Hydrogen and Helium in Rigid Airship Operations
The two primary lifting gases used by dirigible airships have been hydrogen and helium.
Hydrogen is the earth’s lightest element and can be manufactured easily and inexpensively, but hydrogen’s extreme flammability makes it unacceptable for manned airship operations. (In addition to the obvious example of the Hindenburg disaster, dozens of hydrogen-inflated airships were destroyed by accidental fires before hydrogen was finally abandoned as a lifting gas in the 1930s.)
Helium is a relatively rare and expensive natural resource, and because helium is heavier than hydrogen it can reduce a rigid airship’s useful payload by more than half, but helium’s inert non-flammable nature makes it the only practical lifting gas for manned lighter-than-air flight.
Hydrogen and Helium: The Basics
Hydrogen and Helium are the two lightest elements on the periodic table:
Atomic symbol: H (as a gas, H2)
Atomic number: 1
Atomic weight: 1.007
Atomic symbol: He
Atomic number: 2
Atomic weight: 4.002
The atomic weight of a hydrogen atom is approximately 1/4 that of a helium atom, but since hydrogen as a gas exists only as a diatomic molecule (containing two hydrogen atoms) hydrogen gas is approximately 1/2 the weight of helium gas.
The Relative Lifting Ability of Hydrogen and Helium
Although helium weighs twice as much hydrogen, each gas is so much lighter than the air surrounding an airship that helium theoretically provides about 93% of hydrogen’s lift:
Relative lifting ability of 100% Hydrogen vs. Helium
60° F, Barometric Pressure 29.92″ Hg
|Weight of Lifting Gas
(per 1,000 cu. ft.)
|Weight of Air
(per 1,000 cu. ft.)
(per 1,000 cu. ft.)
|Hydrogen||5.31 lbs||76.36 lbs||71.05 lbs|
|Helium||10.54 lbs||76.36 lbs||65.82 lbs|
The actual lifting ability of both hydrogen and helium varies with temperature, pressure, and humidity, and in practical operation it is impossible to achieve or maintain 100% purity of either gas, giving helium about 88% of the lift of hydrogen in actual application. To take account of varying atmospheric conditions and gas impurities, airship designers often conservatively estimated helium’s lift at 60 lbs per 1,000 cubic feet and hydrogen’s lift at 68 lbs per 1,000 cubic feet.
The Effect of Helium on Airship Range and Payload
Because so much of a dirigible’s weight is fixed (in the form of the ship’s structure and engines, called “dead weight,” and required payload, such as crew and ballast) a helium-inflated airship has a much lower useful payload and considerably less range (because it can carry less fuel) than a hydrogen-inflated airship of the same size. For example, when the German-built LZ-126 was delivered to the United States it was inflated with hydrogen, and the ship flew from Friedrichshafen, Germany to Lakehurst, New Jersey nonstop; when the United States Navy operated the same ship with helium (as U.S.S. Los Angeles) its range was limited to 3,925 statute miles and it could not have made the same flight.
The following chart illustrates the dramatic reduction in payload from the use of helium versus hydrogen. (The information is based on Hindenburg’s Flight No. 10, from Rio de Janeiro to Friedrichshafen on April 6, 1936, as reported by U.S. Navy Lt. Cdr. Scott E. Peck.)
|Gross lift/hydrogen (68lbs/1,000 cu. ft.)||215,910||476,000|
|Payload for passengers, mail, freight w/ hydrogen||9,560||21,076|
|Gross lift/helium (60lbs/1,000 cu. ft.)||190,509||420,000|
|Payload for passengers, mail, freight w/ helium||-15,841||-34,924|
Operational considerations further decrease the useful payload of a helium-inflated airship. As an airship rises, its lifting gas expands; an airship that begins a flight with its gas cells fully inflated must therefore release gas as it climbs to keep the cells from bursting. Because hydrogen is easy to manufacture and inexpensive to buy, hydrogen airships often began flights fully inflated to maximize payload and released hydrogen as they climbed. But since helium has always been a rare and expensive gas, helium airships began their flights at only 90-95% inflation, thus reducing payload, to allow their gas cells to expand without releasing helium. In addition, hydrogen airships compensated for fuel burned during flight simply by releasing hydrogen; helium-inflated ships, on the other hand, required heavy water-recovery apparatus (to recover water ballast from engine exhaust), which further reduced the useful payload available for fuel, passengers, and freight.
(Helium blimps do not need to vent helium to maintain equilibrium; they employ internal ballonets, or air sacs, which can be inflated or deflated to maintain the blimp’s shape and buoyancy.)
While the use of helium therefore presented operational challenges, airships of sufficient size were able to operate effectively when inflated with helium. LZ-129 Hindenburg was specifically designed to operate with helium and could easily have conducted transatlantic operations with helium as a lifting gas, and the United States Navy’s rigid airships were also able to fulfill their missions with helium; U.S.S. Akron and U.S.S. Macon were even able to serve as airborne aircraft carriers, carrying embarked fixed-wing aircraft, using the heavier gas.