Nuts & Volts - July 2005

Near Space

Figure 9. BalloonSat closure.

Rubber bands make quick and cheap closures. My near space experiments have demonstrated the ability of rubber bands to function in the ozone, increased UV, and the cold of near space. So, I currently use rubber bands on all closures of my near spacecraft.

Using rubber band closures requires a simple modification to a BalloonSat, adding and gluing two wooden dowels (3/16-inch diameter dowels work well) into the airframe. Cut the dowels 1-1/2 inches longer than the width of the BalloonSat. I found it best if I carved trenches (for the dowels) into the side, or top and bottom walls of the BalloonSat before gluing the airframe together.

After the airframe is glued together, cover the BalloonSat exterior, insert the dowels, and then lock the dowels into place with a little hot glue. The hot glue also prevents the BalloonSat exterior covering from ripping where it meets the dowels.

Styrofoam airframe, the flight line will eventually cut its way through the airframe. To prevent this destruction, BalloonSats have a tube running through their center. I’ve seen both metal and plastic tubes in BalloonSats. The metal ones add unneeded weight and their edges can chafe the flight line. An alternative is to use a ballpoint pen case, as recommended by the University of Idaho.

The plastic is lighter than metal and plenty strong, however, it has a limitation. Most ballpoint pens have bodies only 4-1/2 inches long (their diameter is fine). What if you want your BalloonSat to be six inches tall?

Many hobby stores sell polystyrene modeling materials. So, purchase a package of plastic tubes, 3/16 inches in diameter from your local hobby store. Each tube in the package is over 12 inches long and since they’re made from polystyrene, they’re easily cut with an Exacto knife.

If you place a plastic tube

down the center of a BalloonSat, the tube will reduce the BalloonSat’s useable volume. So, I experimented with placing tubes inside the wall of the airframe and found that it works well. Of course, this means the tubes are off center and will tip the BalloonSat. However, this modification leads to my recommended changes in the BalloonSat carrier, which I describe later in this article.