L2 using LOC 4" Cyclotron - CP Question/Observation

[bibbster]

I'm working on building the LOC 4" Cyclotron for my L2 certification. When I input all the parts, except for the motor, in OpenRocket, the center of pressure is about twice as far back as it should be. Is this a quirk of tube fins in OR, or is there something else amuck?

 

[bobbyg23]

Where should it be?

 

[bibbster]

Based on my understanding, the CP should be aft of the COG about twice the diameter of the body of the rocket, so in this case about 8". OR has it at about 16" behind the COG.

 

[David_Stack]

While it may be a issue with the way Open Rocket is calculating CP, you really need to think of it the other way around.

CP is fixed, based on the physical characteristics of the airframe and the forces acting upon them. CG on the other hand is variable, influenced by weight values within the airframe themselves. Additional nose weight moves the CG forward relative to the CP, a larger heavier motor moves the CG aft, closer to the CP.

For stability the CG should be at least one caliber ahead of the CP as a general rule (rockets exceeding mach need still more margin, short stubby rockets can get away with less).

 

[bibbster]

So I guess 8 had it backwards in my head. By adding the weight of the motor to the rocket file, the COG will move back towards the COP, which will likely put it within the 1 caliber margin.

And perhaps all of the fillet work that will have to be done will help too.

Thanks for getting me straightened out, David.

 

[Tractionengines]

OR has some issues with Tubefins.
1) If using OR 15.03 (old version) there is a note that Tubefin support is experimental. But it seems to give "close enough" performance SIMs

2) If using 22.02 beta version. There is an error with it causing very high CD values... there is a ongoing analysis in the software fourm here, and on the OR site.

Here is a note from @Neutron95 that matches what I gave seen in flight data.

In general, I've found that the 15.03 tubefin model has been a pretty decent predictor of performance, especially for subsonic flights, whereas 22.02 gives results that are about half the altitude predicted by 15.03.

 

[dhbarr]

Other challenges aside, a tubefin where the fins are taller than their diameter will have a CP very close to the top of the fins.

 

[Neutronium95]

OR has some issues with Tubefins.
1) If using OR 15.03 (old version) there is a note that Tubefin support is experimental. But it seems to give "close enough" performance SIMs

2) If using 22.02 beta version. There is an error with it causing very high CD values... there is a ongoing analysis in the software fourm here, and on the OR site.

Here is a note from @Neutron95 that matches what I gave seen in flight data.

I believe that the cause of the excess drag has been found, and it should be fixed in the next beta release. But yes, 15.03 is the version you should be using if you want to simulate a tubefin rocket right now.

 

[bibbster]

Thanks for all of the input guys.

I added the motor which moved the COG closer to the COP. Still isn't the ideal number but it is closer!