According to one estimate, there are only 25 blimps still operating in the world. It is unclear whether this estimate refers only to proper non-rigid blimps or if it includes, for example, the three Goodyear “Blimps,” which are not blimps at all but semi-rigid airships. Either way, airships are an endangered species.
What will it take to bring back these majestic beasts of the sky? Hydrogen.
You may recall hydrogen from its most iconic contribution to popular media, as the lifting gas that erupted in flames in the Hindenburg disaster. The accident killed 35 of the 97 souls on board, plus one ground crewman at Lakehurst, New Jersey. Although air transport fatalities weren’t rare in the 1930s, the incident was seared into public consciousness by newsreel footage of the biggest aircraft ever built bursting into flames, paired with Herbert Morrison’s unforgettable simultaneous eyewitness lamentation: “oh, the humanity.” The haunting image marked the end of the airship era.
But even before the Hindenburg explosion in 1937, there were special interests lining up against hydrogen: the helium lobby. When helium was discovered co-mingled with natural gas in the early 1900s, the Bureau of Mines saw an opportunity to create a little empire for itself. In a demonstration before Congress in 1922, a Bureau representative set fire to two party balloons—one filled with helium, and one filled with hydrogen. The helium balloon gently deflated, but the hydrogen balloon exploded. Members of Congress were startled. How could they ask airmen to fly on airships filled with such an evidently unsafe lifting gas?
Fun fact: pure hydrogen doesn’t burn. It needs an oxidizer—like the oxygen in air. A mixture of hydrogen and air with 4% to 75% hydrogen will burn. More or less hydrogen than that, and it won’t. In a 1969 book recounting the early days of the helium industry, Mines employee Clifford W. Seibel all but admits to having added air to the hydrogen balloon in the demonstration before Congress—that is, to having intentionally misled Congress to secure greater authority for his agency.
The ruse worked. Congress banned the use of hydrogen as a lifting gas in the U.S. military airship fleet. The Helium Act of 1925 banned exports of helium—now a material of strategic importance—and gave the Bureau of Mines broad authority to control helium production, effectively creating the National Helium Reserve that is only just winding down operations.
Aside from flammability, hydrogen and helium have some other differences. At 0ºC and standard atmosphere, hydrogen has a density of 0.0899 kg/m3, while helium’s is 0.1785 kg/m3. Under the same conditions, air has a density of 1.293 kg/m3. This means that at sea level on a 0ºC day, hydrogen provides enough buoyancy to lift 1.2031 kg per cubic meter, while helium can only lift 1.1145 kg per cubic meter of gas. Hydrogen, then, provides about 8% more gross lift than helium does.
This difference in gross lift is non-trivial for airship performance, but by far the bigger effect of helium on airship economics is cost. Hydrogen prices vary widely by production method and location, but conventional hydrogen can be had in some locations for $1.25/kg, which works out to about $0.112 per cubic meter. Helium prices have been wildly fluctuating since Congress instructed the Department of Interior to sell off the contents of the National Helium Reserve, but last year, the private sector price was $7.57 per cubic meter. Helium, in other words, is around 67 times more expensive than hydrogen.
When Congress mandated that the military airship fleet switch over to helium in the 1920s, both the greater expense and the worse performance of helium was fatal. Because helium was in short supply, the military couldn’t afford to vent lifting gas. On the USS Shenandoah, they capped the safety valves. In the wee hours of September 3, 1925, the airship was caught in a violent updraft over Ohio that carried it beyond the pressure limits of its gas cells. With the safety valves capped, the ship was torn apart. 14 crew members lost their lives.
Helium also appears to have caused the crash of the USS Akron, the most deadly airship accident in history. On April 4, 1933, the lower-performing lifting gas did not give the ship enough altitude in a rough storm off the New Jersey coast, and a vertical plunge from a pressure change caused the ship’s aft end to drag in the water. It never recovered. An investigation did not get far as only 3 of Akron’s 76 crew members survived.
Today, although the issue doesn’t come up much, the Federal Aviation Administration (FAA) seems disinclined to approve a civil airship that uses hydrogen as a lifting gas. An advisory circular issued in 1992 states succinctly, “Hydrogen is not an acceptable lifting gas for use in airships.” Other guidance from 1995 says “the lifting gas must be non-flammable.” Dutch and German airship standards that the FAA says they will accept as airworthiness requirements likewise say: “The lifting gas must be non-flammable, non-toxic and non-irritant.”
