Introduction to Airships
Airships are very ancient and poorly understood aircraft. The simple definition of an airship is a powered, steerable aircraft that uses buoyant gas, often helium or hot air, but also hydrogen in the past, to lift some or all of its structure. The "steerable" part distinguishes them from balloons (which have no propulsion mechanism and drift with the wind); the "buoyant gas" part distinguishes them from heavier-than-air aircraft like airplanes and helicopters, which maintain lift by moving airfoils through air rather than buoyant (or lighter-than-air[-at-that-ambient-temperature-and-altitude]) gas.
Airships have been around since 1852, but their commercial and cultural heyday was in the 1920s and 1930s. While limited to few roles today, such as advertising, tourism, surveillance and scientific research, they were once used for exploration, heavy cargo transport, maritime patrol, and anti-submarine warfare.
Most famous of all, however, were the leviathan rigid airships that were used as high-altitude heavy bombers and flying aircraft carriers, or the civilian ships used as luxurious transportation akin to an ocean liner. The sheer spectacle of these rare, gigantic aircraft ingrained them into the public consciousness in the early 20th century, and the disproportionate cultural impact of these ships has continued to this day.
Airships have inspired several tropes, including Cool Airship, Zeppelins from Another World, Airborne Aircraft Carrier, and Sky Pirates. Believe it or not, those last two actually happened. As time passed, powered flight against the wind has been viewed by the public as an impossible dream, then as a promising innovation when it was finally accomplished by an airship that used a steam engine to fly around the Eiffel Tower. By 1915, they had become an instrument of terror and mass murder, and after the Great War, they were redeemed as glamorous high-tech transportation for wealthy passengers and intrepid explorers. With the 1937 crash of The Hindenburg the public considered them unsafe and obsolete. Ultimately, they have become seen as an old technology with surprising new potential.
There are many different types of airship:
Hot Air Airships
Not all Zeppelins were built by the Zeppelin company. However, as stated above, the word Zeppelin has become more or less synonymous with rigid airships regardless of their manufacturer. Those made by other companies included the USS Akron and the USS Macon, the U. S. Navy's flying aircraft carriers, as well as the Schutte-Lanz series used by Germany during World War I. The first Zeppelin shot down over Britain, in fact, was the SL11, a Schutte-Lanz product.
Metal-clad airshipsAn interesting sub-class of zeppelins, with envelope made not of fabric, but of a very thin metal shell. Such metal air-tight skin, while costly and complex to construct, is much more durable than fabric, and could serve longer without need for repairs. It's also non-flammable, which is attractive for hydrogen-filled crafts. A lot of proposals to build metal-clad airships were made since the 19th Century, but only four were actually built, and only one - Detroit ZMC-2, built in 1929 - was actually proven to be an undoubted success. Unfortunately, the Great Depression stopped plans to build more such airships. While metal-clad airships were deemed impractical compared to their fabric-skinned counterparts, they appear frequently in fiction, often in Steampunk or fantasy stories.
Airships are often portrayed in the media as being Made of Explodium, which is somewhat justified, as there were dozens of hydrogen-related airship crashes, but oftentimes even helium blimps will be shown exploding in flames for no reason. The public at large seems to stereotype airships as ruinously expensive, disaster-prone, obsolete aircraft, with top speeds in the single digits, and that can be shot down with one shot from a BB gun. Oh, and the helium is running out. These stereotypes are all either untrue or gross exaggerations. Most of these stereotypes are born out of a warped sense of scale, or simple ignorance, so it's no surprise that their accuracy is spotty.
As for cost, airships are almost always cheaper than comparable aircraft, due to their mechanical simplicity and low power requirements. However, the costs associated with airships can be quite expensive. More primitive models of airship still require mooring masts, ground crews and hangars for storage; these are all expensive, but may or may not counterbalance the low cost of the airship. Hybrids, hot airships and some advanced airships avoid these altogether, as they are such a nuisance.
