Dave, may I suggest that instead of using BB's, you go to Wal-mart and buy fishing sinkers in sporting goods for your ballast.

While you are at Wal-mart, in the home repair area, you can get an aerosol can of foam insulation called "Great Stuff" you can use this to fill your nosecones. Put in a little at a time, in steps or stages so that you do not blow your nosecones apart. Expansion is about 25:1. It is cheap and pre-mixed.

If I had access to RockSim, I would do the calculations and then weigh my finished rocket so that I would know how much weight to put in. If you do not have that, then there is the old school method, where you find the center of gravity of your ready to fly rocket, with the motor in place. Then make a cardboard cut-out of your rocket, as accurate as possible. Find the center of gravity of your cut-out. This will be a rather accurate representation of where the Center of Pressure of the rocket will be. If the CG is at least one body diameter ahead of your CP, if not further, then you should be OK. You can do this all before you foam the nose and then you should be good to go. The cardboard cut-out is scalable also, so find the percentage of the length of the rocket for your CP, and then multiply or divide for the other rocket. I hope this helps.

thanks for the practical info Bear. But what's the reasoning behind the sinker vs. the BBs?

Greater weight, less volume. Therefore, as you establish your center of gravity, you can get more weight further forward, moving your center of gravity further forward, making it so you do not need as much ballast. In some events, this allows your rocket to go even higher. Tungsten is better than lead, but more expensive. Wal-mart has tungsten fishing sinkers also. Uranium is even heavier, but I have an issue with using depleted uranium. Issues like cancer and death. 😯

I would caution against the "Great Stuff" spray can foam.

Most (but not all) of the spray can varieties rely on exposure to air to cure, unlike the epoxy-based 2part foams that (justifiably) cost a bit more. The problem comes when you fill a closed space. The "skin" cures, but the inner foam can stay mushy for a VERY long time. I figured this out the hard way when using it for sound deadening some closed in spots in the body of my car. Ended up having to dig most of it out until air eventually cured it.

Good luck on the refurbs!
-Ken

Another reason for applying it in layers. A little at a time. If you allow it to expand in thin layers, you keep it from getting dense or too dense. I used it to seal a dog house and in a model airplanes. Small amounts and let it set and dry, then do it again, and you void the mush and the increased density so that it stays lite. I never let it get more than about 3/4" thick per layer.

I have both types of foam, but is there an issue with adhesion to the cone wall?

Both stick really well if the surface is clean. I would wash the surface with warm soap and water, then afterwards you lacquer thinner or alcohol, allow to dry and then use your foam. I use ethyl alcohol most of the time.

What about epoxying in a piece of all thread, 5/16", and stacking washers on this. Then you can tailor your nose weight to what you need, and be able to have a way to do tracking in the future. You will also have an anchor point for recovery.

2 part expanding foam sticks to nearly everything. I have adhered epoxy and foam to plastic nosecone in the past by adding about 1/2 cup of acetone to the inside of the nosecone. It then use 150 grit sandpaper to mechanically scour the surface and the acetone gets a nice deep cleaning. I dump that out and rinse with more acetone and then let dry. I've never had the epoxy fail to adhere well after this. Another option for adhesive is PL Premium. It is a polyurethane construction adhesive that sticks very well to plastics. I have built entire rockets using this instead of epoxy.

Edward