When Clemens did the math, he realized that a working model airship of this type would have to be huge. He would need lots of helium--at least 150 cubic feet worth--to carry the weight of the internal wooden frame. That meant lots of interior space. By his calculations, the model would have to be at least 20 feet long. Anything smaller, and the weight of the frame would negate the lift of the gas. But he also couldn’t go any larger; a 20-foot-long airship would just fit, on a diagonal, inside his two-car garage.