Abstract: A lighter-than-air craft, which may be a freely floating balloon or may be an airship with its own propulsion, has an envelope which is filled with at least a substantial proportion of steam to provide buoyancy. Liquid water which accumulates at a low point of the interior of the envelope is passed out from the envelope and is discharged into the atmosphere, and stored ballast is also discharged into the atmosphere. The ballast discharge mass rate may be brought to be approximately equal to the water discharge mass rate, either manually or automatically; and the ballast discharge mass rate may be manually varied within a certain range around the water discharge mass rate. The ballast may be water. A drain valve may be provided for draining off liquid water as it condenses upon the inside of the envelope while preventing gas from escaping or entering, and a boiler may be provided for reboiling the drained off water into steam which is supplied back into the envelope.