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

[Dotini]

The lightest weight ring to give stable flight and maximize apogee.

To that end:

1. Is a bigger diameter with a shorter length more efficient than less diameter and longer?
2. How thin can the ring be and still handle flight stresses?
3. Use support spokes instead of support fins?
4. Should the ring be an airfoil shape?

1) I think so.
2) IMHO, 0.020" with only 3 support fins is bordering on, or past, being too flexible.
3) Yes, but only if the spokes were airfoil (teardrop) sections. It could take two spokes to replace one fin. Difficulty of construction and performance degradation caused by quality control/dimensional variation is an issue. IMHO, coaxial alignment perfection between the ring and the tube is of great importance.
4) Yes, but how? A rudimentary Clark Y profile could be achieved by layering 2 or more plies of sheet PETG. IMHO, when flowing air between two surfaces, such as between a ring and a tube, at least one of the surfaces should not be flat and parallel; a venturi profile would be desirable.

 

[lakeroadster]

4) Yes, but how? A rudimentary Clark Y profile could be achieved by layering 2 or more plies of sheet PETG. IMHO, when flowing air between two surfaces, such as between a ring and a tube, at least one of the surfaces should not be flat and parallel; a venturi profile would be desirable.

Sand / grind / cut the contour by using a lathe,
or
3d print the ring

 

[BABAR]

Spokes may NOT be less draggy than fins.

https://www.apogeerockets.com/downloads/PDFs/launch-lug-drag.pdf
Look at table on page 26.

among the options are 1/8” lug in 1/2”, 1”, and 2” sizes. At least as I read it, the SHORTER the lug, the HIGHER the drag. To me this seems counterintuitive.

spokes also don’t contribute much if anything to stability, and I think would also be much more difficult to attach firmly to the ring fin due to the small contact surface area.

not saying don’t try it, but I don’t think the available data supports it. Also doubt there is any real good simulator data for spokes, although @neil_w often proves me wrong.

 

[lakeroadster]

Spokes may NOT be less draggy than fins.

https://www.apogeerockets.com/downloads/PDFs/launch-lug-drag.pdf
Look at table on page 26.

among the options are 1/8” lug in 1/2”, 1”, and 2” sizes. At least as I read it, the SHORTER the lug, the HIGHER the drag. To me this seems counterintuitive.

spokes also don’t contribute much if anything to stability, and I think would also be much more difficult to attach firmly to the ring fin due to the small contact surface area.

not saying don’t try it, but I don’t think the available data supports it. Also doubt there is any real good simulator data for spokes, although @neil_w often proves me wrong.

I modeled the spokes in Open Rocket.

(2) sets of three spokes: 0.04" thick fins that have a root length and tip length of 0.04"? I updated Dotini's Animist 3 to a set of spokes near the front of the ring, and near the back.

Nearly no change at all in Apogee but drastic change in the CG and CP. Weird.

Note: Both these simulations have 0.50 ounces of nose ballast added, in order to have a stable rocket with all the various motor configurations.

 

[BABAR]

I modeled the spokes in Open Rocket.

(2) sets of three spokes: 0.04" thick fins that have a root length and tip length of 0.04"? I updated Dotini's Animist 3 to a set of spokes near the front of the ring, and near the back.

Nearly no change at all in Apogee but drastic change in the CG and CP. Weird.

Note: Both these simulations have 0.50 ounces of nose ballast added, in order to have a stable rocket with all the various motor configurations.

I slouch corrected.

 

[neil_w]

among the options are 1/8” lug in 1/2”, 1”, and 2” sizes. At least as I read it, the SHORTER the lug, the HIGHER the drag. To me this seems counterintuitive.

I don't think those results with launch lugs can be generalized to cover spokes.

I modeled the spokes in Open Rocket.

(2) sets of three spokes: 0.04" thick fins that have a root length and tip length of 0.04"? I updated Dotini's Animist 3 to a set of spokes near the front of the ring, and near the back.

Nearly no change at all in Apogee but drastic change in the CG and CP. Weird.

Worse than weird: nonsensical. I will look into this, and also see if one of the new beta builds does the same thing.

 

[BABAR]

I don't think those results with launch lugs can be generalized to cover spokes.

true. Also the paper was all computer analysis, as I recall, so I don’t know that it was backed up by any empirical data.

just saying that replacing a long fin with a small spoke may not reduce drag as much as you might expect, and you lose pretty much all the CP effect, except maybe base drag, by changing ring attachment from fin to a spoke.

 

[Dotini]

I modeled the spokes in Open Rocket.

(2) sets of three spokes: 0.04" thick fins that have a root length and tip length of 0.04"? I updated Dotini's Animist 3 to a set of spokes near the front of the ring, and near the back.

Nearly no change at all in Apogee but drastic change in the CG and CP. Weird.

Note: Both these simulations have 0.50 ounces of nose ballast added, in order to have a stable rocket with all the various motor configurations.

I've ordered some streamline section bamboo strut material, 1/16" thick and 3/16" wide. These come from the RC airplane world. I may build a new model with these supporting a ring. I'm thinking of a larger diameter ring with a shorter length, possibly supported by only 3 of these struts. When building ring fin models in the real world, I think it's been very easy for me to end up with a model with the ring less than perfectly aligned with the tube.

Another design refinement might come from cantilevering the ring just aft of the end of the tube.

 

[BABAR]

Another design refinement might come from cantilevering the ring just aft of the end of the tube.

