Trucks of the Sky
By Patrick Ming
January 2nd, 2024
The highly exothermic thermite reaction is well known for its destructive capabilities, able to reach temperatures upwards of 2,500 ॰C. It is this exact reaction that caused the Hindenburg Disaster. With the aid of a spark, the coating of the airship, which contained the necessary ingredients, jump-started the thermite reaction, lighting the 7.062 million cubic feet of hydrogen gas stored within the airship on fire. In a mere 32 seconds, the pinnacle of the era of innovation in transportation was brought to its knees. The event was so disastrous that it was the turning point of the age of airships; the disaster is now recognized as the beginning of the end of the era of blimps and airships.
Even ignoring the potential safety hazards of being on board a giant balloon of highly flammable (H2) gas, airships have not recently been in contention as a mode of cargo transport in the modern era for many reasons. They are big and slow; the Hindenburg could only reach a maximum speed of 84 mph (135 km/h), while modern-day commercial flights can reach up to 600 mph (966 km/h).
However, with recent developments in the cargo transport industry, many aircraft manufacturers have proposed using airships in long-distance transport. Organizations such as Hybrid Air Vehicles (HAV), ATLANT, Lockheed Martin, and many others have begun the work to bring about the new age of air transport.
Currently, trucks are the primary mode of domestic transport in the US, with an average of 250,000,000 freight tons being transported across US highways and road systems annually. With trucks being our dominant mode of domestic transport, it is already as optimized as possible. While a truck is far from as fast as a plane, it is cheaper; conversely, while not as cheap as a barge ship, it is much quicker than waterway transport. Thus, the truck is in the sweet spot of both cheap enough and fast enough. However, this sweet spot only exists for domestic transport, leaving us with only the choice of either a slow but cheap method or a fast but expensive method.
The lack of practicality in using trucks for international transport raises the question of what other modes of transport fit in the sweet spot. The answer lies within the airship, the new trucks of the sky. While airplanes generate lift from their wings and the excessive burning of an abundant source of fuel just to stay in the air, airships effectively get their lift for free, as the buoyancy force is alone generating the lift, meaning no propulsion is necessary to keep the airship aloft.
Today, three main classes of airships are used: non-rigid airships (blimps), semi-rigid airships, and rigid airships. A nonrigid airship is a giant balloon with a small car/gondola attached to the bottom. As a result, the balloon itself experiences tension forces. When scaling up the size (for our purposes, we will treat this as r, the radius of the cylindrical airship) of nonrigid airships to account for increased cargo load, the tension forces experienced by the balloon will increase due to the square-cube law. Another consequence of scaling up a non-rigid airship is the potential for the blimp to deform and struggle to maintain its shape as the volume increases. Thus, scaling up the size of blimps is impractical and dangerous.
The second viable option for cargo transport is the use of semi-rigid airships. Similar to blimps, they only have partially rigid structures within the balloon to give it shape. However, fundamentally, it still functions as just a giant balloon, meaning it has the same flaws as blimps, making it increasingly challenging to balance the safety of the blimp against the volume of cargo transported.
The last viable option is rigid airships. Rigid-body airships have a well-defined rigid internal structure that helps maintain their shape. Unlike the other two classes, the rigid airship does not function as a giant balloon; instead, it functions more like a rigid protective covering around multiple giant balloons called gas cells. These cells can be filled with lighter-than-air gasses or emptied of them to make the airship more or less buoyant. As a result of the rigid structure, the surface of a rigid-body airship does not suffer from tension forces. Another benefit of the rigid-body airship is the lack of scaling limits. Because they are unpressurized, a rigid-body airship can be made of all sizes. Thus, it is more beneficial to scale the volume of the airship for a more significant capability to transport cargo and increase the length of the airship to include more gas cells.
The lift force of the airship is proportional to the volume of gas. In contrast, the drag force is “proportional to some combination of cross-sectional area and wetted area–in any case, it increases with area.” Thus, an airship’s efficiency depends on the lift-to-drag ratio, which can be simplified to the volume-to-area ratio; due to the square-cube law, this ratio will always increase. In principle, the “performance of an airship gets better as it gets bigger. Forever.” Consequently, when rigid airships are scaled to incredibly large sizes, airship-driven cargo transport becomes feasible.
Unfortunately, many problems arise when thinking about the practicality of constructing a legion of rigid-body airships to revolutionize the transport industry. For one, the abundance of helium is a privilege we cannot take advantage of forever. Even if companies like the aforementioned HAV state that the helium in their crafts can be continually reused, with only around 15% needing to be replenished annually, the dangers of the world running out of helium is a grave concern. In 2019, Dr. David Cole-Hamilton, a professor of chemistry at the University of St. Andrews, stated that he estimated that the world would have only around ten years left of helium if proper effort was not put into recycling. If hydrogen gas is chosen instead of helium, the dangers of yet another Hindenburg disaster become a pressing concern. Additionally, though a rigid-body airship can indeed be constructed to carry a payload of 500 tons, it would have to be almost 388 meters long, making it the largest aircraft ever built. To satisfy the world’s demands, a whole legion of these 388-meter-long airships must be built, which will be both costly and challenging.
For over 90 years, airships have been underappreciated for their potential to revolutionize the cargo transport industry. Though many concerns are to be addressed, with innovation and sustainable practices, the return of airships to the skies may indicate a new era of global transport. Let us celebrate the inauguration of the new trucks of the sky!
