Center of Pressure in Open Rocket

[ScienceC]

I have been trying to calculate the Center of Pressure of a model rocket with and without fins using open rocket. However, whenever I take away the fins the center of pressure seems way too low. I calculated the CP to be 9.22 inches by hand (by using the areas and distances of a 2D cross section), however open rocket has been telling me its 2.09 inches, which doesn't seem right. I'm brand new to this software so is there something I am doing wrong?

Thanks,
Chase

 

[dhbarr]

At M=0.3

 

[ScienceC]

Thanks, I did not notice that! However, I changed it to M=0 and I am still getting the same value. Is there something else that I am doing wrong?

 

[neil_w]

It's possible the speed of measurement is indeed an issue. In general, I find OR gives weird results when there are no fins. I don't understand the details of the Barrowman equations to know why. Again, maybe that M=0.3 contains the answer.

 

[dhbarr]

I'm not sure whether or not OR will let you plot a sim result without a motor selected. Pick an MMX and plot static vs. dynamic CP? Just spitballing here.

 

[rocketguy101]

I calculated the CP to be 9.22 inches by hand (by using the areas and distances of a 2D cross section)

Did you use the 2D area of the body tube in your calculations? Classic Barrowman calculations neglect the body tube because "For small angles-of-attack less than 10 degrees, the force on any cylindrical body portion is so small it can be neglected" [quote from Centuri technical report TIR-33]. I haven't read the OR technical paper in a long time, so I'm not sure if they are using the direct Barrowman equations, or some modification like RockSim uses.

 

[mikec]

The cutout balance method is much more conservative than Barrowman. You shouldn't expect them to produce the same answer.