Another finless rocket

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Dotini

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Taking into consideration Reynolds number, boundary layer, laminar flow, detached flow, turbulent flow and all the myriad details of aerodynamics is no straightforward matter for this model rocketeer. Sometimes a simple test in the field can help. My recent launch of the Animist revealed outstanding stability and performance compared to its weight, power and the other rockets it was tested against, with no trace of overstability or weathercocking. One key question I wanted to investigate was the minimum distance between the outer diameter the ring and the major diameter of the airframe tube. By convenience of the parts available, the distance I tested for was 0.42". For my next model, I will retain about that distance, but scale up the diameter of the ring and tube.
 

jqavins

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I suspect the difference in diameters should increase as both diameters do. Should it scale linearly? Probably not; nothing is linear with this stuff. But it's probably not a fixed difference in diameters either. My gut says it's somewhere in between the two, but my gut has proven itself to be just as ignorant as my head.
 

Dotini

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I suspect the difference in diameters should increase as both diameters do. Should it scale linearly? Probably not; nothing is linear with this stuff. But it's probably not a fixed difference in diameters either. My gut says it's somewhere in between the two, but my gut has proven itself to be just as ignorant as my head.
By convenience, I will be using a BT70 section coupler as the ring, and a BT50 tube transitioning to a BT20. I think that works out to about 0.57". So yes, a bit of an increase in gap. Thanks for your feedback!
 

BABAR

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Since it seems like you're speculating, I'll join in. If anyone knows, I will withdraw all such speculation.

On the first point, I'd be surprised to find sealing the tube is a negative, since the plume continues to fill the volume behind the rocket as said volume increases.

But if the tube is sealed at the bottom end and not completely filled by the rocket the flame will precede the rocket out the front; we've all seen pictures of that, I'm sure. So I would think the finish would be badly scorched. One might consider using a sabot to fill the tube and protect the rocket until it emerges. Which would increase the pressure behind the rocket and any gun barrel effect, as well as protect the paint.

Regarding the second, I think you've got it right, 99%. The difference with a rocket that's on a guide and also in free air is that the fins or GDS (or whatever else) begin operating, as you said, before the end of the guide. The two means of assuring straight motion overlap. With a tube launch there is a hand off from one to the other with no overlap. Is there a moment after emerging from the tube that, even though there's enough speed for the fins or whatever to work, the necessary air flow geometry is not yet established? A hand-off transient? And if so, does it last long enough to do any mischief? (If it's scant milliseconds then simple inertial will keep the rocket going straight enough.)
Agree that tube launch does have an instantaneous “handoff” (i like the term!) from material guided (rod or tube) to fin guided stability, whereas the hand off is gradual with a rod or rail. But in both cases there is also a component of intertia, the rocket not only has forward momentum but also orientation inertia, it takes some energy to deflect it i. Pitch, yaw, and roll, and that takes time. Hence my observation that even the most unstable rockets usually make it a few feet or more above the rod or rail before going catawampus, and yes I did have to look up the spelling and Im not sure experts agree

 

Dotini

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Gerald Bull, the assassinated supergun engineer, and others before him, developed the idea of pressurizing the tube to extreme levels by detonating additional charges as the projectile passed up the tube.
 

jqavins

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But in both cases there is also a component of intertia, the rocket not only has forward momentum but also orientation inertia, it takes some energy to deflect it. Pitch, yaw, and roll, and that takes time.
Didn't I say that?
And if so, does it last long enough to do any mischief? (If it's scant milliseconds then simple inertial will keep the rocket going straight enough.)
I don't know how many milliseconds inertia will get you when there are no fins at all (as opposed to inadequate ones). Maybe it's even a thousand of them. Two thousand I doubt, and if it's only a few I'm sure you'd get away with it.
 

BABAR

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Didn't I say that?
I don't know how many milliseconds inertia will get you when there are no fins at all (as opposed to inadequate ones). Maybe it's even a thousand of them. Two thousand I doubt, and if it's only a few I'm sure you'd get away with it.
Yup, you did. I missed it.

I still think best be is an open tube (both ends)

From a practical standpoint, it makes igniter hookup a heck of a lot easier. Closed tube base end certainly possible, but more complicated.

And if a rocket gets stuck on a rod or wire, the damage is mainly to blast plate or as @kuririn showed, the legs of the pad. If the rocket gets stuck in the tube (which I think is likely more common than a rod or rail stck), likely will torch much of and potentially ALL of the rocket.
 

