Airships

For many of us, airships occupy a sort of odd speculative space left open where materials science, aviation, engineering, computerization, and air traffic control have all improved massively while airships themselves have seen comparatively little use. That leaves a lot of room for argument and a handful of startups that promise that everything is fixed now and they can slot neatly into this low carbon, slower than planes, faster than ships, with fewer transfers, cargo or passenger niche.

The interesting thing is that airships didn't actually vanish with the Hindenburg, though there are certainly fewer examples in operation, and many of those that remained were military. Still, these airships give us some solid evidence backing up the kind of improvements we expect from massively more powerful motors, better materials, etc.

Airships had a number of high-profile accidents in the early 20th century that stunted their development considerably, but it’s very easy to forget that airplanes at the time were far less safe. Not only did airplanes of the 1900s-1930s crash at a higher rate than airships, but when they did, their fatality rate was about double. Those crashes were far more significant for airships, though, because they were like the jumbo jets, supersonic airliners, or space shuttles of their time—huge, resource-intensive megaprojects that aren’t really able to be iterated on and tried again so easily as a relatively tiny airplane design.

even hydrogen airship accidents (which are far more lethal than helium airship accidents) were about half as lethal as airplane crashes of the same time period. Considering this was the 1900s-1930s, that’s a really low bar—aviation didn’t become even remotely safe until about the 1970s—but it’s worth noting that Zeppelin’s been flying its NT model airships for nearly 30 years without a single fatal accident.

it’s true that the overwhelming majority of airships were hideously underpowered. That’s down to the lacking engines of the time period, though. Some airships carried as much as 17 tons worth of engine, but none made more than 4,500 horsepower collectively. The airship pictured above has 32 electric motors, totaling about 10,000 horsepower, for a collective weight of less than half a ton. The reason airships are only just now beginning to be built again (with the largest airship built since 1938 being an electric rigid airship undergoing tests right now in San Francisco) is because aviation is a fiendishly difficult, expensive, and risk-averse industry to attempt a startup in, and airships being far more efficient than planes or helicopters was not considered an important enough thing to prioritize to justify spending hundreds of millions to get them going again.

That hasn’t been true since the 1950s. The U.S. Navy demonstrated that a properly designed airship can actually operate in blizzards and thunderstorms far more reliably than fixed-wing and rotary-wing aircraft, with a mission readiness rate of 88% in inclement weather.

An airship’s ability to land and take off in strong winds is directly proportional to its speed. Back in the ‘50s and ‘60s, U.S. Navy radar blimps were taking off and landing in blizzards and thunderstorms with over 40-knot winds. In practice, all-weather Navy airships were able to operate in worse weather conditions (besides just wind) than Navy helicopters because airships had a number of other characteristics that made it safer for them to do so, such as having more stability, being able to divert far greater distances to alternate landing zones, and having vastly greater endurance before running out of fuel, allowing them to wait for better visibility, precipitation, or wind conditions.

Hence, Navy blimps were able to operate in blizzards and thunderstorms that grounded all fixed-wing and rotary-wing aircraft. There wasn’t anything particularly special about the Navy blimps either — they had de-icing gear, variable-pitch propellers, sturdy tricycle landing gear, and reasonably powerful engines that gave them a top speed of 82 knots. As a rule of thumb, an airship can land and take off in wind speeds that are about half its top speed.

An airship designed to have a 200-knot top speed could thus theoretically land and take off even in a 100+ knots hurricane, though obviously no one would ever be crazy enough to do such a thing, nor would it be desirable — just because its engines could make enough headway in a hurricane to be able to land or get in the air doesn’t mean that it would appreciate being blasted with a bunch of flying debris near the ground. In practice, it would do what all airships and helicopters have done when confronted with a hurricane—simply go around or wait it out.

Even historical airships, which were incredibly crude aircraft with structural insufficiencies we can now readily identify, managed some impressive feats in high winds: The Graf Zeppelin once intentionally steered into a typhoon over the Pacific Ocean to try and pick up a tail wind to help speed it on its way during it's round-the-world flight in late summer of 1929.

airships have already been used to carry entire scientific platforms to the tops of rainforest canopies, which would have been damaged by a helicopter’s downwash? Or that landing only one landing wheel on a small circle and the pilot picking up a champagne bottle from a narrow plinth are both games used in recreational airship sport-flying competitions? The airships that did so didn’t even have thrust vectoring. Something like the LCA60T pictured above devotes an entire 66% of its propulsive power to vertical and lateral thrust vectoring, and has the same operational wind limits as a helicopter aircrane and a normal tower crane as a result. The LCA60T — an airship currently under development as a flying crane, that has the same operating wind limits as a helicopter or conventional crane. It can do its hovering cargo operations 250-320 days out of the year, depending on the location.

Modern airships address changes in weight in several ways, probably the simplest of which (aside from releasing the lift gas, or heating it during flight and letting it cool on the ground) being to just fly the ship heavier than air by the weight of the payload. With the structure still buoyed by helium, it remains quite efficient even while supporting the cargo with aerodynamic lift and/or vectored thrust, and then you can simply offload the payload at the destination, assuming it’s not able to take on any return cargo or extra fuel or water ballast or anything of the kind — sort of a “deliver your max payload to the middle of nowhere and come back” solution, which should hopefully not be needed too much in practice.

