Monday, June 25, 2012

The Thing of Dreams


1. Prelude:
The gossamer-like plastic film did little to hide the intricately crafted balsa frame underneath, net-like, daylight bent and refracted as it streamed through. Tethered by a thin black thread, the latex balloons inside provided enough aerostatic lift to buoy the craft upward, with another 80 or so grams of excess lift to spare. This was its first and only test flight, thus far.

Actually, it was only half a craft, the front half of what I had originally intended to be a twelve-foot-long flying model rigid airship. A Zeppelin, of sorts.


2. A Thing of History:
In precise terms, a rigid airship should only be called a Zeppelin if manufactured by the Luftschiffbau Zeppelin firm or one of its subsidiaries, though the term is commonly employed to mean any rigid airship. In numbers, a total of somewhere over a hundred rigid airships have been built and flown by various countries, notably Germany, England and the United States, since 1900, but most of them by the Zeppelin firm itself. Three were build in the United States for the U.S. Navy, the latter two by a consortium of Goodyear and Zeppelin engineers.

There were also secondary players in the rigid airship field. Shutte-Lantz, a competitor firm in Germany during the First World War, proved to pioneer many technological innovations that later found their way in the rival Zeppelins, which held more political favor with the German government. There were also several dozen rigid airships built in Great Britain, which was only second to Germany in the number of rigid airships produced.

There are currently no rigid airships in existence, the last one flown being the LZ-130, named the Graf Zeppelin II, dismantled in 1940, along with the unfinished LZ-131, under orders by Hermann Goring. The National Socialists took little liking to these floating behemoths or the anti-Nazi Zeppelin organization, but understood the value of a propaganda tool when they saw one. In the end, the enormous quantity of duralumin girders and steel wire that made up the frame of the airships proved more valuable as war materiel for fighters and bombers. Their time as a weapon of war or vehicle for long distance passenger transport was past, with the advent of long range passenger aircraft after the war sealing their fate.

The rigid airship is a thing of the past, few people alive today ever having seen, much less flown in, one. They exist only in history books and the imaginations of airship fans such as myself.

I've been aware of balloons and airships since childhood, and have amassed a small library of books on the subject, many of them now out of print. I've also harbored this idea - a wild dream, really - of building a flying model rigid airship. A pipe dream, perhaps; in the days past, when the Navy flew nonrigid airships, their crewmen were called "helium heads"; certainly there must be a bit of helium floating around inside mine.


3. A Thing of Gravity:
Balloons and airships float because their total mass weighs less than the volume of atmosphere that they displace. Gravity, pulling down on the myriads of gas molecules that make up our atmosphere, serves to buoy up a less dense balloon or airship. A balloon, released from its tether, will accelerate in an upward direction, opposite the pull of gravity, until it finds equilibrium with the density of the surrounding atmosphere. Some balloons, large in volume and constructed of thin plastic films, can rise to over twenty miles into the stratosphere, to nearly the edge of space; aerostatics, the subject is called, in contrast to the heavier-than-air science of aerodynamics.

Balloons and airships are aerostats, their lift coming from the force of gravity pressing down on the surrounding atmosphere. They employ anti-gravity technology, to the chagrin of UFO-buffs who complain "that's not what we mean by anti-gravity". Aerostats and submarines have this in common, that they both employ the force of gravity upon a more dense surrounding medium - air or water - for their buoyancy. A toy party balloon displays anti-gravity technology, but you can't argue that with the true UFO believer. Place such a balloon into outer space, away from the atmosphere, and it will not continue to accelerate outward; it requires the force of gravity upon a surrounding medium for its upward motion.

A liter of helium gas will lift about one gram. A liter, for us Americans who need to be reminded, is a cube that's 100mm on a side, nearly 4 inches, about the length of a long cigarette. A gram weighs about as much as several paper clips. There are about 453 grams to a pound.

The lift produced by lighter-than-air gasses is easier to understand in larger volumes. Take a thousand cubic feet of helium - enough to fill a ten-by-ten-by-ten foot room - and it will lift about 68 pounds. Now you're talking real lift. That same volume of hydrogen gas will lift around 72 pounds, about 8 percent more. But if you need to lift a really large amount of weight, enough to account for engines, fuel tanks, passenger's cabins, chef's kitchen and supplies of food and wine bottles - even an aluminum grand piano - then you need an accordingly large volume of lifting gas. One gram per liter amounts to 250 tons of lift for an airship the size of the Hindenburg's 7 million cubic feet of lifting gas volume. But the Hindenburg, as we're reminded every year around the anniversary of its demise, was not filled with helium gas.

Helium is rare, and expensive. All of the helium in the universe comes from the fusion, by the stellar furnace inside stars, of hydrogen atoms. A very small amount of helium has been produced artificially, as a byproduct of thermonuclear explosions. If you blew up enough H-bombs, you could create enough additional helium to float a few party balloons. Yet, the helium that we blow into those party balloons, all of the helium available to us, comes from certain natural gas wells, sequestered deep underground since the days of the earth's formation, from interstellar material made up of the stuff of exploded stars, aeons ago. Balloons are not only anti-gravity devices, but the substance enabling their lift comes from outer space.