The durability of an airship is hard to explain. People confuse being easy to hit with being easy to shoot down, which is an entirely different matter. Counter-intuitively, airships are actually incredibly resistant to attacks because they are fragile. Artillery shells, rockets, bullets, even missiles pass straight in one end and out the other; they don't detonate against fabric. Airships' nearly nonexistent radar and heat signatures also make missile targeting difficult. The resulting entry and exit holes of missiles and bullets are so tiny in relation to the sheer size of the craft, and the gas is under so little (if any) pressure, that it's almost negligible. Even hydrogen airships did not explode when shot with ordinary munitions, or even incendiary bullets — fire inside thousands of cubic feet of hydrogen, but without oxygen, is useless. Fighters equipped with special explosive-incendiary bullets to combat them had to concentrate on one spot and empty one or two drums of ammunition before enough hydrogen would leak out to mix with atmospheric oxygen and ignite. A hybrid airship's durability is proportional to its volume, and is greatly augmented; it can fly using aerodynamics like a plane. Testing has shown it will take about two hours for 500 bullets to put a small blimp in danger of crashing; midsize Zeppelins can endure thousands of holes.
Even non-hybrid airships can lose a large quantity of helium and still fly; a military K-class blimp once had its gas vent stick open at sea, and it managed to fly all the way back to base despite losing 25% of its helium. British airship R33 could be flown back to base after being blown for a night by a storm over the North Sea with first 2 gas cells broken in April 1921. The Zeppelin LZ-39 was ambushed by flak cannons and artillery fire twice over the Ukraine, had all aft gas cells breached, had the forward gondola and engines blasted off the hull, yet still managed to slowly fly hundreds of miles back to Germany. Part of airships' durability stems from their loads of redundancies — lots of gas cells, many engines, several steering gondolas, etc. Some new hybrid airships are even capable of flying with no lift gas at all, essentially doubling as airplanes.
Airships have a reputation for being disaster-prone and/or vulnerable to the weather. Whilst this was certainly true for hydrogen airships 90 years ago, it should also be noted that these were, well, hydrogen airships 90 years ago. Modern airships are actually quite safe, and compare favorably to helicopters note . It's hard to make generalizations, though, because airships are just like airplanes when it comes to safety: an ultralight plane is less safe in inclement weather and crash situations than a 747. In the same way, some hot airships are blown backwards in a light breeze, but on the other hand, in the 1950s an American naval blimp program manned an Arctic airborne early warning station continuously for 10 days in the winter. The weather was the area's worst in years, grounding all other military and commercial airplanes with combinations and variations of ice, snow, rain, fog, and 65 mph winds. The airships went on sorties that lasted for days, in extremely heavy icing conditions, and succeeded in their mission with no crashes, effectively crushing the misconception that airships are inherently more vulnerable to weather than other aircraft. In fact, their natural buoyancy, lengthy endurance and ability to maneuver at low speeds makes them very well suited to ride out storms.
In fact, airships were so well regarded for operating in inclement weather in their heyday that several high-profile airship disasters were paradoxically caused by this because of the resulting overconfidence. American military airships USS Akron and USS Macon were lost because they were flown in weather that was unsafe for them, exacerbated by their highly experimental and thus unreliable construction; while British Zeppelin R-101 and Soviet semi-rigid V-6 Osoaviakhim both crashed into mountains while speeding through the storm at night, R-101 due to a navigational error and V-6 because of faulty maps. In both cases the night and bad weather prevented the crews from noticing their error until it was too late.
There are basically three determining factors to the safety of an airship: speed, control and structure. The speed of airships determines how much wind they can withstand. As a rule of thumb, wind exceeding the top speed of the ship in flight puts it in danger, and wind exceeding half its top speed makes landing dangerous. Control is a function of maneuverability. In terms of maneuverability, airships can be divided into two categories: ones with vectored thrust, or the ability to direct the force of the engines up and down and/or side-to-side, and those that lack it. Structure is simply the design and materials an airship is made of, and the amount of aerodynamic stresses and loads they can endure.
Lastly, there is indeed a helium shortage — but that is often mistaken for the helium running out. Rather, the current helium crisis is due to problems with delays with new production facilities, reserves being abandoned, existing outdated production facilities breaking down, and increasing demand. Waste factors in as well; the overwhelming majority of helium — which is used for things like industry and medicine — is perfectly recyclable, but is instead discarded. The United States alone has enough proven helium reserves in natural gas to last sixty years, and the country's unproven reserves are estimated to be 1,000 times that — not accounting for improvements in extraction methods, recycling or additional reserves. Furthermore, the other noble gases krypton and xenon are commercially extracted from the atmosphere; helium is more than 5 and 65 times as abundant than them in the atmosphere, respectively. Helium is constantly escaping the atmosphere into space, but it comprises a stable, significant percentage of it because it is constantly seeping from the ground due to radioactive decay. It is more expensive to distill helium from the atmosphere with current methods, but not enough to jeopardize certain competitive airship operations. Natural gas helium is currently around $.07 per liter, and if extracted from the atmosphere, it is estimated to be roughly three times as energy-intensive to produce. Even so, Helium remains a surprisingly small portion of the cost of an airship — the initial fill-up, which is the single largest gas-related expense, is typically only a small percentage of the cost of the ship itself, and airships leak very little Helium per annum and avoid venting it at all costs, so the cost of keeping it inflated is relatively negligible.