The remaining hurdles are in terms of the lack of available experts and sufficient funding to undergo a years-long research and development program for a large airship, but it has long been established in World War II and the Cold War that airships can be engineered to serve as safe, practical, low-cost alternatives to conventional aircraft where speed isn’t a priority.

Much in the same way we know that it is possible to build reliable, profitable high-speed rail, even if the concept of such a thing seems wildly out of reach to people in places where it doesn’t exist.

This section gathers broad categories of design and intended use

https://canadiandefencereview.com/arctic-sovereignty-airships-for-the-arctic/

Don’t forget the exponential growth curve of the square-cube law. It’s a double-edged sword. Small airships are not competitive with other aircraft or trucks, but large ones are. Small and midsized airships are indeed niche, but the largest modern airships under consideration have payloads of 200-1,000 tons, depending on the design and manufacturer. The largest cargo planes today carry about 100-150 tons of cargo. That, in concert with large airships’ increased efficiency, would allow them to pose a credible threat to a decent chunk of shipping, particularly for higher-value cargoes and somewhat more time-sensitive ones, such as fresh fruit and seafood. It would be more expensive than a ship, but cheaper than a plane, and currently the gulf between those two modes of transport is so vast that there are several profitable efficiencies to be found, once they’re actually built out. The “built out” is the hard part. Additionally, airships’ optimal speed increases drastically over shorter route lengths, due to the effects of fuel weight on payload and productivity. For 5,000 nautical miles, a typical rigid airship carrying 100 tons of cargo has an optimal cruising speed of 63 knots/72 mph. For 2,000 nautical miles, it’s 82 knots/94 mph. And for short-haul trips of 300 nautical miles, it’s 145 knots/167 mph. Do note those are the optimal cruising speeds, not the top speeds. Airships benefit from having reserve power capacity to account for headwinds without losing speed, in this case, the NASA study assumed a 15 knot headwind was reasonable, and calculated the optimal cruising speed (accounting for engine size, structural weight, fuel, etc.) accordingly. However, this study was done some time ago (mid-1970s), and modern propulsive systems have gone down in weight and up in efficiency tremendously since then. That could change the optimal cruising speed and feasible degree of excess power capacity for an airship, since those speeds are primarily dictated by the trade-off between fuel load and speed of cargo throughput. More fuel burn means faster, but also less cargo carried due to the weight of the fuel, hence why the optimum for shorter range is so much faster. For example, some modern airship designs assume a cruising speed of 115 mph is ideal over distances of several thousand nautical miles, rather than the 1970s optimum of 72-94 mph over similar distances. That’s not a trivial difference—to take an airship to 115 mph requires about four times as much power as that same airship traveling at 72 mph. Some modern designs just go ahead and keep the efficiency gains as savings rather than pressing to go faster, though. It really depends on the application.

Modern airships can outperform helicopters in pretty much every respect save for size. That’s why modern cargo airship designs are targeting the roles currently held by heavy transport helicopters first and foremost—in the most difficult and expensive part of getting a business off the ground, they perceive that as the matchup that is most favorable to them. An airship is overwhelmingly more efficient than a helicopter, can carry vastly more, and costs less to operate. They have far greater range, and operate in similar or worse weather conditions than a helicopter. They’re also far easier to convert to zero-emissions operations. The practical upper speed limit for a rigid airship is 200 knots, whereas most cargo helicopters cruise between 80-160 knots. With thrust vectoring, modern airships like the Zeppelin NT are also capable of maneuvering like a helicopter, which aids greatly in VTOL operations. Even in terms of speed, airships and airplanes have remained in similar positions since the 1930s—the cruising speed of a DC-3 is about 180 knots, and for an airship of that time period, it was 70 knots, or about 40% the speed. Today, the cruising speed for most airliners like the 737 is around 0.8 Mach, or 453 knots, but a Boeing study found the most productive cruising speeds for an airship carrying 100 tons for 300 nautical miles is 180 knots, which is still about 40% the speed. Granted, the optimal cruising speed for an airship does dip considerably over greater distances, with that same 100-ton-payload airship design’s optimal cruising speed dipping to 110 knots over 5,000 nautical miles, but many planes don’t fly that far anyway, and it’d still handily beat a helicopter carrying only 8 tons at 140 knots, but which would have to stop 17 times to refuel over that same distance, or over 200 times to carry the same amount the same distance.

Aside from carrying more weight, they could also carry things far larger, like wind turbine blades, prefab buildings, radio towers, etc. They can also hover, which is very useful, as evidenced by the fact that extreme STOL airplanes haven’t successfully replaced helicopters despite being wildly superior in practically every other way.

Navy airships I mentioned had about 1/3-1/2 the operating costs of planes with a similar payload capacity. More to the point, though, airships wouldn’t necessarily be competing with cargo planes primarily, but rather cargo helicopters—which cost at least ten times as much as normal air freight per tonne/km. They can also just plain do things that no airplane or helicopter can do at any cost, such as carry giant wind turbine blades and other outsized cargoes.

