Physicists, engineers or anyone else with such arcane knowledge: I want to determine whether it's feasible to fill a hot-air balloon with helium, rather than heating air each time the balloon is used. I emailed someone in the county's hot-air balloon club, but he blew me off with "The balloons are filled with hot air, not helium." No foolin'? Is there someone out there who would know how to find out how much helium is needed to lift X weight?