BABAR

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Gerald Bull, the assassinated supergun engineer, and others before him, developed the idea of pressurizing the tube to extreme levels by detonating additional charges as the projectile passed up the tube.
Timing is everything!
 

Dotini

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Yup, you did. I missed it.

I still think best be is an open tube (both ends)

From a practical standpoint, it makes igniter hookup a heck of a lot easier. Closed tube base end certainly possible, but more complicated.

And if a rocket gets stuck on a rod or wire, the damage is mainly to blast plate or as @kuririn showed, the legs of the pad. If the rocket gets stuck in the tube (which I think is likely more common than a rod or rail stck), likely will torch much of and potentially ALL of the rocket.
Somehow I think the founding fathers of model rocketry would frown upon us launching finless rockets from tubes. The idea of offending Vern Estes or G Harry Stine seems almost as heretical as offending Madison or Jefferson by violating the Constitution. That said, I must hurry back to the shop for more glueing on my two new ringtails. :rolleyes:
 

BABAR

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Somehow I think the founding fathers of model rocketry would frown upon us launching finless rockets from tubes. The idea of offending Vern Estes or G Harry Stine seems almost as heretical as offending Madison or Jefferson by violating the Constitution. That said, I must hurry back to the shop for more glueing on my two new ringtails. :rolleyes:
Maybe, maybe not. I don’t have an inside information on either of these gentleman. I suspect however that as long as it was provably safe, was allowed by both the safety code and applicable local laws, fun and/or cool, they would be fine with it.

ringtails are fun. You should try this, it’s a nice small field rocket. Easy scratch build for someone of your skills.

 

Dotini

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Highly encouraged by the test results of my first two ringtails models, I'm building two more for further experimentation. Here's what I have so far:

DSC00077.jpg

Left, 0.21 oz, BT20/BT50 transition, rings supports are 1/16" basswood skinned with 65 lb cardstock. The ring will partially enshroud the aft end of the tube and engine.

Right, 0.41 oz, BT50 tube with motor mount, skinned ring support vanes. The "advantage" sought here is the volume within the ring will not be intruded into by the tube or engine.


Edit:
DSC00086.jpg


Rockets now mocked up with all parts partially fitted.

On left, 0.81 oz., on right 1.16 oz. due to motor mount, heavier baffle, nose cone and ring support structure. Both can carry a 6" streamer.
 
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jqavins

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Nice looking designs. My only suggestion is to make the next one with much smaller pylons (ring supports). The pylons on the smaller one are probably sufficient fins on their own, and on the larger one they most certainly are. With the ring doing the stabilizing, the pylons needn't be anyhing more than is needed structirally. They could be sticks if they and their attachments are strong enough.
 

Dotini

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Nice looking designs. My only suggestion is to make the next one with much smaller pylons (ring supports). The pylons on the smaller one are probably sufficient fins on their own, and on the larger one they most certainly are. With the ring doing the stabilizing, the pylons needn't be anyhing more than is needed structirally. They could be sticks if they and their attachments are strong enough.
It was my supposition from reading Milligan that proper fins needed to be about 1.5 x the major tube diameter, and the tube between 10 and 20 x the tube diameter in length for a basically stable rocket.
 

jqavins

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When conventional fins are used, yes. But these are not meant to be fins. The ring does the fins' job instead, so it just needs to be soundly attached. Or it can be there just to look cool, which it does, but beware of becoming overstable. (Overstable is when the static margin is so great that the rocket weathercocks really badly.)

Are you using OpenRocket (OR) for these designs? OR doesn't treat ringtails as "fins" (yet), so it underrates the stability in designs like this.

@neil_w uses ringtails a lot, and is also an OR expert. Neil, how do you handle this?
 

neil_w

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Neil, how do you handle this?
First off, I create a second OR model that is purely intended for CP and flight simulation, vs. the original model that is intended for parts listing, construction details, and appearance.

In my "flight" model, I replace the rings with an equivalent number of fins. I have my own way of doing this that is based purely on my own intuition (and therefore probably wrong), vs. the way that was described in an Apogee POF a long time ago. Here's what I do. I imagine that if anything it is overly optimistic with regard to effect on CP.