With ships, they can compete sometimes (fresh food, high-value manufactured goods, etc), with freight trains, definitely not, but trucks? The largest airships can compete with trucks in terms of cargo cost per ton/mile, and are considerably faster, in addition to their capability to carry things too bulky and/or too heavy for a truck. That won’t detract from trucks’ ability to transport things last-mile, of course, but there’s certainly some useful applications.

Gas cells are a very important safety feature as they introduce redundancy, similar to the watertight bulkheads in a ship or submarine. They’ve allowed several historic airships to survive catastrophic damage that would have destroyed a plane or nonrigid airship, such as attacks on World War One Zeppelins like the LZ-39, which survived repeated bombings by airplane. 20-lb high explosive bombs are akin to a modern Sidewinder missile’s warhead, and it managed to survive four of them and keep flying. It also helped during accidents, like when the British R33 collided with its own mast during a storm, and whose skeleton crew managed to fly it through the storm safely despite missing most of its bow.

double hull of inert gas to keep out the oxygen that hydrogen needs to mix with in order to form a flammable or explosive mixture. That’s how fuel tankers were rendered safer after the SS Sansinena explosion, and airliners as well after the TWA Flight 800 explosion. Carbon dioxide and nitrogen, respectively, are used to inert the empty spaces in partially full fuel tanks, which would become giant fuel-air bombs otherwise.

Airships actually benefit far more from electrification than other aircraft, for a number of reasons—which are many and varied, but basically boil down to the advantages of electric propulsion not being particularly helpful to airplanes and helicopters, while the disadvantages exacerbate their greatest weaknesses.

For airships, it’s the reverse—they’re greatly aided by the benefits of electrification, and the disadvantages of electrification aren’t particularly harmful to airships, or are even beneficial instead.

For example, airplanes and helicopters are greatly disadvantaged by the fact that batteries and fuel cells either don’t lighten at all or lighten far less than a kerosene fuel tank, which can be reduced by tens of tons over the course of a flight, making it much more efficient. By contrast, airships greatly appreciate a constant, unchanging weight since that allows them to operate more efficiently without having to compensate for changes in buoyancy.

The astronomical improvements in aviation safety would more than make up for the difference in safety between hydrogen and helium, such that a properly designed modern hydrogen airship would be incomparably safer than a historical helium one, but that doesn’t change the fact that hydrogen is always going to be more dangerous.

Traditionally Airships had to dock at a mooring mast (of which there were several types) or shelter inside a hangar. This is because an unpowered airship is basically a huge sail, and is likely to drift. Landing them on the ground was a huge and dangerous undertaking which involved landing parties of hundreds of men physically pulling the airship down to the ground by ropes. Attaching them to a mooring mast involved the tower crew and the airship crew both lowering lines which would be linked together by a ground crew so the tower could winch the airship in.

With improvements to maneuverability and control over buoyancy modern airships are far more controllable and can dock or land on their own.

Not all airships are designed to land, but those that do have such a light footprint they often land on completely unimproved grassy fields. A modern airship like the Lockheed-Martin P-791 can use its landing gear to stay fixed in place on the ground without any external support equipment with up to 40 knots of wind down the nose or 25 knots of wind from any other direction. A Cessna needs to be tied down at 25 knots to keep from being flipped over. They have also landed on lakes, beaches, swamps, ice floes, and aircraft carriers. Some of the new designs, such as those of Lockheed-Martin, have no ground infrastructure or crew requirements whatsoever.

Not all Airships are designed to land. Some, like flying crane designs such as the LCA60T, will dock at a mooring mast instead. The idea here is that the airship attaches nose-first to the tower and is allowed to freely rotate around it like a weathervane in the wind. This ensures that it always has the lowest possible exposure to the wind.

Modern mooring masts are almost disappointingly simple and are often deployed as part of a large truck.

When hooked up to a mast truck, airships can stay put in 70-90 knots of wind — and anything past that, they’d have to evacuate the area, because higher wind speeds than that would be a hurricane or tornado.

A new option that allows the best of both worlds is a large rotating platform design called a Boyant Aircraft Rotating Terminal or Depot (BART or BARD). This design allows for the convenience of landing (perhaps for loading and unloading cargo) while still allowing the airship (and the platform it's anchored to) to turn so it's facing into the wind.

Hangars are to airships as drydocks are to ocean vessels — they can be located on cheap land, since they don’t need to be visited very often except during initial construction or intensive tear-down maintenance overhauls/refits, which only happen rarely. Modern Airships are designed to spend almost their entire lives outside.

The best layman-accessible compendium on the various airship projects over the years, past and current, is Peter Lobner’s excellent “Modern Airships” series of articles, which are given a handy index and general airship industry overview/airship science summary here.

The best source for understanding airship science, economics, and design from a far more technical perspective is the Feasibility Study of Modern Airships, a vast, multi-phase, multi-part study for NASA and the Department of Commerce conducted in many separate parts by Boeing and Goodyear Aerospace. These can be found on NASA’s archives for free.