  A light-weight "inflatable" tensioned-membrane-structure vacuum container is
proposed and its stability is analyzed. The proposed structure consists of a
pressurized lobed cylindrical "wall" surrounding a central evacuated space.
Stability is analyzed by discretizing the system and diagonalizing the second
derivative of the potential energy. The structure is found to be stable when
the pressure in the wall is greater than a critical pressure. When membranes
are non-elastic, the critical pressure is found to be greater than the pressure
required for equilibrium by a factor of 4/3. When membranes have only finite
stiffness, a first order correction to the critical pressure is found.
Preliminary experimental data shows that a stable structure can be made in this
way, and that the observed critical pressure is consistent with theory. It is
also found that such structures can be designed to have net positive buoyancy
in air.
