How to create / simulate a ring fin in OR..?

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Tim51

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Hi all

Does anyone know how to construct a ring fin (i.e. a single one that circles the body tube, not clusters of tubular fins) in Open Rocket?

Cheers,
 
I believe this is likely a case where you need to not worry about what the picture looks like, and worry about performance in the sim.

That said, I would start thinking about using a number of fins to approximate the area of the ring fin using regular fins. Adjust weight accordingly.
 
Thanks all - very useful pointers. I'm sort of half way there in so far as about 3 years ago my son and I built a LPR ring fin design from scratch which, following swing tests, flew very well on a single Klima D9-5 motor to (we guess) around 800 ft (see picture). We've been talking recently about building an upscale HPR version, ideally with a 4" airframe, and a slightly wider ring fin to accommodate the rail. I'm presuming I'll have to scratch build that blunt NC as well, which should be an interesting learning experience..

WP_20161212_18_22_53_Pro.jpg
 
I was going to chime in and point out the PoF article, but that's already been done.

If I'm planning an unconventional rocket, I'll make one model physically accurate for a parts list, and another with equivalent bodies to try and get a valid flight simulation. Unless its an Induction tube, in which case there is no equivalent bodies...
 
Yeah, this. Can OR make said fins transparent?

No.

I have long wished for the ability to independently tag which components are shown visually, and which are incorporated in the simulation. That would solve a lot of these issues.

I wish the OR guys had a more active presence here.
 
I wish the OR guys had a more active presence here.

They should, considering OR webpage says this:

"The official support forum for OpenRocket is the Rocketry Electronics and Software forum at The Rocketry Forum."
 
At this time, there are two methods of creating ring fins... using an externalized "Inner Tube", and a multiple body tube method.

For these methods, Each separate part (all fins will be considered one part) has been given a separate color for clarity.

The inner tube method will give you an accurate appearance, but the ring it generates doesn't get calculated aerodynamically.




The multiple body tube method breaks down the inner body tube into separate segments. This method also uses an externalized inner tube (in this case the red section) to simulate the body tube under the ring fin, the ring fin is a body tube itself.



The inner tube method would need a phantom body tube to "attach" fins to it (such as you find with the Ram Jet), whereas the multiple tube method wouldn't need it.

Both methods can be decorated with "decals" to give accurate appearances.

View attachment OpenRocket Inner Tube Ringfin Method.ork

View attachment OpenRocket Multiple Tube Ringfin Method.ork
 
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At this time, there are two methods of creating ring fins... using an externalized "Inner Tube", and a multiple body tube method.

For these methods, Each separate part (all fins will be considered one part) has been given a separate color for clarity.

The inner tube method will give you an accurate appearance, but the ring it generates doesn't get calculated aerodynamically.




The multiple body tube method breaks down the inner body tube into separate segments. This method also uses an externalized inner tube (in this case the red section) to simulate the body tube under the ring fin, the ring fin is a body tube itself.



The inner tube method would need a phantom body tube to "attach" fins to it (such as you find with the Ram Jet), whereas the multiple tube method wouldn't need it.

Both methods can be decorated with "decals" to give accurate appearances.

Does either method give accurate analysis? That is what is important with a new design.
 
No, because inner tubes are not accounted for in the sim.

Ok, that is what I thought. So really it is only useful for making a pretty picture versus predicting anything having to do with the stability and performance of the rocket.
 
Ok, that is what I thought. So really it is only useful for making a pretty picture versus predicting anything having to do with the stability and performance of the rocket.

While flawed, of the two methods, the multiple body tube method would likely be the more accurate version.
 
Ah - does RockSim allow accurate aerodynamic ring fin simulations? (I've never tried it..)

Without commenting on its accuracy, we can at least say Rocksim at least recognizes ring fins as a design component and attempts to simulate them. I'd assume that they're at least reasonably approximated.
 
I think Rocksim does a more refined job of locating the center of pressure. OpenRocket uses much stricter geometry definitions and gives a conservative (forward) CP position.

That being said, Rocksim wigs out going through Mach (or so I heard), and OpenRocket behaves better.

Still, the more versatile geometry allowable makes me consider getting it one day so I can perhaps characterize odd designs and extrapolate for scale-up.
 

The photos are showing a tube fin rocket...

attachment.php


Ring fins are different...

25179111131_c43f57f31a_c.jpg


BTW, I still need the thicknesses of the fins, and the width of the black bands so that I could re-do this file (it was lost in the HD crash back in September).
 
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I could not find the RocSim files on my own computer. I am thinking that some designs got moved around or lost when I installed RocSim9 I think with a new key. I just now went back to my RM-1 thread and saved the RocSim files that are there. I am attaching the files here. The RM-1 is a stubby rocket and has that umbrella shield far forward. I ended up using a lot of nose weight. I wonder if RocSim was too conservative, however, the model has flown twice and it does fly well.

Ring fins are different...

OK, yes, I see that ring-fins are different.

View attachment RM-1 Recon Craft A-1.rkt

View attachment RM-1 Estes.rkt
 
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I think Rocksim does a more refined job of locating the center of pressure. OpenRocket uses much stricter geometry definitions and gives a conservative (forward) CP position.

That being said, Rocksim wigs out going through Mach (or so I heard), and OpenRocket behaves better.

Still, the more versatile geometry allowable makes me consider getting it one day so I can perhaps characterize odd designs and extrapolate for scale-up.

Rocksim is not as bad at Mach as folks like to say. That said, I cannot comment on it above Mach 1.5 or so as I do not have data to compare- I have not flown faster, so I do not have data to judge.

Like any other sim let's start with GIGO- garbage in garbage out. Meaning that if your sim is not accurate you will get crap results.

Next, there is variation in motor performance. When motors are certified a certain number are burned. Motor files are made from a certain data set. Throw in variation and error will happen.

Next point, both Rocksim and OR are based on Barrowman. One of the basic assumptions of Barrowman is that rockets are subsonic. Anything that goes transonic or faster is technically breaking that assumption. The equations are extended, but how accurately?

Next- tweaking your sim to match launch conditions. Wind, temperature, launch angle all play large in reality. Your sim will be better if you adjust for these things.

So, consider your sim good if you typically get within 10% of predicted altitude. Consider yourself really good if you typically get within 5%.

Rocksim user since much earlier versions.
 
Unmodified Barrowman, not Dahlquist or Galejs? That would surprise me.

Why would that surprise you? Rocksim has straight Barrowman, and the Rocksim method, which is an extension of Barrowman. No mention of Dahlquist or Galejs. If they were involved, or their work was used don't you think it would be mentioned?

OR is still based on Barrowman too. Likely an extension, but developed by who? Verified as legitimate and accurate by who?

I strongly suggest that folks who have not done so read the Barrowman report to understand the assumptions and possible limitations. Link follows, and username and password are both guest.

https://www.rocketryfiles.com/?p=home&d=Technicalarticles/
 
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