It should also be mentioned that helium was adopted on a large scale after the Hindenburg disaster "proved" hydrogen too dangerous, but the Hindenburg was, again, an old design using outdated technology. In the current age, with our enormous advancements, it's perfectly possible to build extremely safe hydrogen-filled airships. The reason why this is not done isn't practical — it's just a matter of public fear.
The Science of Airships
Airships operate on two foundational principles: buoyancy, or Archimedes' Principle, and the Square-Cube Law. Airships do not float because they are light; on the contrary, they can weigh hundreds of tons. Rather, they float because their average density is reduced by their lift gas, which means that their weight is displaced by the helium and supported by the denser air. This is why airships can't float into space. The second principle, the Square-Cube Law, states that the volume of an object goes up cubically with linear increases in size. That's why an airship only slightly larger than another can have double the volume of the smaller one, and it's also why Zeppelins carry seemingly disproportionately huge loads in comparison to blimps and balloons.
Helium gas can carry about a kilogram (2.2 pounds) per cubic meter, roughly the same as hydrogen, despite having double the molar weight (4x the atomic weight but hydrogen forms diatomic molecules). The difference in buoyancy is about 8%. Hot air at the typical temperature of 110 degrees Celsius or so can carry a third as much as helium can. For much of airships' history, helium was undiscovered or unavailable, forcing them to use flammable hydrogen instead. This, in conjunction with very primitive construction and safety methods, caused dozens of crashes.
Due to the obvious size incentive, airships are quite large. The Hindenburg was the largest, at 804 feet in length and 135 feet in diameter. The interior was somewhat like a cross between a fancy hotel, a luxury train, and an ocean liner, with staterooms for 70 passengers. It contained two hundred thousand cubic meters — or over seven million cubic feet — of hydrogen gas, which provided 250 tons of lift — at a time when the world's heaviest airplane had a maximum takeoff weight of 56 tons. It could fly at about 130 km/h, or 85 mph. Airships in general are slightly slower than helicopters (hybrids about the same), and between 1/4 and 1/5 as fast as a jet airliner. The world flight endurance record is held by a Cold War-era military airship called Snowbird, which stayed aloft with a crew of 13 for ten and a half days, and crossed the Atlantic twice in that time. Even small blimps are efficient enough to stay aloft for more than 24 hours at a time.
Altitude is an extremely important factor when it comes to lift; because air becomes increasingly rarefied at higher altitudes, the relative lift of helium is reduced drastically. Compounding these restrictions is the "pressure height" of airships. The air ballonets that regulate hull pressure (and in Zeppelins, the space the gas cells have to expand) are also a limiting factor. The helium gas expands as the airship rises, gradually filling out the hull. To go beyond this point, gas must be vented, but this is extremely dangerous, as when the airship descends again, air equalling the vented helium will reenter the hull and weigh it down. In practice, this means that most advertising blimps have a flight ceiling of 10,000 feet, Zeppelins can reach 5-25,000 feet, depending on the type, and specially-designed high altitude airships with tiny payloads, huge ballonets and little helium can reach 60-110,000 feet. How much an airship can lift depends greatly on its maximum altitude.
The inside of a large airship is unlike any other aircraft. Because airships are both much larger and much less structurally dense than other aircraft, combined with the fact that they can carry larger payloads than any other type of aircraft, they must have very large, spacious interiors in order to facilitate proper weight distribution throughout the hull. This is the primary reason why the Hindenburg stands to this day as having the most interior space of any aircraft, just the A deck alone had more floor space than all 3 decks of the world's largest passenger jet, the Airbus A380. On military and commercial craft, this space is dedicated to long corridors with fuel, crew accommodations and supplies, interspersed with aircraft hangars and cargo bays, respectively. On luxury craft like the Graf Zeppelin ll and the Hindenburg, this space was devoted to promenades, staterooms, dining rooms, lounges, bars, and so on and so forth. The gondolas of large airships were quite amazing, they were large, wrapped with windows from floor to ceiling. The airship would be controlled with a spoked helm and circular engine telegraphs, just like on a nautical ship, and the officers and The Captain would be dressed in full naval regalia — justified, as they were usually recruited from the surface fleet.