Helium is not the only lighter-than-air gas. Air itself, if heated sufficiently, expands to become lighter than its surrounding, cooler atmosphere. The most commonly seen lighter-than-air manned vehicle is the hot air balloon, which have become so synonymous with lighter-than-air flight that many people, when confronted with the term "gas balloon," will stare at you with that blank look and exclaim, "What's that?" Besides helium, there are other lighter-than-air gasses such as neon, methane and ammonia, and steam has also been shown to be a viable lifting gas; but for a variety of reasons these are all deemed less desirable for practical reasons.

The most common element in the universe is hydrogen. One proton and one electron. But in its gaseous form it combines to form a two-atom molecule, H2. It also combines easily with many other atoms like carbon, nitrogen and oxygen, such that there is virtually no pure hydrogen found on earth, a fact of which the advocates of fuel-cell technology have to be periodically reminded. But hydrogen can be separated from its companion molecule with the application of electrical or chemical energy; hydrogen gas is thus an energy storage medium, but not an energy source. Despite its ubiquitous presence, it needs to be separated from its bonds with other elements, using some other source of energy, in order to become useful.

Hydrogen was first used as a balloon lifting gas in 1783, the same year that the Montgolfier brothers flew their first hot air balloon. In the future, if our precious helium reserves run out, we will have to resort to once again employing hydrogen as a lifting gas for balloons and airships. Actually, manned ballooning in Europe has continued to use hydrogen all of these years since the Zeppelin airship Hindenburg burned upon landing at Lakehurst, New Jersey, in 1937. More recently, manned free balloons filled with hydrogen are once again being seen floating in the skies of the United States, specifically at the America's Cup gas balloon race, in Albuquerque. The Space Shuttle, now grounded, used liquid hydrogen fuel in its main engines. There is a technological legacy from the space program around the safe handling of large quantities of the volatile gas. It's something we can do, if we had to.


4. A Thing of Guidance:
Airships are balloons that can be propelled and steered, hence the term dirigible. They use propellers and some sort of engine. Their lifting gas provides them buoyancy, while their engines provide them forward propulsion and steering. The first airship to demonstrate controlled free-flight was built in France in 1884 and employed an electric motor and batteries for propulsion. Electric powered model blimps are commonly seen today at indoor stadiums and sporting events; the more things change, the more they stay the same, it would seem. If their engines were to fail, an airship would continue to be buoyant as a free balloon. Contrast this with heavier-than-air vehicles, which require forward propulsion in order to generate aerodynamic lift.

Thirty six people died in the crash of the Hindenburg, thirteen of them passengers, the only passengers ever lost in 30 years of airship service. The typical airliner crash consumes the lives of hundreds of people, but airliners continue to fly, or so is argued by Helium Heads.

In the arcane terminology of airships, they are classified as either pressure type or rigid type, referring to the means by which their shape is maintained. Pressure airships require the pressurization of their internal lifting gas to maintain the rigidity of their envelop or hull. There are two common subtypes, blimps and semi-rigids. Blimps have no rigid structures on the gas-inflated envelop, other than thin battens usually employed in the nose, and are the most commonly seen type of airship; while semi-rigids used a pressurized envelop mated to a rigid external keel, from which engines and control cars were attached, or more recently a pressurized envelop with an internal rigid framework within the lifting gas itself, the latter configuration currently being employed by Zeppelin in the form of two airships, one of which, named the Eureka, flies out of Moffett Field, near Sunnyvale, California, former home to the last of the giant Navy rigid airships of the 1930s.

Of all the various types of airships, the Zeppelin-type were the most intricately engineered, and thus the type most desired by me as a pattern for a flying model. They were composed of an outer, self-supporting framework, usually made of an aluminum alloy braced by myriads of steel wires for rigidity, and covered with an outer skin of doped fabric. The lifting gas was contained in separate internal gas balloons, called gas cells, cylindrical in shape to more efficiently fill the airship's internal volume. This division of the airship hull into individual gas cells provided a measure of safety against the likelihood of a leak or other catastrophic failure, in much the same way that naval war vessels employ compartmentalization as a means to maintain watertight integrity. The lift from the gas cells was transferred to the hull by a complex series of wires and nets. The framework was typically composed of polygon-shaped rings of stiff but lightweight girders, connected by longitudinal girders that provided the craft its profile shape, and braced by myriads of stiff wire. These structures were elegant combinations of compression-absorbing girders and tension-absorbing wires arranged in finely balanced concert. A large rigid airship, such as one of the Hindenburg-class, could weigh in at 240 tons dead weight, yet when buoyed to neutral buoyancy by its internal gas cells could be lifted off the hangar floor by the force of but one finger. Flying such craft was as much art as science, a series of finely subtle adjustments to trim, buoyancy, steering and engine power required to maintain control.