I suspect that will work, in fact it may blow the sims out of whack, but I am not sure which way.

if the ring hangs past the tail, the jet nozzle flow may suck flow inward through The ring and accelerate it (suck probaly isn’t the right word, call it a pressure gradient that increases flow through the ring). That could exaggerate the rings effectiveness… or not,

think Lil’ Augie

http://www.spacemodeling.org/jimz/eirp_10.htm
very different from your plan, but has a nozzle inside a ring with airflow to avoid the Dreaded Krushnic Effect (all due respect to Mr, Krushnic, but sounds like something out of the Simpsons.)

 

[Dotini]

 

[Dotini]

While waiting for parts, I decided to see if I could graft a supplementary ring fin to an Apogee Apprentice fin unit sufficient to stabilize a BT-60 model. The unit is normally sufficient only for a BT-50.

Shallow notches were filed into tip edges to locate PETG ring accurately in place. Ring is cut from flat stock.

 

[Dotini]

Here's my latest attempt on the OP question with BT-60, C power and ring fin, below on right. Granted, the ring is supplementary to the Apogee Apprentice base unit's fins, but which alone are not sufficient for a 12:1 length/diameter BT-60 rocket...I think.


On left, heavily bashed Mongoose, testing if enough fin for BT-55 model.

 

[lakeroadster]

Here's my latest attempt on the OP question with BT-60, C power and ring fin, below on right. Granted, the ring is supplementary to the Apogee Apprentice base unit's fins, but which alone are not sufficient for a 12:1 length/diameter BT-60 rocket...I think.

View attachment 479325
On left, heavily bashed Mongoose, testing if enough fin for BT-55 model.

I'm sure they will both fly just fine... no ring needed.

 

[Dotini]

I'm sure they will both fly just fine... no ring needed.

I follow no God, guru or prophet in this realm or any other. However, I have taken a pledge to abide by the bedrock NAR safety code, which I do take quite seriously. Whether rightly, wrongly or simply naively, I have tried to build all my models in accordance with Basic rocket stability design criteria as exemplified in Fig. 2-6 of Van Milligan's Model Rocket Design and Construction.

 

[lakeroadster]

What are you expecting to learn from launching these 2 rockets?

Are you going to do a swing test? My point is... they both look to be very stable birds.

 

[Dotini]

What are you expecting to learn from launching these 2 rockets?

Are you going to do a swing test? My point is... they both look to be very stable birds.

The basic idea of both these rockets is to start from a humble, inexpensive and quick to build fin can (one is reversed) and see how much I can bash and accomplish with them.

Both carry an altimeter; I will record and video my flights as usual. Both are adjustable for nose cone weight. Each has the BT-50 section beefed up against crumpling, one internally, the other externally. Currently I am experimenting with smaller and smaller parachutes and quicker descent rates, including using the Xform. I'm looking for ways to shorten my models down to 12:1. I will be replacing the payload section on the BT-55 model with a nosecone capable of carrying the altimeter. The BT-60 model already has the altimeter in the nosecone. I will swing test them. But I realize swing testing is not always practical or gives correct results. Another objective is to experiment with color/visibility schemes. At some point I will use one as an upper stage. Once I refine a design that pleases me, I will take it to a BT-70 size. Bottom line, acquire experience and have fun doing it.

 

[BABAR]

Not being a sim software user, my roughly accurate mind sim is struggling a bit with what seems to be a relatively gracile fin

you are probably already familiar with Larry Brand,s tube fin page

https://www.rocketreviews.com/larry-brand-page.html
main point being

I understand that yours are rings, not tubes, and I am not aware of ANY data on optimum length to diameter on rings ( which also is likely heavily dependent on the body tube diameter, boat tails, and overhang off the rear). My lack of knowledge certainly doesn’t mean the data doesn’t exist, lots of things I don’t know. Perhaps more to the point, seems from front to back much shorter relative to diameter than many of your previously successful models.

 

[Dotini]

Not being a sim software user, my roughly accurate mind sim is struggling a bit with what seems to be a relatively gracile fin

you are probably already familiar with Larry Brand,s tube fin page

https://www.rocketreviews.com/larry-brand-page.html
main point being

I understand that yours are rings, not tubes, and I am not aware of ANY data on optimum length to diameter on rings ( which also is likely heavily dependent on the body tube diameter, boat tails, and overhang off the rear). My lack of knowledge certainly doesn’t mean the data doesn’t exist, lots of things I don’t know. Perhaps more to the point, seems from front to back much shorter relative to diameter than many of your previously successful models.

Yeah, I've built lots of models over 50:1 on the path of horizontal spin recovery and Magnus effect. But now it's the season to be stubbier, down to the 12:1 limit. Stability will now be more critical, correctly sized fins more important, CG/CG more critical to define. I'm eliminating components like baffles, shelves and payload bays, getting down to basics. But I do like the look and feel of models with transitions, so I will work with those for a while. After a period of parachute denial, I'm also facing that challenge. I'm still pondering @jqavins tube fins, though. I will very likely do some more work in this arena; try to get them as small and sturdy as possible. Another thing I'd like to study is endplates on conventional fins.

 

[Dotini]

To bring the OP question closer to a resolution, I've designed an experiment to compare the results achieved by the Swedish Ghost Rocket to a new generation of ringtails with rings located fully behind the tube, a.k.a., "Venturi Ringtails".

The SGR will be modified in length and weight to bring it into very close similarity to the new rockets, except of course in the location of the ring. It will also get a clearcoat to enable wet field flights.

All three rockets will be randomly flown multiple times each on the same test session, to enable exactly the same conditions for all flights. I have enough C11's for up to six flights each.

Each flight will be recorded by Jolly Logic Alt2 and by video.

Naturally, this is the wet and windy time of year, so no firm date for the test can be announced at this time.




Left to right, modified Swedish Ghost Rocket, "Venturi Ring" I, "Venturi Ring" II.