Spin test and tube fins

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Gunstar

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Is the spin test valid for testing a tube fin rocket (ie: tie a string at the CG, load it with a motor and swing it around in a circle and check that the nose goes first)? I made one that is a 12 inch long bt20 with 6 1 inch bt20 tube fins. According to openrocket it should be very stable, but it fails the spin test. I'm wondering if the circular motion of the spin test works against the stabilizing effect of the tube fins.
 
Can you give us an idea of the overall weight & where OR says CG, CP should be?

A tubefin in that configuration should be pretty straightforward.
 
I went ahead and launched it anyway (making sure I had as much clearance as possible) and it flew great. My conclusion is the spin test is not a good way to test a tube fin rocket for stability.
 
Could be hard to swing it with a long enough string to avoid the effects of the curved flight path. You can probably double check with the cardboard cut out method, which will probably tend to overestimate required tube fin diameter.
 
Tube fin rockets are kinda funny that way. They don’t have much lateral stabilizing force, which means that they don’t recover well from lateral forces (ie moving sideways through the air as in a string test) but they also fly straighter with less weather-cocking. Bottom line is that the air has to flow THROUGH tube fins but flows PAST regular fins. So unless you manage to get it going in the direction of the spin test and get air going through the tubes, it won’t correct. Likewise if your string is too short, the deep arc will limit the air going through. So, as others suggested, use Rocksim or OpenRocket to find the CP (assuming the newest version of OR supports tube fins - last time I looked it didn’t and it would’t let you put enough fins - 3 per tube - to work around).
 
The current ersion of OR does support basic tube fins but you are limited to 90deg ends. Check out the OpenRocket wiki for TUBE FINS if you are interested in more interested in simulating more creative tube fin profiles.
 
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