BT-60 / C11 / Ring Fin: How High?

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Dotini

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In the (BT-55) 6th Observable, we had a rocket which greatly exceeded expected levels of performance in the category of altitude. .

Now we have a new iteration in work, this one with larger BT-60 fuselage and the same 3" ring fin and 24 mm motor mount. There are now only four fins instead of six, and in place of 1/8" balsa there are now built-up assemblies of polystyrene T-sections and flat PETG plastic sheets of 0.020" thickness. It has a payload section for the altimeter, and recovery is by conventional parachute.
DSC00315.jpg




DSC00314.jpg
 

Dotini

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Provisional length is now revised down to ~30". Reason: to ensure target weight of < 6.0 oz is easily enough met. This will come at the cost of some visibility, so this length figure may increase as a clearer picture emerges of exact finished weights.

Question: Currently an Estes 18" plastic parachute is specified, but is this overkill? Can I safely come down a size to 15" or even 12"?

Question: Currently a Jolly Logic Altimeter 2 is carried in an aftermarket pouch loosely located in the payload section. Does this device require additional cushioning, or could it do with less?
 

lakeroadster

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Modeled in Open Rocket using an Estes C11-3, BT-55, ring fin simulated per Bruce Levision's instructions, overall weight of 6 ounces and overall length of 30 inches. Surfaces specified as smooth.

Simulation shows an apogee of 296 feet with the ring fin. ( 780 ft on a D12-5 )
The same rocket, but with 3 small balsa fins and no ring fin, has an apogee of 667 feet. ( 1,534 ft on a D12-7 )

Ring fins look cool, but they add significant weight and drag.

An 18" chute will give a ground hit velocity of about 13.7 ft/s. Anything around 14 is pretty much ideal for a rocket you don't want banged up, IMO,
 
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Dotini

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Modeled in Open Rocket using an Estes C11-3, BT-55, ring fin simulated per Bruce Levision's instructions, overall weight of 6 ounces and overall length of 30 inches. Surfaces specified as smooth.

Simulation shows an apogee of 296 feet with the ring fin. ( 780 ft on a D12-5 )
The same rocket, but with 3 small balsa fins and no ring fin, has an apogee of 667 feet. ( 1,534 ft on a D12-7 )

Ring fins look cool, but they add significant weight and drag.

An 18" chute will give a ground hit velocity of about 13.7 ft/s. Anything around 14 is pretty much ideal for a rocket you don't want banged up, IMO,
Thanks for all that information! I will certainly take the chute recommendation very seriously.

However, and unfortunately, computer program simulations of ring fin performance appear to be in their infancy. My BT-55 6th Observable (37", 4.8 oz) made nearly 500 feet on its first flight with a C11-3, and it had 6 fat fins supporting the ring. Please see more in the thread on the 6th Observable. I have a 41 second video of the flight, but it's too big to insert here.

My new ring fin (undoubtedly excessively large) currently measures 15.92 square inches and weights 0.26 oz, and I can easily reduce it to as few as 11.7 square inches. I believe that works out to near the same as bare, untouched balsa of between 1/16th" and 3/32", or about the same weight as papered 1/16th balsa, IMHO.
 
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BABAR

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This will likely recovery well with a streamer, which may be an advantage given your stated limited field size. Especially if the motor sticks out the back a touch, it will likely impact on the motor casing.

as a test bed, cut some windows in the sides of the ring, all clockwise adjacent to the fin spokes, and you have
Something like


I’d still put a streamer on it, but with a good swivel you will get a component of horizontal spin recovery.

I don’t know if I’d have the guts to go full Monty on it, ditch the streamer, put a long body tube on it, tape on the nose cone and put a puff port up front and trust in full horizontal spin.

Edit, Duh, you’ve already seen this, sorry
 
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BABAR

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Not sure if you want a free you tube account, I usually post my flights to you tube and then paste them into my TRF posts.
 

Dotini

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This model is nearly complete, lacking only the payload bay bulkhead, screw eye and launch lugs. Current weight with 18" parachute, C11-3 and altimeter is 4.88 oz.

