2:1 is more effective then so be it. I was also thinking of beveling the leading edge at something in the ballpark of 30° from the body tube wall, and the trailing edge in the ballpark of 75° degrees. My gut says the critical length for the L:D would be almost the outboard length where the tube is fully closed, since the flow restriction may be somewhat effective a little bit before that. Perhaps Kraken's designer can shed some light on that.

As I think more, of the leading face of the tube is beveled to such an angle, the root edges would end up really long with outer measure being three times the diameter, so more like 2:1 might look batter after all.
View attachment 479825
View attachment 479826
Of course, these plane trapezoids are rough stand ins for the actual shapes.

Absolutely. And, lest anyone forget, I was teasing Neil a bit, and he's totally welcome to post ideas here if he wants to.

I really like that third one. Funky.
I dashed something together a few years back out of paper towel tubes. Always intended to go back and make something similar out of fiberglass.
 

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jqavins

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Too uncool, not sleek enough?
Different sleekness/coolness aesthetic. No claim of outright superiority.

Also, unless I'm mistaken, the mechanism at work with a tube fin is sufficiently different from that of a conventional fin that comparisons like this are not valid.
 

BABAR

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Different sleekness/coolness aesthetic. No claim of outright superiority.

Also, unless I'm mistaken, the mechanism at work with a tube fin is sufficiently different from that of a conventional fin that comparisons like this are not valid.
Au contraire !

I am with @Dotini on this one.

I suspect the lateral component of tube fins is by far the most effective segment (same is true for standard fins as well, but tube fins have a relatively large lateral component unique in that it is relatively parallel to body tube, as opposed to standard fins which are perpendicular.)

if you were to use a sweep, I suspect the most bang for your buck would be a neutral forward edge with a backward delta on the rear edge, with the caudal tip of the delta on the OUTSIDE edge (although this does make it more prone to breakage unless you go rear eject.)

interestingly this set up may place the nozzle plume partially inside the fins. My MindSim wonders if this might augment/improve the efficiency of the fins. (Since forward end is open, this may be the Anti-Krushnic effect.)

Also, by putting the sweep further back, my understanding of rocket physics says the further back the surface area, the greater the effect on CP (more efficient.)

the data that says that tube fins have the same drag per surface area but are less efficient is interesting.

certain segments of tube fins, particularly those abutting Each other placed circumferentially around the body tube (the classic/easiest is 6 tube fins same diameter as the central tube) are understandably ineffective, particularly those that abut the central tube and those that abut each other where Two sides abut each other that segment likely acts as a single piece RE center of pressure, but since double thick causes more drag.

@neil_w or anyone in peanut gallery, is there any recognized name for the ring rocket attachments to the main body? I guess you could call them struts or spokes, but when they are long and thin (basically just like regular fins, except somebody glued a ring on the end), they have two effects.

first, they DO function simply as fins themselves (unless you make them with extremely short chords). Unless the ring is unduly heavy, which may occur with a 3D Printed ring, if the rocket is stable with just the spokes without the ring, it will be likely stable with the ring. If the ring is a lightweight material like the rings @neil_w and @Rktman make out of balsa, the ring will likely greatly increase/improve stability Margin.

second, depending on how many you have, they subdivide or segment the ring into noncylindrical “Tubes”. For two or fewer spokes, probably the spokes have little effect on airflow. Somewhere starting I am guessing between 3 and 6 spokes, the spokes themselves constrict the cross sectional area of the “tube” and start causing both an increase in drag and probably a decrease in effectiveness (approaching the “closed cylinder” equivalent of a long tube fin), at a certain point depending on the chord of the ring and the number of fins, as the latter approaches infinity (and likely a far smaller number, like 8-20) the ring probably acts as a closed disc.

I suspect this is very difficult for sim programs to “comprehend” and therefore accurately simulate.
 

neil_w

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@neil_w or anyone in peanut gallery, is there any recognized name for the ring rocket attachments to the main body? I guess you could call them struts or spokes, but when they are long and thin (basically just like regular fins, except somebody glued a ring on the end), they have two effects.
I've seen "pylons". I don't think I use any single consistent name for them myself, I just call them whatever pops into my head at the moment.
 
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