In contrast, the interior of a small airship or blimp is anything but luxurious, particularly for military blimps. Because the engines of a blimp are typically mounted to the gondola, it is noisy and uncomfortable. The severe restrictions on weight meant for military airships that food was sometimes cooked over hot exhaust pipes, and latrines were simply a can, a curtain, and a rubber hose. The interior was either too hot, or too cold. Bunks were a luxury, hammocks were the norm. On the bright side, like large airships, small blimps do have spectacular views, being fitted with huge panorama windows.
The Origin of Airships
Airships were a dream of early aviators even before the first manned balloon flew in 1783. In 1852, Henri Giffard first flew in a tiny 113,000 cubic foot blimp powered by a 3-horsepower steam engine which weighed several hundred pounds. It had a less-than-impressive top speed of six miles per hour. Airships would then go on to be powered by electrical motors and eventually gas engines. By 1897, David Schwartz invented the first rigid, metalclad airship. But it was not until the 1900 flight of Count Zeppelin's LZ-1 that airships began to move out of the experimental stage. At first, they were used by the military for surveillance purposes, until Zeppelin founded the world's first passenger airline, DELAG, in 1909. During World War I, Zeppelins, blimps and semirigids were used by many nations. They enjoyed much success in naval patrol and surveillance niches, but German attempts to use Zeppelins as high-altitude heavy bombers rapidly became too costly when the British defenders, after many failures, finally came up with a successful countermeasure — the incendiary bullet. They were replaced by fleets of Gotha planes. The development airships experienced during the war was vital for their later commercial service.
Dawn of the Golden Age of Airships
After the war, the military and the public at large became fascinated with airships. This ushered in the "Golden Age of Airships," which began in the mid 1920s and ended by 1940. In the interim, militaries successfully experimented with creating true airborne aircraft carriers. Advertisers found small blimps useful for carrying brand logos, and the Russians used them to carry cargo to far-flung, remote regions. But the most famous and popular use was as luxury transportation. The Graf Zeppelin, the most successful airship in history, become a beloved international icon. It circumnavigated the world many times faster than the airplanes before it. She travelled to the north pole, visited great cities and monuments, and eventually settled into a transatlantic route, becoming the world's first transcontinental airliner. Her safety record was perfect, and she flew over one million miles. Many other airships saw success during this time, but all was not well...
The Fall of Airships
Though the Graf Zeppelin and the Zeppelin airline in general became known as an icon of safety, like all aircraft of the time, there were still safety problems. The size of airships had increased rapidly, but the materials, design, flight procedures and technology of the early 20th century were all still extremely primitive. Even the Americans, who had switched to Helium instead of hazardous Hydrogen, were met with crushing tragedies due to the loss of their two sister aircraft carriers Akron and Macon. Both were lost in storms at sea, the Akron due to crew error and a malfunctioning altimeter, and Macon due to a flawed post-design modification to the tail fins, combined with improper handling. Meanwhile, the British unsuccessfully attempted to bring their "Imperial Airship Scheme" to fruition. The British government's bid for the project was the R101, which was the world's largest and most luxurious skyliner to date. However, due to appalling build quality, flying headfirst into a storm in spite of warnings, insufficient testing and egregious leadership incompetence, the R101 crashed on her maiden voyage (hold on, this sounds extremely familiar...) The competing design, R-100, despite being very well-built, was subsequently scrapped. After this rather inauspicious series of events, the wreckage of the doomed R-101 was collected and reforged into a brand-new airship, this one surpassing even the doomed R101 in sheer size and luxury: the LZ-129 Hindenburg. These events marked the beginning of the end. The demise of the Hindenburg brought an extremely abrupt end to the rigid airship. Its explosion was literally the penultimate flight of any passenger Zeppelin, and by 1940 not a single one remained.