Support fin shear panel size is 0.7" high by 1" long. Ring fin is 3" diameter by 1.375". length. Overall length is 26".
Swedish national (Sharpie) colors are in honor of Scandinavian friend Don who is organizing a reunion of friends from Roosevelt High School, class of 1967. We will gather at Dahl field and launch this amongst other rockets, then retreat to the local pub. I will swing test this model some time after sunrise today.

DSC00316.jpg

Nosecone is donated from an unborn Baby Bertha kit. I intend to acquire a slightly longer balsa nosecone for this current model.

DSC00317.jpg

There is a Qualman crossover baffle of 0.13 oz (located just forward of the left cradle support in photo above) which serves as shock cord anchor and prevents wadding and chute from shifting too far aft.

DSC00318.jpg
 
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BABAR

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Nice.

I am a big fan of rounded or other “soft” nose cones for rockets of “yet to be proven” stability.
 

Dotini

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Nice.

I am a big fan of rounded or other “soft” nose cones for rockets of “yet to be proven” stability.
Yes, I am slowly chipping away at the length/diameter ratio, fin area and distance of the fin from the body. So for sure a swing test is in order, as I mentioned above.

This is my 7th ring fin model, and boldest yet in the proportional spacing of the ring to the body. All previous models have been quite stable. I feel very confident this one will be no exception. But my process calls for a swing test. So be it.
 

BABAR

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The extra long “SuperRoc” dimensions are I believe mandatory for Back Slide Recovery. I don’t think they are required for stability ring fins

the Sprite is a good example

from
7E24EB6C-C4A8-4FF8-AA11-A6AF70A895F3.jpeg


rings that are minimally larger than the rocket body AND are not below the tail of the body can constrict the air flow between the two . Especially when associated with increased number of “spokes” or attachment fins (your support panels) they start acting as tube fins. At some point as the cross sectional diameter to length ratio decreases (think long skinny tube fins) effective air flow ceases and they act like capped cylinders.

my mind sim predicts you will have a great flight.
 

Dotini

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rings that are minimally larger than the rocket body AND are not below the tail of the body can constrict the air flow between the two . Especially when associated with increased number of “spokes” or attachment fins (your support panels) they start acting as tube fins. At some point as the cross sectional diameter to length ratio decreases (think long skinny tube fins) effective air flow ceases and they act like capped cylinders.

my mind sim predicts you will have a great flight.
When air flow between a body and a control surface is constricted, it accelerates, as in a venturi. Clever designers of racing cars, amongst others, are known to take advantage of this effect. :)
 

BABAR

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When air flow between a body and a control surface is constricted, it accelerates, as in a venturi. Clever designers of racing cars, amongst others, are known to take advantage of this effect. :)
True for funnels and other shapes that DO actually constrict, as defined by a wider forward / entrance diameter larger than some point more tailward.

“constrict” may have been a poor word choice on my part. Regardless, ring fins start acting like tube fins as the cross sectional segmental area decreases (between ring, rocket body, and support panels) decreases, and this is in some way related to ring size and length, tube diameter, and number of spokes (probably not directly but roughly). The DECREASE in area gets worse with larger body tubes and increased number of shear panels (aka spoke fins), the area INCREASES and air flow improves with bigger DIAMETER rings.

since most tube fins and rings are of constant cross sectional area along their length, they will not (IMO) significantly increase velocity, they WILL generally increase drag. They MAY redirect flow, which is why Lil’ Augie rocket CHAD stages easily.
 

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3E53E386-D5C0-4927-B951-866913BCD1AB.jpeg

I will put the NAR report on the Krushnic effect at the tail of this post.

basically, if you have a recessed motor mount where the distance from the motor nozzle to the actual tail of the rocket is roughly greater than or equal to the body tube diameter, the motor rapidly loses effective thrust.

in the Lil Augie, the sustainer and possibly even the booster motors are more than one rear tube diameter above the base of the rocket. BUT, the rear tube actually functions kind of like a ring fin, most importantly it is OPEN forward/nose-ward. Not sure I know if it is push me or pull you regarding the nature of the air flow, in any case sufficient air passes between the forward body tube and the rear body tube/ring to PREVENT the Krushnic effect.

I also can’t remember the name, but I do recall someone building a finless rocket with ducts along the airframe that routed airflow around the motor, I think it was in Sport Rocketry.


