OpenRocket accuracy question

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Walldiver7

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Alright, so here's the situation: I was building a scratch design and after (of course) cutting the fins and beveling them, I discover that the motor I based this design had been loaded into OpenRocket incorrectly. The weight of the motor was actually much higher than what was loaded into OR..... and thus, my scratch design was not going to be stable.

So, not wanting to cut new fins, I started looking at what could be done to achieve 1 cal stability. I discovered that if I turned the rocket into a split fin design the cal stability went way up.... To clarify, on OR I cut the fin into two separate fins (typical split fin design) and spaced them only 1/8" apart. My question: Is OpenRocket accurately stated the stability here? It went from a relatively low figure to way above 1 cal. I also found it interesting that the spacing between the fins (1/16 to 1/2 of an inch) didn't seem to make much difference. If OR is correct, then I've saved myself from having to cut a new set of fins!! The downside is... it scrubbed off about 3,000 feet from the apogee with the split fin design.

Can I conclude that the extra drag the split induces is the reason that the stability increased?? Thanks for the help!
 
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Can I conclude that the extra drag the split induces is the reason that the stability increased??
Unlikely, since drag (and airfoil shape) are not really factors in the stability equations. I'd suspect a bug in the handling of split fins. But you need to post the ORK files for anyone to have a useful opinion.
 
Wow, that's really weird. I'll take a look at the guts of the code to try to understand what's going on.

As for the motor mass error, that comes straight from Thrustcurve.org. What's the correct mass?

Kevin

Here are the two files: View attachment 247135
View attachment 247136

I'm not sure if this is how to attach files.... let me know if this didn't work.
 
The CTI Pro75 1590

Which motor exactly? Was it grabbed from the rasp file or an rse? Not in position this weekend to fix, but will early next week once I verify. Please PM me with details.

Mark- Thrustcurve Editor


Mark Koelsch
Sent from my iPhone using Rocketry Forum
 
Which motor exactly? Was it grabbed from the rasp file or an rse? Not in position this weekend to fix, but will early next week once I verify. Please PM me with details.

Mark- Thrustcurve Editor


Mark Koelsch
Sent from my iPhone using Rocketry Forum

Mark, This was the CTI Pro75 7545 M1590-P motor. It wasn't imported by me... this was just part of OR's database. OR shows the weight as 5,223 grams while the published (CTI) data states that the Loaded weight is 6,076 grams (propellant weight 3,590g and the Burn Out weight is 2,316 grams).
 
Wow, that's really weird. I'll take a look at the guts of the code to try to understand what's going on.

As for the motor mass error, that comes straight from Thrustcurve.org. What's the correct mass?

Kevin

The correct mass would be 6,076 grams.
 
Wow, that's really weird. I'll take a look at the guts of the code to try to understand what's going on.

As for the motor mass error, that comes straight from Thrustcurve.org. What's the correct mass?

Kevin

I'm sure glad I brought this up; I was about to cut slots in all the fins! ha! I thought I had reached rocket deity status for discovering this "fix" !!!
 
Can I conclude that the extra drag the split induces is the reason that the stability increased??

No. This is a consequence of the Barrowman equations. The equations conclude that two half fins are more effective than the whole. What the Barrowman equations (and Openrocket and Rocksim (at least V7 which is what I have)) do not do is correct for the loss of effectiveness in the trailing fin. I don't think anyone has researched that well enough to be able to calculate the results.
 
No. This is a consequence of the Barrowman equations. The equations conclude that two half fins are more effective than the whole. What the Barrowman equations (and Openrocket and Rocksim (at least V7 which is what I have)) do not do is correct for the loss of effectiveness in the trailing fin. I don't think anyone has researched that well enough to be able to calculate the results.

Thanks for the explanation. This means I WILL be cutting new fins... bummer.
 
Kevin,

Did you ever determine what was causing this??

Wow, that's really weird. I'll take a look at the guts of the code to try to understand what's going on.

As for the motor mass error, that comes straight from Thrustcurve.org. What's the correct mass?

Kevin
 
We have pushed and broken the boundaries/ assumptions of the Barrowman equations quite a bit. Barrowman is designed for subsonic, a certain range of fin number. I do not have the list handy, but if read the Barrowman report many of them are listed.


Mark Koelsch
Sent from my iPhone using Rocketry Forum
 
I think the "Rocksim" method uses some sort of finite element discretization of the actual design geometry, such that any fin configuration can be computed, and Barrowman limitations are avoided. Not sure. It might be described in a long-ago Peak of Flight newsletter.
 
I think the "Rocksim" method uses some sort of finite element discretization of the actual design geometry, such that any fin configuration can be computed, and Barrowman limitations are avoided. Not sure. It might be described in a long-ago Peak of Flight newsletter.

I do not think that Rocksim implements any of the findings of NACA report 1307. At least V7 doesn't.
 
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