A non-rigid airship, commonly known as a blimp, maintains its shape through the internal pressure of the lifting gas it contains, unlike rigid airships that rely on a structural framework. Blimps are utilized for activities like advertising, surveillance, and observation due to their stable, low-speed flight characteristics. Hydrogen is the lightest element and provides the greatest buoyancy, yet modern blimps exclusively use helium. This choice to use a heavier, more expensive gas instead of a lighter one suggests that performance is secondary to other, more pressing considerations.
The Physics of Buoyancy
The ability of an airship to lift off the ground is governed by Archimedes’ principle, which states that an object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced. For an airship, the displaced fluid is the surrounding air, and the lift is generated by filling the envelope with a gas significantly less dense than air. Hydrogen is the least dense gas available.
Helium is nearly twice as heavy as hydrogen. Despite this difference in mass, the actual gross lifting power of helium is only about 8% less than that of hydrogen. This marginal performance advantage of hydrogen is significant for payload capacity and range.
The Critical Difference Flammability
The defining factor separating hydrogen and helium is their chemical nature, specifically their behavior when exposed to oxygen. Hydrogen is a highly reactive molecule that burns readily and can explode violently when mixed with air in certain concentrations. The combustion reaction of hydrogen is extremely energetic, making it a severe hazard in the event of a leak or structural breach.
In stark contrast, helium is a noble gas, rendering it chemically inert. This atomic stability prevents helium from engaging in combustion or any chemical reaction with oxygen in the air. Therefore, even if a blimp’s envelope were to tear open in flight, the helium would simply dissipate harmlessly into the atmosphere. The choice of helium completely eliminates the risk of a catastrophic fire or explosion during operation, mooring, or maintenance.
The inherent safety of helium allows airships to operate closer to populated areas and to carry passengers with confidence. A small leak of hydrogen could rapidly create a volatile mixture with the air inside a hangar or near an engine, whereas a helium leak only results in a loss of lift.
The Defining Historical Event
The widespread adoption of helium was accelerated by the catastrophic loss of the German rigid airship Hindenburg in 1937. While the Hindenburg was a rigid airship, the disaster demonstrated the extreme danger of using hydrogen as a lifting gas for passenger air travel. The speed and scale of the resulting fire, which consumed the massive craft in mere seconds, shocked the world and solidified public fear of hydrogen-filled airships.
This event effectively ended the commercial era of hydrogen airships. The immediate and lasting impact on aviation policy was a global shift towards mandatory use of non-flammable lifting gases for virtually all civilian airships. The United States, which had already begun using its domestic helium supply for its military airships, became the primary source for the safer gas.
The Economic and Supply Reality
While safety is the primary driver, the decision to use helium is complicated by economic and supply challenges. Helium is a non-renewable resource, typically extracted as a byproduct from specific natural gas wells, making its availability geographically restricted and finite. This scarcity results in helium being significantly more expensive than hydrogen, which is abundant and can be produced cheaply through various methods.
Helium atoms are so small that they can escape through the material of the airship’s envelope. This constant leakage means blimps require frequent and costly “topping off” with the expensive gas to maintain internal pressure and shape. The decision to use helium, therefore, represents a conscious, expensive trade-off where safety is prioritized far above operational cost and maximum lifting performance.