OR-Fins on a boattail?

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vahpr

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How do you guys model a fin/boattail as on a BB or Arcas? I can see maybe using fin tabs, offsetting the fin set off the back of the parent (body tube) but there's gotta be a better way.

Thanks.
 

Scott_650

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Use a “ghost” - transparent with 0 mass - body tube.
 

K'Tesh

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You mean like this?

1631381845632.png


Or this?

1631382239217.png



You find a tutorial, and experiment....
 
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Scott_650

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K’Tesh isn’t known as the Open Rocket Chuck Norris for nothing! His tutorials will show you the way…
 

vahpr

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You mean like this?

View attachment 481341

Or this?

View attachment 481342


You find a tutorial, and experiment....
Both actually, 2 different rockets which incorporate both designs.

Thanks for the link, I'll check it out.
 

vahpr

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OK K'Tesh, using the ghost tube method, it looks like this. Are there any issues that you know of with this method regarding the sim calcs? For example, the fins are attached below the airframe so the span is reduced unless you compensate (which I did). Anything else to be aware of? Thanks.
 

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K'Tesh

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OK K'Tesh, using the ghost tube method, it looks like this. Are there any issues that you know of with this method regarding the sim calcs? For example, the fins are attached below the airframe so the span is reduced unless you compensate (which I did). Anything else to be aware of? Thanks.
OR's flight performance will likely be skewed somewhat by the extra surface area that has been "buried" inside the body tube. If you can get your fin to meet its Phantom Body Tube (PBT) at a single point (first and last coordinate being 0.0 /0.0) (for the rear one it might be actually 0.0001/0.0), the less "extra" you'll have, and the more accurate OR's sims should be. That said, I don't know (or do) the maths behind OR's calculations so I can't vouch for how accurate or inaccurate the sim will be for dealing with the boattail. Basically, my logic is the more accurate the info you can give the program, the more likely the info you get out of it will be accurate, within the capabilities of the program (the opposite of "Garbage in, garbage out.").

Another reason to make your fin shape accurate is that you can use the OR generated fin templates to cut your fins. If your shape includes the extra part that is buried inside the body tube, you won't be able to do that (without modifying the templates post generation).
 

Jeff Lassahn

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You can also try to track down a pre-release version of the new "it's gonna get released any time now" OR version. It does fins on noses and transitions natively so you don't need the weird tricks.
 

rharshberger

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You can also try to track down a pre-release version of the new "it's gonna get released any time now" OR version. It does fins on noses and transitions natively so you don't need the weird tricks.
Been waiting on this version for a long time, then maybe I can get my Nike Hercules noseweighted properly.
 

K'Tesh

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You can also try to track down a pre-release version of the new "it's gonna get released any time now" OR version. It does fins on noses and transitions natively so you don't need the weird tricks.
Keep in mind that you have to have the right version of Java to get it to work properly, and the right version of Java *MIGHT* break 15.03
 
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