If you're looking to launch 15 fuel per second, and keep the balls at least an inch apart from each other, the speed at the top of the arc must be at least 7.5 ft/s, if your shooting interval is quite regular. (That is, keep a 6 inch separation between ball centers.) If it's more of a random timing, you would need to go significantly faster. Perhaps if you had two launchers running in parallel...
