The Technological Principles of Airships – Part 1
The optimism of airship advocates is based on scientific principles of which most people are only mildly aware. Understanding how helium or hydrogen gas allow an airship to float, and other principles of how airships work, enables us to appreciate what giant airships could do, if they were built using modern technology. Economic analysis of the giant airships begins with a clarification of their potential capabilities based technological principles.This post and the following three introduce the technological principles of airships.
Principle 1. Avogadro’s Law, Gas Buoyancy, and Aerostatic Flight
Airship technology begins with Avogadro’s Law. As illustrated below, balloons of the same size, temperature and pressure that contain different gases, do not weigh the same. The weight of the gas in these balloons is (roughly) proportional to the size of the gas molecules. In this example, the oxygen balloon is eight times heavier than the helium balloon.
Like fish in water, we rarely consider the weight of the air around us. It may come as a surprise, but a cubic meter of air weighs more than a kilogram. As per Avogadro’s Law, its weight is proportional to the molecules that comprise air that is 78% nitrogen, and most of the rest oxygen. A quick peek at the periodic table of the elements reveals that two gases in particular, hydrogen and helium, are made up of molecules much smaller than nitrogen and oxygen, so these gases are lighter than air. By replacing the air in a container with these lighter gases, the container becomes lighter. As a rule, for each cubic meter of air replaced the container generates about one kilogram of lift.
The buoyancy principle for a gas-filled airship is similar to the operations of a submarine, which controls buoyancy by substituting water for air (to go down) or vice versa (to go up) in its ballast tanks. Since the density of water is about 1,000 times more than air, for every cubic meter of water a submarine expels from its ballast tanks, the vessel becomes one ton (1000 kilograms) lighter. Compressed air is stored onboard the submarine, which can be released to push water out of the ballast tanks. If enough water is replaced by air, the submarine gradually floats to the surface because its overall density is less than the water around it.
The lift of an airship is directly proportional to the weight of the air that is replaced by helium or hydrogen. As an easy way to envision this relationship, an electric stove occupies about one cubic meter of space. This much gas provides one kilogram of lift. So, in the case of the giant Zeppelins that had a gross lift of 150 tons, their size would be about the same as 150,000 kitchen stoves stacked together. In fact, the largest Zeppelins were about 250 meters long and 38 meters in diameter.
Consequence 1.1. Aerostatic flight – Lower energy consumption
The ability of airships to float in the air is called aerostatic flight. They require no expenditure of energy or effort, in contrast with the aerodynamic flight of airplanes and helicopters. Over half the fuel consumed by airplanes and helicopters is used just to remain airborne.
Airplanes need to have air moving over their wings to remain aloft. Airplanes cannot stop, and if they slow down below some minimum velocity, they start to descend. Helicopters fly by passing air over their propellers to create a pressure differential equal to their weight. Essentially, the propellers are rotating wings. A helicopter can hover in one place but it has to burn a lot of fuel to do it. Airships can turn off the engines and hover indefinitely. Engines are required only for maneuvering or forward motion.
Being able to hover opens up unique applications from tourism to operating like cranes to acting as flying warehouses. Being able to land at their leisure also means that airship can wait out the fog or other bad weather, and stand in a queue before landing at a busy station. Being able to fly relatively slowly also gives an airship range flexibility. Because of the speed’s effect on drag, an airship travelling at half the velocity can go approximately twice as far on the same amount of fuel.
Consequence 1.2. No need for runways – Infrastructure independence
Another advantage of aerostatic flight is that it eliminates the need for runways in order to land. Airplanes must fly fast to stay aloft and when they land, they are still moving at high velocity. They need long runways to slow down, typically over one kilometer in length, and longer as airplanes get larger. Runways are expensive to build and to maintain.
Airships do not need runways because they can take off and land vertically. The degree of infrastructure independence can vary depending on design choices and ground-handling methods, but some airships are likely to be able to “land anywhere,” taking off and landing from any large area of unimproved land, or body of water. Aerodromes, mooring bases or stations of some sort are needed for refueling and transshipment, but airship infrastructure should be much less expensive than airports. Some airship hangars are required for inspection, maintenance and repair, but such hangars operate more like the drydocks of ocean-going ships than airplane hangars. Most of the time, the airship will be flying from base to base earning its keep.
Consequence 1.3. Huge cargo bays – Few cargo restrictions
Related to airships’ gigantic size is the huge spatial capacity of their cargo bays. If a cigar-shaped airship is 200 meters long and 40 meters in diameter, a hold of 25 by 10 meters could be easily accommodated. No other transport modes except ships and barges can readily accommodate such large cargo. Trucks are limited in size by the width of highway lanes, the height of bridge-clearances and the bends in the road. Trains can carry heavier weights, but the rail infrastructure also limits the size and dimensions of freight carried. Airplanes are the most limited, because of their narrow fuselage and runway requirements.
Airships are widely recognized as a promising future transport option for extremely bulky project cargo, such as wind turbines, giant transformers and reactors. Moreover, they can carry full loads of low-density cargo like furniture, running shoes and insulation. This kind of cargo is carried rather inefficiently by existing modes because it exhausts the spatial capacity of trucks and airplanes before they reach their weight capacity. Highway weigh stations may lead people think that most trucks are easily overloaded. In fact, about 80 percent of all trucks are underloaded because they reach their volume limit before exceeding their full weight potential.
Consequence 1.4. Extremely large lightweight structures – “Flying tents”
For their enormous size, airships must be very light, yet strong enough to withstand large forces. This is so counterintuitive that it is worth invoking the analogy of a camper’s tent, misleading though it is in some respects, to help train the imagination to grasp the engineering concept.
Anyone who has gone camping has experienced the “magic” of a tent that is so light that it can be carried in one hand, but so large that a family can sleep in it comfortably with all their backpacks. The tent structure maintains its shape even though most of the material that comprises it is, of itself, shapeless. A camper’s tent ingeniously maximizes the ratio of volume to weight thanks to smart engineering and light-weight materials.
Airships require the same sort of “magic”: very large, and ultra-light. G.K. Chesterton once wrote, as a metaphor for mythology, that “there are no rules of architecture for a castle in the clouds,” but for an actual, practical castle-in-the-clouds, the architecture needs to be all the cleverer for the difficulty of the feat. Just as the architecture of a camper’s tent is much smarter than that of a brick wall, airship technology is deceptively sophisticated.
By looking at the pictures of the Graf Zeppelin or the Hindenburg, you would not guess their resemblance to tents. The taut fabric is as flat and opaque as metal so they look solid. In fact, only the skeleton was made of rigid aluminum rings. The covering was just painted cotton canvas. The interior was stressed with cables and filled with gas cells that occupied most of the space.
The extreme need for lightness makes materials crucially important for aerostatic flight. For example, carbon fiber, which has been available since the 1960s, is a material stronger but much lighter than steel. It is not as widely used, because it is expensive and difficult to shape in manufacturing processes. But it is a top choice among aspiring airship builders. Advances in materials are one of the reasons why airships built today would have advantages over those built in the 1930s. A booming airship industry, in turn, could be expected to catalyze further development of new materials for better airships.