[calebkruse]

I've launched one of these balloons before. Made it from Colorado to the Gulf of Mexico and started making the turn up the coast before losing contact overnight.

The balloons are standard mylar balloons, but slightly bigger than average. About 2.5 feet in diameter. In order to allow the balloon to climb to ~30k feet, you need to leave a lot of room for the gas to expand. I'd say our balloon was only about 15-20% full at launch. You put just enough helium in the ballon so that it has about 1 gram of lifting force.