It was the loss by a hydrogen fire of the Italian-built Roma, after it struck a high tension line at Langley Field in February, 1922, which fixed the policy of "helium only" for U. S. Army and Navy airships. The Army's C-7 was the first airship to use helium. In building the Pilgrim in 1925, Goodyear followed the same policy--even though it had to pay $125 a thousand cubic feet for helium while it could have obtained hydrogen for $5 per thousand.
Further improvements and increasing volume of production brought the cost down in time from $125 to less than $20, and helium expense became relatively unimportant in providing safety for Goodyear's airship operations.
Important too during this period was the Army's development of tank cars for transporting helium. A large item of helium expense was freight, the cost of hauling 130 pound metal containers which held 170 to 200 cu. ft. of the gas. It took 250 such containers to inflate Goodyear's smallest ship, the Pilgrim. The tank cars hold 200,000 cu. ft. of gas, almost enough to inflate two Goodyear airships.
Experiments with specially woven fabric and the use of synthetic rubber cut down the losses resulting from diffusion, and where formerly it was necessary to remove the helium and purify it every six months, diffusion losses were cut to one or two per cent a month, with purification needed only every other year.
In addition to increasing safety, helium permitted improvements in airship design. The wartime craft had its control cars suspended by cables from finger patches cemented to the outside of the bag. But with helium ships the car could be built into the bag, attached by an internal catenary suspension system to the top of the gas section. Each exposed suspension cable, no matter how small, creates parasitic resistance from the air, so that the removal of yards of steel and rope had the result of increasing the speed of the ship with the same horsepower.
The second set of major improvements centers around the mooring mast. The mooring mast idea was not new. The British had built the first ones during the World War for its large rigid ships, found that a ship attached to it would swing easily, like a weather vane, continuing to point into the wind, and that a well streamlined ship would hold securely even in winds of great velocity.