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Kill Bill Build
#1
I will be starting my Kill Bill project.  It will be a 5.5" split fin design that will be designed to fly to about 10K feet on a M.

One thing I learned that may be useful is that Rocksim (version 7) does not calculate CP correctly for split fin designs using either Rocksim or Barrowman equations.  Probably the problem is in the Barrowman equation.

The way to prove is to create a rocket with two sets of SQUARE fins that are split.  Move the forward set of fins so that they are just touching or overlap the rear set.  Note the CP location and/or margin of stability.  In my rocket design the CP was 89" Barrowman, 92" Rocksim.

Now delete the rear set of fins and elongate the front fin to a rectangle that has the same shape and dimension of the set of split fins before.  In my case the CP changed to 80 Barrowman, 85 Rocksim.  The CP should have been identical to the split fin model because the fin geometry was exactly the same.

The difference between these calculation was almost 2 calipers in the Barrowman case and 1.5 calipers in the Rocksim case.  That is a lot of discrepancy especially if you are designing for a 1-2 caliper stability margin.

If any has newer versions of Rocksim I'd be interested if it has the same problems with split fin designs.  If so, I would trust Rocksim or Barrowman equations for modeling CP in split fin designs.
John Derimiggio
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#2
I have V9.1 and it will handle a split fin design. you could either upgrade to 9.1 or build it in 7 and send the file to me and I can bring it up on 9.1 and let you know where everything actually is. Actually upgrading to 9.1 is way cheaper than getting the full version of 9.1 by itself.

When I was purchasing v9 I couldn't find my key to put v7 on my laptop, so I sent in the $$ to get the full version. Apogee called me to tell me that their records showed that I had v7 and that it would be cheaper just to upgrade. I told them my predicament about the loss of the key and they said no problem. they said to just download the file and they would give me a new key online. Since then I checked for upgrades and saw the v9.1 was an available upgrade and installed that at no cost. V9.1 does a lot of nice things that v7 can't do. You should really just upgrade to it.
I just wanna fly ROCKETS!!

Dutch
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#3
We have V.8 here in TCR World HQ and it gets it wrong as well.

Also What did I do? why you wanna kill me?
Bill Clune L2 | NAR#88583
MARS Club|SRC|METRA|MDRA - Forever
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#4
We have the latest RockSim version and it does calculate the split fin properly.  I checked this against the PML Endeavor which is a split fin model.
Jim Goggins NAR  L3
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#5
Oh, I forgot to tell you that Apogee only charged my CC for the upgrade as well. Great customer service.
I just wanna fly ROCKETS!!

Dutch
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#6
I downloaded version 9 and it had the same error for me.  Did anybody with v9 try the rectangular fin test I described?
John Derimiggio
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#7
For you RS9 owners who says it works correctly,

Attached here is RS file of a test rocket with a pair of 10x10 fins.  Note the CP.  Now delete one set of fins and change the other set to a 20x10.  It should have the same CP location but it does not.  It is off by 5 inches!

It does not work correctly.

Another test you can to is slide one set over the other and perfectly overlap the fins. Note the CP.  Then delete one fin set. The CP should be the same but it is not by a wide margin.
John Derimiggio
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#8
(09-17-2010, 06:34 AM)Jim Goggins link Wrote: We have the latest RockSim version and it does calculate the split fin properly.  I checked this against the PML Endeavor which is a split fin model.

That just means PML used RS and got it wrong as well.
John Derimiggio
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#9
It's nice to see these observations. I had the trial Rock sim V9 and it was doing some weird things with tube fin designs. It had me thinking this don't work right. I finally just trusted Larry Brands design on the tube fin design. He puts CP on tube fin designs at the front edge of the Tube fin can. I wasn't even happy with the calculated CP on my LOC Doorknob kit, that I used for my first L2 cert attempt. It seemed the CP was too far forward. I checked and double checked, same result. Even Mike Dutch told me it seemed too far forward. I finally said, to hell with it, it's a kit, it's supposed to fly even with bigger heavier motors than this in the tail. So I stuck a motor in and it flew just fine. The only way to get a REAL CP is with a wind tunnel. Anybody got one??
David Haas
NAR #13780
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#10
I also tried this with the V9 free trial with weird results.  Mike said it worked in 9.1 - could it be a difference in the two versions?  (v9 demo vs. 9.1 registered)

Dale
Dale Stoyer - L3
NAR #91256
TRA #13499
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