Abstract: A collapsible fuel tank for aircraft, the tank having rigid top and bottom elements and foldable rigid side walls containing therein a non-vented fluid-impervious elastomeric bladder. When fluid is pumped into the bladder, it distends to deploy the top, bottom, and side walls whereby a box-like structure whose rigid walls protect the bladder is erected. The withdrawal of fluid from the bladder causes it to contract such that the walls are folded and the tank thereby collapsed into a space-saving volume. The tank can be mounted on the outside surface of an aircraft and the contraction of the tank as fuel is expended effects a reduction of the aircraft frontal area to reduce drag.