After the devastating tragedy that ended rigid airship travel just as it was beginning, Helium blimps continued on, defending Allied convoys from Axis submarines with stunning success during World War II. During the war, they escorted over 89,000 ships, losing only one to a submarine attack; as convoys that weren't protected were sunk left and right by U-Boat wolf packs. American blimps enjoyed the highest mission readiness of any aircraft during the war. After the war ended, American blimps continued to be used for a few decades as airborne early warning radar relays and sub-hunters, but that niche was driven into obsolescence with the adoption of nigh-undetectable nuclear submarine and ICBMs. In the meantime, Goodyear had continued operating a handful of blimps to advertise its brand. This stability — or some might say, stagnation — went relatively undisturbed until the invention of the hot air airship in the 1970s, and their subsequent rise in advertising popularity, but even then, it was limited to the advertising niche. Many times during this fallow era from the mid-40s to the late 90s, there were attempts to revive the airship beyond its advertising role. These attempts generated a little press, but few generated any flying prototypes. One that did reach prototyping stage was the disastrous Forest Service Helistat, which consisted of four chopped-up, obsolete helicopters affixed to a spindly frame and a Cold War-era blimp envelope. The helicopters destabilized, and it fell apart and burst into flames and shrapnel within seconds of taking off, killing the pilot. Another attempt was the Aereon project, which was the subject of the popular book The Deltoid Pumpkin Seed, by John McPhee. It flew, and was a sound concept, but it failed to get funding. With each attempt at a revival never getting off the ground, airships became increasingly associated with Vapor Ware, and few regarded them seriously anymore.
By the 1980s, airship technology had atrophied to such an extreme, even advertising was proving to be complicated for airships. Then, a British company called Airship Industries introduced the next generation of airships. Their successful "Skyship" series of airships are in use to this day, and employ advanced composite materials, ducted fans, and vectored thrust. These airships were vastly more competent all-around than their World War II-era Goodyear counterparts, and could do more than simply advertise — they can carry tourists in their spacious cabins, conduct research, and perform long-range scouting duties. Soon after, the Zeppelin Company, which had expanded into other markets following the Hindenburg crash, reentered the airship business with the advanced semi-rigid Zeppelin NT. The NT, which has recently been updated and adopted by Goodyear to replace their aging blimps, also employs vectored thrust and composites, which drastically cut the crew and maintenance costs while increasing performance.
Shortly after this mini-revival of the 80's and 90's, the Pentagon became keenly interested in airships during the 00's. The major airship project of that decade, ultra-heavy-lift WALRUS program, was cancelled and subsequently revived in a different iteration by DARPA working with NASA and the Worldwide Aeros Corporation. During late 00's and into the 10's, the two primary airship developments, Blue Devil II and LEMV, were both cancelled. However, the private market is producing far more flying prototypes than any previous revival schemes were ever able to. This can be attributed to an influx of practicality: airship startups that had gone defunct, such as Cargolifter, attempted to build gigantic, impractical airships weighing in the millions of pounds from scratch, with next to zero experience or prototyping. Now, however, established companies such as Lockheed Martin, Hybrid Air Vehicles and Worldwide Aeros Corporation are developing hybrid airships in the smaller, more economical 20-50 ton capacity range. Solar Ship, a Canadian company, has set their sights even smaller — they've produced three prototypes of a very small, solar powered flying wing hybrid airship that competes against small helicopters and bushplanes. After entering the market with these, then those companies will work on scaling the designs up for efficiency. Arguably the most significant development milestone has been the ongoing certification and liscencing process for production-model cargo airships; the creation of a new regulatory category of aircraft is currently under way.
Because they were available for far longer and were much more forgiving in terms of structure, aerodynamics and payload than airplanes, airships were experimentally used for a wide variety of niches — sometimes with the same ship. While some of these applications took off quite well, others went over like a lead Zeppelin. Here's a brief summary of the uses that airships saw, as well as their performance.
- Advertising: Possibly the most lucrative and famous commercial use for airships, nearly as old as airships themselves, one which has sustained them in periods of obsolescence in other fields. Hot air airships in particular are incredibly cheap and profitable for advertising; for $80-$200,000 you can buy a hot airship, and advertise very effectively with little continual investment compared to far more expensive conventional ads and commercials.
- Luxury Liner: While the ships themselves were astonishingly luxurious, to a degree unmatched to this day, in practice the results were decidedly mixed, thanks to hydrogen. The Hindenburg was profitable, a first for the Zeppelin Company, but unfortunately they were all scrapped due to the fire hazard before any could be used with nonflammable helium, so it's unknown how they would have fared in that niche.
