noun
or bal·lon·net |balə|nā
( -s )
Etymology: French ballonnet, diminutive of ballon
: a gas-tight compartment of variable volume within the interior of a balloon or airship used for controlling its ascent or descent and for maintaining pressure on the outer envelope so as to prevent deformation and kept inflated under the control of valves by a blower or by the action of wind caught in an air scoop