How can I find the center of pressure of a model rocket?

[Anonymous]

I made a custom model rocket a few days ago and I posted some pics online.  A lot of the rocketry community gave me crap that my fins are too small, and my nose cone was not the right shape.  

I made a CAD of the rocket in inventor and I would like to run a simulation so I can see what it will look like when the air is flowing over my rocket.  Hopefully this test will show me that everything will be OK and I am good for lauch... But I have no idea where to start.  I am a noob and I have never done a dynamic simulation of any sort.

Thank you!

Here's a STEP file of the rocket if it helps people, as of now I don't have the density of the parts set because I am still trying to figure out how to add custom densities.  I know that there are somethings about the rocket that don't seem right, like the nose cone that doesn't go over the sides of the tube - but that was a machining error on the actual rocket and I wanted to make the CAD as similar as possible.

[Anonymous]

I downloaded Autodesk CFD Simulator 360 2013 (or something like that) which seems to be the product I am looking for, but I have no idea of how to use it.  Could someone please explain how I can get the little lines of air to flow over my model like they show in all of the demo photos of the product?

Thank you

[luiww]

The centre of pressure is the centre of gravity if the whole assembly is enclosed.  Simply add solid to the individual parts, making sure the material is homogenous for all the parts.  I use the default which is water.  Then click centre of gravity from the view tab.  I found it to be 10,452in from the base.

[Anonymous]

Thank you for looking at my model, but the center of pressure and center of gravity are indeed different.  I have been experimenting with Simulator CFD and I think that I will be able to use  that to find my answer.

[hosford]

Take a look at this:

http://exploration.grc.nasa.gov/education/rocket/rktcp.html

if you try "Mechanically determining cp" it sounds pretty easy.

[graemev]

The quick and dirty way would be to create a new part from within the assembly file.  Make a sketch on a plane that is parallel to the axis of the rocket.  Project the profile of all the rocket's external components onto the plane.  Exit the sketch and extrude the resulting (closed) profile.  The new part should look like a silhouette of the rocket.  Now look for the iProperties of the new part, click on the Physical tab, update if necessary and the centroid should be listed.  Note that value and compare it to some datum on the rocket.  (The tip of the nose cone or a tail fin may be handy references.)  Just be sure you have the correct absolute dimension if crossing an origin plane.  As a quick check, make a new sketch on the silhouette part, make a circle at the calculated centroid that does not cross the outer profile, exit the sketch, extrude (either cut or add), and recheck the iProperties for the new centroid.  If the dimension hasn't changed the circle is at the correct location.

It's not CFD accurate, but it's close.