A door system for a movable structure, the movable structure comprising a plurality of spaced apart frame members and at least one tensioned structure membrane extending between at least two of the spaced apart frame members and forming an interior space under the spaced apart frame members, the interior space having at least one open end (160, 170), the door system comprises: first and second axles arranged coaxially and attachable to the structure at first and second sides of the open end (160, 170) of the interior space; a first door module comprising: a first frame extending from a first end of the first frame hingedly attached to the first axle, to a second end of the first frame hingedly attached to the second axle; and a first tensioned door membrane, separate from the at least one tensioned structure membranes, supported by the first frame, wherein the first tensioned door membrane is stretched taut over the frame of the first door module; and a second door module comprising: a second frame extending from a first end of the second frame hingedly attached to the first axle adjacent to the first end of the first frame, to a second end of the second frame hingedly attached to the second axle adjacent to the second end of the first frame; and a second tensioned door membrane, separate from the tensioned structure membranes, supported by the second frame, wherein the second tensioned door membrane is stretched taut over the frame of the second door module; the first and second door modules (230, 250) being rotatable about the first and second axles between a closed position in which the first door module extends across an upper portion of the open end (160, 170) and the second door module extends across a lower portion of the open end (160, 170), and an open position in which the open end (160, 170) of the interior space is substantially unobstructed by the first and second door modules (230, 250), and in which the second door module is nested under the first door module in an essentially vertical position in the open position, uncovering the entire opening in the end of the structure, and the second door module is not nested under the first door module in the closed position. first and second axles arranged coaxially and attachable to the structure at first and second sides of the open end (160, 170) of the interior space; a first door module comprising: a first frame extending from a first end of the first frame hingedly attached to the first axle, to a second end of the first frame hingedly attached to the second axle; and a first tensioned door membrane, separate from the at least one tensioned structure membranes, supported by the first frame, wherein the first tensioned door membrane is stretched taut over the frame of the first door module; and a first frame extending from a first end of the first frame hingedly attached to the first axle, to a second end of the first frame hingedly attached to the second axle; and a first tensioned door membrane, separate from the at least one tensioned structure membranes, supported by the first frame, wherein the first tensioned door membrane is stretched taut over the frame of the first door module; and a second door module comprising: a second frame extending from a first end of the second frame hingedly attached to the first axle adjacent to the first end of the first frame, to a second end of the second frame hingedly attached to the second axle adjacent to the second end of the first frame; and a second tensioned door membrane, separate from the tensioned structure membranes, supported by the second frame, wherein the second tensioned door membrane is stretched taut over the frame of the second door module; a second frame extending from a first end of the second frame hingedly attached to the first axle adjacent to the first end of the first frame, to a second end of the second frame hingedly attached to the second axle adjacent to the second end of the first frame; and a second tensioned door membrane, separate from the tensioned structure membranes, supported by the second frame, wherein the second tensioned door membrane is stretched taut over the frame of the second door module; the first and second door modules (230, 250) being rotatable about the first and second axles between a closed position in which the first door module extends across an upper portion of the open end (160, 170) and the second door module extends across a lower portion of the open end (160, 170), and an open position in which the open end (160, 170) of the interior space is substantially unobstructed by the first and second door modules (230, 250), and in which the second door module is nested under the first door module in an essentially vertical position in the open position, uncovering the entire opening in the end of the structure, and the second door module is not nested under the first door module in the closed position.