DISCLAIMER: THIS COULD VERY WELL BE WRONG, YOU SHOULD PROBABLY GO READ THAT POF ARTICLE INSTEAD

The goal is a fin set that will behave similarly to the ring. In order to do that, the fin set needs to have:
a) same total area as the ring
b) same amount of leading and trailing edge as the ring
c) on average, the fin area at the same distance from the body as the ring
d) same position on the body as the ring

So here's the process:
  1. Calculate the circumference of the ring, call it C
  2. Calculate difference in diameter between the inner BT and the ring, call it ΔD
  3. Create a trapezoidal fin set with the following parameters:
    1. Number of fins = C / ΔD (round to nearest)
    2. Root chord and tip chord = length of ring
    3. Height = ΔD
    4. Sweep length = 0
    5. Position on body = same as ring
    6. Fin cross section = square
    7. Thickness = same as wall thickness of ring
    8. Material = same as ring
  4. Fudge the values a bit, to account for the error introduced by the rounding in step 3.1.
This satisfies my four requirements above. If you did it right the mass of the fin set should be just about the same as that of the ring; for extreme preciseness you can override the fin set mass to equal that of the ring.

This approach ignores fin/body interaction (present with the fin set, absent with the ring), and the additional drag of the air squeezing through the gap between the ring and the body. My guess is, much like tube fins, if the ratio of ring length to the size of that gap gets too high, air stops flowing through there and the ring contributes nothing My rings tend to be short and do not (I don't think) approach that situation.

So here's an example for the ring on Biohazard, which is a BT80 ring wrapped around a BT55.

Ring:
1615920777523.png

"Equivalent" fin set:
1615921039757.png


That's it. I'm happy to hear all the reasons why this is wrong.
 

Dotini

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I'm appreciating the erudite discussion of the finer points of ringtails. It motivates me to do better.

So I took my model to the vertical belt sander for a few aggressive seconds and removed as much of my offending structures as I could. I will try to avoid excesses in the future. Even so, I do not feel it is entirely fair to call them true fins. In my noobish opinion, since these are not even half the diameter of the tube, I feel it more appropriate to refer to them as vanes.

My goal is to get the ring entirely behind the tube and the engine casing, yet ensure structural integrity on landing. I've done this with sticks, yet they are wispy and no easier to ensure axial alignment of the ring with the tube.

Question: If a ringtail rocket seems prone to weathercocking, what is the remedy? More weight in the nose? Less? Canards?


DSC00089.jpg


DSC00014.jpg
 
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KenECoyote

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I've been away too long! Beautiful and incredible builds Dotini! :clapping:
 

Dotini

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I've been away too long! Beautiful and incredible builds Dotini! :clapping:
Gee whiz, thanks! My craftsmanship really is nothing special, but I do confess to working a bit off the beaten path.

DSC00092.jpg

Just completed, the Birdie and Mega-Vortico.

These will fly tomorrow, if the winds permit, down on the beach in front of my fishing cabin on Hood Canal.
 

KenECoyote

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Gee whiz, thanks! My craftsmanship really is nothing special, but I do confess to working a bit off the beaten path.

View attachment 455705
Just completed, the Birdie and Mega-Vortico.

These will fly tomorrow, if the winds permit, down on the beach in front of my fishing cabin on Hood Canal.
The Mega Vortico is one of my favorites! I've flown it many times and it's always a crowd pleaser, however you shouldn't fly it in windy conditions. I actually just recently built a second one I plan to attach whistles to. 😁

IMG_20210316_183525579.jpg
 

KenECoyote

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Question: If a ringtail rocket seems prone to weathercocking, what is the remedy? More weight in the nose? Less? Canards?
The more stable a rocket is, the more it will weathercock since it wants to flow with the straight airstream. I'm not sure of the best way to adjust for that (maybe heavy, not overstable rocket on high boost?), but I am currently working on a system of internally weighted forward fins to keep rockets travelling vertical even in wind and first tests have been fantastic.
I'll be posting it on TRF likely by next week, but have already posted some on the Estes FB group
First post
 
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KenECoyote

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Also most of us fliers would adjust for wind by aiming the rod a bit into the wind so it does weathercock, but at apogee, the chuted rocket gets blown back...kind of an art. :)
 

jqavins

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Question: If a ringtail rocket seems prone to weathercocking, what is the remedy? More weight in the nose? Less? Canards?
Less weight in the nose or more weight in the tail will move tne CG down. Canards will move the CP up. Either of these will reduce the static margin, which is usually a challenge to be overcome, but in the case of overstability it's what you need. And remember, I said you need to be careful of overstability, not that I think you have it. I really don't whether you do or not.
If you'll forgive me, it looks even better this way.
Also most of us fliers would adjust for wind by aiming the rod a bit into the wind so it does weathercock, but at apogee, the chuted rocket gets blown back...kind of an art. :)
Really radical weathercocking that comes from extreme overstability is what turns an ordinary model rocket into a cruise missile. I'm not so sure that turning the launch into the wind will help. I guess I'm not so sure it won't help, but it doesn't feel right.

