First time I've clicked on a Google link in a long, long time; they changed their format.
Probably Helium, but it could be steam.
www.researchgate.net: Advances in Steam Aerostation
Quote:
Such an airship filled with steam lift gas could of course be powered by a conventional gasoline or dieselaeronautical engine, but the intriguing possibility arises of using a steam engine. The spent exhaust steam from the engine would naturally be discharged into the envelope to replenish the lift gas. Since a steam airship must in any case carry a boiler for reboiling the condensate water, and since the envelope itself would function as the condenser for the steam engine, only the actual steam engine itself would be required in addition. A steam reciprocating engine can be quite lightweight and is very reliable, and its high torque and low rpm characteristics are very suitable forairship application. Moreover, maneuvering thrusters could be driven by steam vane motors, which are very light indeed, so they could be mounted at any convenient point on the envelope. (ed: Else plasma actuators, obv.)
....
Any possible actual development of a steam airship is a long way off, but the general outlines of what might be possible are emerging. Within limits, the picture is encouraging. First, the obvious disadvantages of using steam lift gas in an airship are relatively low lift, and the necessity for reboiling the condensate water. The advantages are cheapness, and the ability to deflate the airship after each flight, thus obviating the need for a hangar or mast. A rigid airframe would sacrifice this second great advantage of steam lift gas while preserving all its disadvantages, and so I think that the idea of a rigid steam airship is a non-starter.
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So why not a semi-rigid? The Plimp:
Egan Airships: The PLIMP™ Invention
But compare the maneuverability of the Roboloon:
At 1:06 watch it cut the propeller half way and coast to it's destination.