Tracy Barnes, a 22-year-old University of Minnesota student, is a former Army paratrooper who got tired of always coming down. After discharge from the Army he decided, for a change, to go up instead. Aided by his mother in the family backyard at Wayzata, Minn., he got together a sewing machine, some nylon cloth and two tanks of propane gas, and proceeded to make a 70-foot hot air balloon that actually carries him aloft. Thus far Barnes has made six flights, the highest to 8,000 feet. He regulates his flight by the amount of propane gas he burns: the hotter the air, the higher he goes. "It's so serene," he says. "All I can hear is the faint hissing of the burner." Someday, Barnes hopes, hot-air ballooning will become a popular pastime. "My balloon only cost me $300," he says, "and it will support as much as 300 pounds." Barnes's mother is also optimistic. "After parachuting," Mrs. Barnes says, "a balloon looks safe."