I believe you mean the center of pressure, as the center of gravity is just finding the balence point of the rocket, this can be done by balencing the rocket on the ruler, the cente of pressure can be roughly estemated using the cardboard cutout method, which is balencing a cardboard template of your rocket on a ruler and finding that point, it gives you a very conservitve result, and, usually resulting in an over stable rocket. The barrowmans stability equations is the last method, but it assumes the drag your body tube and that you have 3 or more fins, finally there is the extended barrowmans equation which takes into account the body tube and less than 3 fins probobilty
Beat you but there is somthing extremely strange, I got .71 stabitlty with extended barrowmans but got 3.41 on the cardboard cutout methodI presume you mean where you want the CG to be. Ideally, it should be one caliber (largest airframe diameter) ahead of your center of pressure (CP), which is similar to the neutral point (NP) in model airplanes.
The simplest (and most conservative) way to calculate the CP is the Cardboard Cutout method. Simply cut out a silhouette of your rocket in a piece of cardboard and balance it on a knife blade. The place where it balances is the CP at a 90 degree angle of attack. Because this is the farthest forward the CP can ever be, your rocket will always be stable if your CG is further forward than the CP.
The absolute best way to determine the CG of a rocket is to install the heaviest motor you plan to use in it, and then balance the rocket on a straightedge.
You gotta love tHe archive.... One of these days I'll have to check it out
I always like to set my cg on planes at 33% it makes it more fun to fly when they are touchy.And in the model airplane world 33% of the chord back from the leading edge is an approximation of where it should be, not necessarily where it is. Actually that's a bit aft for many airplanes....
John's answer is the way to go to find out where it IS for a given rocket. Some of the discussion about simulators and cardboard cutouts and cranking through the Barrowman equations sheds light on where it should be.
Some of the discussion about simulators and cardboard cutouts and cranking through the Barrowman equations sheds light on where it should be.
And adding on a little bit to the end of Troy's post, c.g. location is NOT found through the use of Barrowman eqns or cardboard cutouts. Those tools are for estimation of c.p., or center of pressure.
Of course..Fair enough... I should've been more clear and was thinking in model airplane terms where one moves stuff around to get the balance point in the right place relative to the aerodynamic configuration of the airplane rather than in model rocketry terms where often the aerodynamic configuration may be in play as much as the mass distribution within it. But either way, correcting a tail heavy (or insufficient stability margin) situation can be done the same way - add nose weight. Using the CP calculations just suggest where the CG should be by using CP as a necessary reference point, which is what I was thinking.