Of course, another thing that would help is more rod exit speed. As the saying goes, "In thrust we thrust."
 
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Sooner Boomer

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Gee whiz, thanks! My craftsmanship really is nothing special, but I do confess to working a bit off the beaten path.

View attachment 455705
Just completed, the Birdie and Mega-Vortico.

These will fly tomorrow, if the winds permit, down on the beach in front of my fishing cabin on Hood Canal.
A Birdie! Wow, I haven't seen one of those since the mid-70's.
 

Sooner Boomer

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Nice looking designs. My only suggestion is to make the next one with much smaller pylons (ring supports). The pylons on the smaller one are probably sufficient fins on their own, and on the larger one they most certainly are. With the ring doing the stabilizing, the pylons needn't be anyhing more than is needed structirally. They could be sticks if they and their attachments are strong enough.
Sort of like...




 

teepot

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The Mega Vortico is one of my favorites! I've flown it many times and it's always a crowd pleaser, however you shouldn't fly it in windy conditions. I actually just recently built a second one I plan to attach whistles to. 😁

View attachment 455714
What kind of whistles? I've been trying to come up with a design that whistles.
 

KenECoyote

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What kind of whistles? I've been trying to come up with a design that whistles.
Currently testing! I got some cheap emergency whistles off Amazon and sawed & grinded off the clips...may attach to the Turbo Vortico with Command strips. The whistles are very loud, but usually the rocket motor is louder and it's the coast phase where you might hear something like the Screaming Mini rocket whistles. I figured the TV should whistle as it rotates on descent.

Another whistle design I may make is with the same whistles inside the nose cone or as "fins".
 

neil_w

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That's a great idea, the Turbo Vortico really spins up by the time it gets to Apogee, seems to have a reasonable shot at actually making a noise people can hear.

I still have a video of one of your Turbo Vortico flights around here somewhere (F15-0), fun flier.
 

Dotini

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Gee whiz, thanks! My craftsmanship really is nothing special, but I do confess to working a bit off the beaten path.

View attachment 455705
Just completed, the Birdie and Mega-Vortico.

These will fly tomorrow, if the winds permit, down on the beach in front of my fishing cabin on Hood Canal.
Today these birds flew successfully in zero wind next to a saltwater fjord. Attaining maybe 90' the Blender landed a few inches from the pad, and made a pleasant whizzing noise. The Mega Vortico shot far higher amid loud noises and stinking smoke from its 24mm engine. That was highly entertaining. Three launches of ringtail die Glock with greater power and added tail ballast were all disappointments. Back to the drawing board on that one. No models were damaged, as the wet sand treated them gently.

DSC00103.jpg

Hood Canal, Washington State. Here, close to low tide - and not a very low one - is about 250' of open beach between the tideline and the treeline briefly available for rocketry. In the background is the Olympic Range. Visible to left is a pier of Sub Base Kitsap. Just a few miles west over the opposite shore (a narrow peninsula) is Dabob Bay and Bill Gates's vast compound.
 

BABAR

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I believe that tube and ring fin rockets are less prone to weathercocking than standard orthogonal fin rockets, relative to the surface area of the fins. Part of that may be that portions of the fin surfaces are “shielded” from a cross wind by the nature of the circular construction, for example your rocket with the internal linear/orthogonal fins (which function as struts for the ring) totally contained WITHIN the ring. You don’t get something for nothing, things and tubes also tend to be a little less efficient, meaning you end up with more drag for the same change in Cp compared to orthogonal fins.

I’d say you hit an extreme with rockets 2 and 3 in pic 2 post 76. If those didn’t weathercock, I think your other designs should do fine.

I just launched my Birdie a month ago, fun little flier.

you can fly relatively symmetrical rings with only one strut (you can go with zero but it will corkscrew), but structurally that’s a tough build. I think even with two struts you need a pretty strong ring. Three or four is structurally easy, beyond 4 I think is mainly for style rather than structural integrity.

your designs are cool. Keep it up and have fun!
 
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