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Hindenburg was originally designed for helium, heavier than hydrogen but nonflammable. Most of the world's supply of helium comes from natural gas fields in the United States, which had banned its export under the Helium Control Act (1927). Eckener expected this ban to be lifted, but to save helium the design was modified to have double gas cells (an inner hydrogen cell protected by an outer helium cell).[1] The ban remained however, so the engineers used only hydrogen despite its extreme flammability.[2] It held 200,000 cubic metres (7,062,000 cu ft) of gas in 16 bags or cells with a useful lift of approximately 232 t (511,000 lb). This provided a margin above the 215 t (474,000 lb) average gross weight of the ship with fuel, equipment, 10,000 kg (22,000 lb) of mail and cargo, about 90 passengers and crew and their luggage.
The Germans had extensive experience with hydrogen as a lifting gas. Accidental hydrogen fires had never occurred on civilian Zeppelins, so the switch from helium to hydrogen did not cause much concern. Hydrogen also increased lift by about 8%. After the Hindenburg disaster Eckener vowed never to use hydrogen again in a passenger airship. He planned to use helium for the second ship and went to Washington, D.C. to personally lobby President Roosevelt, who promised to supply the helium only for peaceful purposes. After the annexation of Austria in March 1938, U.S. Secretary of the Interior Harold L. Ickes refused to supply the gas, and the Graf Zeppelin was also filled with hydrogen.