Let’s pause here to ponder the absurdity of this position. Flight operates using four forces: lift and weight on the vertical axis, thrust and drag on the horizontal. What the FAA is saying is that you can use a flammable material to generate thrust, but you cannot use that material to generate aerostatic lift. The agency would happily approve a hydrogen-based propulsion system for an airship. Using fuel cell propulsion, depending on the design range, a fully hydrogen-based airship would have to carry about the same mass of H2 on board as fuel and as lifting gas. It is roughly the same amount of flammable material, and for thrust it’s okay, but for lift it is not.
Still, hydrogen can’t burn without an oxidizer, a fact that gives rise to continued debate about the Hindenburg disaster. If hydrogen was responsible for the initial fire, it must have been due to an uncontrolled gas leak that allowed hydrogen to mix with air. Two eyewitnesses reported seeing the fabric cover on the upper port side fluttering as if gas was escaping before the fire started. The mixture of hydrogen and air may have been the initial source of fuel for the blaze, but it seems to have spread quickly because the outer fabric hull was treated with a highly flammable compound chemically similar to thermite.
Assuming the fire was caused by leaking hydrogen, then it is only fair to acknowledge that similar leaks of fuel lines have caused disasters on airplanes. What has improved aviation safety is not the removal of flammable fuels, but increased rigor in design, manufacturing, monitoring, and maintenance. There were many airship accidents in the first third of the 20th century, and few of them had anything to do with hydrogen. The airplane safety record of the period was not much better. All sorts of mistakes were made because both technologies were early in their development. With modern engineering standards, there is no doubt that hydrogen could be made a safe lifting gas.
Why does this matter if there are only 25 blimps in service today? Because it is still possible that, under more suitable rules, there is a market for thousands of airships. The Hindenburg disaster marked the end of passenger airship travel, but it was not due to safety alone. The DC-3 was introduced in 1936, the year before the accident at Lakehurst. It could take passengers cross-country in 18 hours with only 3 refueling stops. After the war came the Constellation, with its 5,400-mile range and pressurized cabin that allowed it to fly above the weather. These iconic airplanes made airships entirely impractical for passenger travel.
If airships were to make a major comeback, it would be in cargo service. Airships are too slow for human travel, but cargo containers don’t get fatigued on long flights. In fact, they spend weeks or months on large container ships today, stopping at every port along their way. Point-to-point airship service for cargo could get containers anywhere in the world in a few days, rather than in weeks or months, at a cost much lower than putting them on a 747 freighter. It’s a sweet spot in freight service that could attract a lot of demand in the middle category of value-to-weight ratio: cargo like machinery, equipment, and parts.
Cargo airships would need to be big—bigger than the Hindenburg. Airships are blessed and cursed with a square-cube law: the drag of the airship, which is proportional to its cross-sectional area, scales with length squared, but the volume of lifting gas, and thus the gross lift, scales with length to the third power. Therefore, the lift-to-drag ratio, a critical parameter in aircraft performance, gets better as the airship gets bigger.
Ginormous airships require a lot of lifting gas—perhaps a million cubic meters. Given the excellent economics of airships this big, the demand could be high–thousands of airships would be needed to carry only a small percentage of today’s container volume. This means the airship industry would require billions of cubic meters of lifting gas to meet demand. Helium is the second most abundant element in the universe, but it is relatively rare on Earth. If we were to try to meet this airship demand with helium, it would require on the order of total known helium reserves.
Helium, of course, has other critical uses besides airships. Among other applications, it is used to cool magnets in MRI machines, to create a safe environment for arc welding, to maintain purity in semiconductor manufacturing, and to pressurize propellant tanks for rocket engines. Attempting to use helium also for a large fleet of cargo airships would be expensive, as it would significantly bid up the price of the gas, and it would disrupt these other industries that are heavy helium users. It’s better to use hydrogen for airships instead.
The FAA’s prohibition on hydrogen as a lifting gas is only guidance, and in any case, the agency has wide leeway to change its mind. Virtually every aircraft certification program has waivers and special conditions negotiated with and agreed to by the FAA—essentially new rules based on a study that shows the negotiated condition provides for an adequate level of safety. It’s possible that an airship program targeting cargo operations—particularly if it were uncrewed—could get approval for hydrogen as a lifting gas.
But with decades of guidance to the contrary, the FAA has discouraged the return of the airship in the use case that makes the most sense. The only way to get an answer to the hydrogen question is to invest millions of dollars in starting an airship program and engaging the FAA on a certification basis. In turn, investors shy away from regulatory risk, especially when it falls outside their area of expertise.
This, sadly, is how innovation dies. This is what we have to overcome if we want to recover the airship and watch thousands of them dancing across the sky.
CNBC
New Atlas
Popular Mechanics