I can’t recall the name of ANOTHER
3E53E386-D5C0-4927-B951-866913BCD1AB.jpeg
rocket at the moment (probably something from Odd’L rockets, maybe @hcmbanjo or @jadebox can correct me), but I think there was a tube fin rocket where the tubes were intentionally canted off kilter to induce spin on ascent. Why? Simply because it looked cool, it certainly did nothing to improve speed or altitude.



 

Dotini

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View attachment 474498
I will put the NAR report on the Krushnic effect at the tail of this post.

basically, if you have a recessed motor mount where the distance from the motor nozzle to the actual tail of the rocket is roughly greater than or equal to the body tube diameter, the motor rapidly loses effective thrust.

in the Lil Augie, the sustainer and possibly even the booster motors are more than one rear tube diameter above the base of the rocket. BUT, the rear tube actually functions kind of like a ring fin, most importantly it is OPEN forward/nose-ward. Not sure I know if it is push me or pull you regarding the nature of the air flow, in any case sufficient air passes between the forward body tube and the rear body tube/ring to PREVENT the Krushnic effect.

I also can’t remember the name, but I do recall someone building a finless rocket with ducts along the airframe that routed airflow around the motor, I think it was in Sport Rocketry.


I can’t recall the name of ANOTHER View attachment 474498rocket at the moment (probably something from Odd’L rockets, maybe @hcmbanjo or @jadebox can correct me), but I think there was a tube fin rocket where the tubes were intentionally canted off kilter to induce spin on ascent. Why? Simply because it looked cool, it certainly did nothing to improve speed or altitude.



Interesting!

I believe I have been able to achieve anomalous altitudes with these ring tail rockets:
DSC00319.jpg
DSC00320.jpg

I conjecture it may have to do with base drag reduction.
 

BABAR

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Interesting!

I believe I have been able to achieve anomalous altitudes with these ring tail rockets:
View attachment 474501View attachment 474502
I conjecture it may have to do with base drag reduction.
Love to see some data. Maybe build two otherwise similar rockets, one with a ring and one without, same motor, same mass, use open rocket to get smallest fin size/panel size as low as possible, and do a fly off with altimeters.
 

lakeroadster

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Love to see some data. Maybe build two otherwise similar rockets, one with a ring and one without, same motor, same mass, use open rocket to get smallest fin size/panel size as low as possible, and do a fly off with altimeters.
Or just one rocket, make the ring fin removable. That eliminates any variables, except for allowable variations in motor thrust.

As long the second flight with the ring is done on the same day, in relatively close time as to the first flight without the ring, that should provide good actual flight data.
 

BABAR

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Or just one rocket, make the ring fin removable. That eliminates any variables, except for allowable variations in motor thrust.

As long the second flight with the ring is done on the same day, in relatively close time as to the first flight without the ring, that should provide good actual flight data.
I like this. I’d suggest that while the weight may be slightly greater with the ring, it will be minimal, certainly if it is stable without the ring and flies higher WITH the ring you have proved your point.

FWIW, my mind sim suggests it won’t, but this isn’t the first time I would like my mind sim to be wrong.
 

BABAR

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At least as to the ring IMPROVING efficiency. I wouldn’t be surprised to see your rockets go higher than a SIMULATOR program predicts. I’m not sure the sims fully replicate rings and tubes all that well.

I mean, it’s not like it is rocket scien……. ooops, I guess it is.
 

Dotini

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I'm accumulating quite a bit of anomalous data. I will acquire more as I learn to use my new altimeter. But I'm not going to shoulder the burden of advancing model rocket theory, design and construction all by myself. If any of this matters, then some other rocketeers are going to have to turn off their computers and start building and testing rockets. So, let's get off our butts and get with it!
 

lakeroadster

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I'm accumulating quite a bit of anomalous data. I will acquire more as I learn to use my new altimeter. But I'm not going to shoulder the burden of advancing model rocket theory, design and construction all by myself. If any of this matters, then some other rocketeers are going to have to turn off their computers and start building and testing rockets. So, let's get off our butts and get with it!
I hear crickets 🦗🦗🦗🦗🦗🦗🦗🦗

Since it's your theory, and you have the rocket(s) to make the test a reality, why not:
  1. fly the ring finned rocket, twice, and record the altimeter data,
  2. then take an X-Acto knife, surgically cut off the ring and then fly it again, two more times, and record the altimeter data,
  3. post the results here and show us the error in our ways.
No need to even talk about computers and simulations, that's a separate issue all together.
 