- Heavy Cargo: First done successfully with the Soviets on a limited scale, typical airships have not been very suitable for heavy cargo operations. They can carry very heavy loads, and don't need runways, but their landing sites are a limiting factor — they require a large ground crew, large specialized infrastructure (hangars, masts, etc.), and delicate ballast exchange so their buoyancy doesn't make them uncontrollable during offloading. However, hybrids are completely the opposite; they're actually now being sought after because they require little to no infrastructure to take off, land, and offload cargo. In addition, they're fuel-efficient (which translates to long range), 1/3 the cost of similar airplanes and 1/10 the cost of similar helicopters, and are able to carry up to 500 tons of cargo for Hindenburg-sized ships.
- Science and Research: Airships have consistently shone in research applications. They are quiet, cheap relative to other aircraft, stable, slow, and perhaps most importantly, can stay up in the air for extended periods. Blimps were even used to test atomic bombs. The US Navy's last airship, the MZ-3A, was used until 2013 as as a "flying laboratory", and the European PEGASOS Zeppelin is currently conducting atmospheric tests. The AS-300 also proved hot airships could conduct research; it carried around huge detachable study platforms with teams of scientists that could be deposited directly on the jungle without harming it. NASA has also shown an interest in airships as flying astronomical observatories, carrying telescopes above the thick lower layers of the atmosphere. They are also cheaper and more flexible than space-based telescopes.
- Surveillance: Airships have been used for surveillance ever since they were adopted by the military over a hundred years ago, and today they show no signs of slowing down. Aerostats — essentially kite blimps — remain the most dirt-cheap, low-risk surveillance system around, and the go-to for the US and Israeli militaries. Helicopters are noisy, expensive, have zero loitering ability, and are often shot down. Jets are even more expensive, though they have better endurance and less vulnerability. Satellites have an ungodly price tag, and are literally incapable of remaining over an area (at a distance where you can actually see anything). It's no wonder the 19th century option is preferred.
- Offensive military: Death from Above meets the Cool Airship, in theory. In World War I, the Germans in particular were gung ho on using Zeppelins to crush the Entente's will by bombing cities like Paris and London. Airships proved poorly suited for the task. A Zeppelin can take an immense amount of punishment, but that was rendered moot once the British — after many, many failures and a depressing 0% success rate — finally devised a method to shoot down a Zeppelin: the incendiary bullet. By war's end a third of the armada had been shot down, thanks to this invention — even though airships of the time carried as many as twenty-four AA guns and an autocannon, they were rarely able to drive away the squadrons of fighters and their incendiary bullets. It didn't help that Zeppelins couldn't be produced in as great of numbers as planes, and their hangars were giant bombing targets to the enemy.
- Defensive military: Airships used in a defensive strategy proved vastly more effective than using them as an offensive weapon. In World War II, American anti-submarine blimps escorted 80,000 ships, and only lost one of their protected vessels. In World War I they were used for the same thing, but the records are fuzzier on their exact success rate. However, defensive airships became obsolete in the 1960s after the Soviets switched from bombers and diesel subs to ICBMs and nuclear subs.
- Some people may have noticed balloons in pictures of D-Day after the landings. A special type of airship (or balloon really since it was unpowered) known as a Barrage Balloon was used as an area-denial system for lower flying aircraft. Each balloon was anchored to something on the ground, be it a ship, truck, or something else, with steel cables. Any flying craft trying to go that low would get chopped up by the cables. They were fairly effective against dive-bombers or for defending a small hardened target that required precision bombing, but during the Blitz they were rarely more than a minor inconvenience; it doesn't really matter if you have to fly a few thousand feet higher and lose some accuracy when your target is an entire city.
- Airborne Aircraft Carrier: An unorthodox combination of the above two, and in some ways superior to both. Invented in 1917, the aircraft carrier airship was originally intended to use its parasite fighter airplanes as a diversion or protection so that the then-lightened airship could ascend out of range and escape. Eventually, the carriers evolved from Zeppelins launching airplanes hanging off the keel to a launch-and-recover "trapeze" system connected to an internal hangar bay. The rationale evolved as well: the airship would use its immense range and endurance to serve as a kind of flying airbase to the aircraft, by proxy extending their range and endurance. Famously, two sister carriers of US Navy, the USS Akron and USS Macon, crashed in separate storms, with Admiral Moffet, the chief of US Naval airship program, being killed aboard the Akron, but both were due to human error, rather than being a poor concept. Still, as aircraft's ranges grew, the carriers were no longer necessary to fly sorties out at sea, and they became obsolete as well.