The stratospheric airships have large inflatable membranes capable of retaining the lifting gas. Moreover, they are equipped with additional interior bags that contain air, adapting the internal pressure to the one outside so that the airship maintains its shape with minimum effort. In other possible architectures helium is enclosed in the inner bags (gas bags) while the air is confined in the outside membrane, which gives the aerodynamic shape.
In any case, the thin membrane (whose thickness can be as low as few tens of microns) must withstand the pressure differences that, integrated, counteract the weight of the car (Archimedes' Principle). Structural design is a compromise between minimum stress in the fabric and the need for an aerodynamic form.
Models and optimization programs have been developed that allow designing the best forms that ensure structural integrity and that facilitate manufacturing from flat fabric patches. The program is able to add static aerodynamic forces and rigid structural elements inside, which cooperates in the progressive concentration of loads and therefore produces an optimal design.