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Dotini

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a. Maybe build two otherwise similar rockets, one with a ring and one without, same motor, same mass, use open rocket to get smallest fin size/panel size as low as possible, and do a fly off with altimeters.
Bad idea. All my ring tails are designed or at least intended to be unstable with only the support fins in use. Maybe half the size they'd need to be according to Van Milligan's chapter on stability.

When I swing test the model I'm working on now, I will test it with and without the ring. If it's stable without the ring, it's a failure right at the start, and I'll wad it up and throw it away. Or at least trim the fins to past the point of instability.

IMHO, to deliberately build a rocket with a ring fin that is stable without it is a purposeless exercise in superfluity.
 

lakeroadster

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You previously stated, in regard to ring fin designs:

I conjecture it may have to do with base drag reduction.
Now, when a proposal to prove that theory is discussed, you state:

IMHO, to deliberately build a rocket with a ring fin that is stable without it is a purposeless exercise in superfluity.
In regard to
 
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lakeroadster

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A conjecture is not a theory.
Ye ole internet rabbit hole... can somebody toss me a line?

When I swing test the model I'm working on now, I will test it with and without the ring. If it's stable without the ring, it's a failure right at the start, and I'll wad it up and throw it away. Or at least trim the fins to past the point of instability.
I propose the reality is the exact opposite of your conjecture. It's not base drag reduction at all. In reality a ring fin design has so much drag, it's making your unstable rocket, stable.

Take that same design, remove the ring fin and increase the fins until it is stable and it will outperform your ring fin design... every time.
 

Dotini

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Ye ole internet rabbit hole... can somebody toss me a line?



I propose the reality is the exact opposite of your conjecture. It's not base drag reduction at all. In reality a ring fin design has so much drag, it's making your unstable rocket, stable.

Take that same design, remove the ring fin and increase the fins until it is stable and it will outperform your ring fin
design... every time.
This much I have bolded may well be true. But it would be no fun me. I greatly enjoyed building and flying the rocket the way it is, which is unique. Nobody on Earth has one like it. That it makes the RockSim look silly is merely a bauble, a bonus pleasure. RockSim said my rocket would go up 131'. Yet, in the real world, it went up almost 500'.
 

BABAR

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This much I have bolded may well be true. But it would be no fun me. I greatly enjoyed building and flying the rocket the way it is, which is unique. Nobody on Earth has one like it. That it makes the RockSim look silly is merely a bauble, a bonus pleasure. RockSim said my rocket would go up 131'. Yet, in the real world, it went up almost 500'.
Many people building typical 3FNC and 4FNC rockets have found sim programs helpful. As we get further from simple rocket designs, the sim programs, which are guesstimates to begin with, become less accurate. I never would have guessed that a launch lug is a MAJOR cause of drag, and while I may have guessed that an elliptical fin may be lowest drag in the subsonic speed range, a clipped delta is pretty close.

two possible questions here.

The first is whether your rockets outperform the expected altitudes from the simulation programs? Quite possibly true. I did not find that all together amazing.

however, if you are suggesting that ring fins are more efficient than planar fins I think that is something that would be great if it’s true, but it is not currently obvious. Then as I have mentioned many facts of rocket aerodynamics are not intuitively obvious to many of us mortal minded amateurs .

I am currently working on “Go With The Flow”, an air brake recovery rocket that theoretically should have infallible deployment, no pull bands, and self corrects with elastic brake/blade retention for early or late ejections .

but how about this? I will build a ring rocket to your specs, except I will initially build it with fins long enough to be stable without the ring. I will fly it 3 times with altimeter. I will round but not taper the forward and leading edges.

I will then cut off the excess outer hemispan of the fins and add the ring, so final rocket should be at your specs. Then weight the rocket, adjust if possible for any changes, and fly it again

would this be an acceptable test of your conjecture?


love to see you having fun, even cooler if you can turn rocket science on its ear!
 

BABAR

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BTW, Go With The Flow has failed miserably twice, the 36 inch fins are too flimsy with 1/16” balsa.
 
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