Long distance air travel was pioneered, in the 1920s, by the famous German airship Graf Zeppelin, including the art of "pressure pattern flying," whereby major atmospheric storm systems were steered around in such a direction as to provide a more energy-efficient tailwind, the added distance often being of negligible consequence to the flight's overall duration, which would often be from Germany to Brazil, across the south Atlantic, but which also included a round-the-world flight. Today, pressure pattern flying would not be permitted by the various governmental agencies that regulate international air travel. Instead, intricately mapped air travel corridors are predefined with no regard for the vagaries of weather, with modern jet aircraft flying above most of the world's weather, rather than through it, as would be required of the much lower flying airships.


5. A Thing to be Built:
Since large amounts of aerostatic lift require large volumes of gas, it's important to see the opposite as also being true, that small-sized airships, such as a model, generate a very modest amount of lift. There's this fact of airship engineering that's impossible to avoid, the Inverse Square Law which, when applied to airship design, implies that if you double the size of an airship, its volume - and consequently its gross lift - will increase eightfold, while its surface area - and consequently its dead weight - will only increase fourfold, meaning that its lift-to-weight ratio becomes more efficient as it gets larger. But the inverse is also true: as you make an airship smaller, it becomes less efficient aerostatically such that there exists a lower limit to how small such a model can be built and still be able to float. That's the regime I found myself working within when I started this project, some years ago.

Zeppelins were built of girders made from stamped aluminum alloy, braced by steel wires drawn in tension. But a model of the size I anticipated, being small enough to practically store and transport yet large enough for significant amounts of lift (about 12 feet in length), required a much different bill of materials than aluminum alloy and steel wire. Balsa wood - the stuff of the model aircraft enthusiast - and ultra-lightweight tissue or plastic film would be required, along with novel construction techniques. Once an opportune period of time became available, I suddenly started work on the project with feverish energy.

One initial problem to overcome was how to measure the weights of all these tiny parts, which individually could weigh less than the resolution of a typical inexpensive digital scale. The solution I came up with was an inverted T-beam counterweight balance that used plastic gram cube weights as a reference, and whose readout was calibrated as the resulting angle of incline of the inverted T-beam. Using this improvised scale, whose prototype was initially built from Starbucks wooden stir sticks and black sewing thread, I was able to weigh parts with a resolution of a fraction of a gram.

Next came the problem of converting the design of the full-sized Zeppelin airship hull into corresponding structures of main-rings and longitudinals that could be assembled, one piece at a time, into a somewhat accurate representation of the historic fact. I chose ultimately not to pursue a true scale model, but rather to employ a design easier to construct that still illustrated the fundamental principles of Zeppelin construction. This resulted in an airship hull whose profile was more cylindrical in shape, similar to the earliest pre-World War I Zeppelins, rather than the teardrop-shaped streamlined hulls of later. This decision proved much easier to implement in the construction phase, since entire segments of the cylindrical hull could first be preassembled flat and later joined together.

There were some interesting learnings from this project, that I had not anticipated beforehand, despite much planning. I found that by inadvertently including a bit too much glue to each balsa joint resulted in a consequent dramatic increase in the structure's overall weight, due to the large number of individual glue joints employed. Accuracy and precision of construction were thus of monumental value. Another interesting observation was the strength of the overall structure as compared to the fragility of each piece of balsa or sewing thread taken individually. The term "rigid" is truly appreciated when examining in-hand the completed airframe, as it appears to be held together as much by abstract design principles as it is by myriads of tiny drops of glue.

Once the framework was constructed, I used the built-up front half of the model as a testbed for examining the various options of covering material. In the end I decided on testing two types of material, the one being condenser tissue - an ultra-thin tissue used as the dielectric of capacitors - and the other being Ultrafilm, a clear plastic film much thinner than Saran Wrap, used in covering the wings of indoor model aircraft of the type that can fly for many minutes in the cavernous interiors of gymnasiums and blimp hangars. In the end result, the condenser tissue proved to be lighter in weight, closer in appearance to the historic airships, but more delicate, while the Ultrafilm proved easier to apply and more resistant to mishandling.

One of the fundamental principles of the Zeppelin design that I intended to illustrate in this project was that of compartmentalization of the airship's hull into individual gas cells. But further testing and analysis showed me that, for models of such a small size as this, the combined surface area of all the individual gas cells would represent a significant weight increase, as compared to filling the entire volume of the model with one large gas cell. However, since the airship was intended to be stored and transported in two halves, to be assembled at the flying site, I decided it best if comprised of two large gas cells, one for each hull section. This weight-saving decision, arrived at well into the project, required significant design changes to the hull's main-rings, with the consequence that the already built front hull section would have to be scrapped, and a newer design would have to be built from scratch.

My personal life situation soon changed, such that I had less time and space to devote to the project. The partially completed prototype sat dormant in its white hangar box in a relative's garage for years, through the stress of summer's heat and winter's freeze. I would periodically revisit in my mind the now dormant project and my childhood dream of a flying model Zeppelin, and organize my copious notes, but inertia and the force of life's demands proved overwhelming. Then yesterday, in the midst of working on this piece, I went over to the garage, dusted off the hangar box, unlatched its lid and took a peek inside. I expected to see a rat's nest of broken balsa and knotted threads, but was surprised to see the prototype essentially intact, its framework sufficiently hardy to resist my handling as I took photographs, its partially covered hull just as I'd left it, years ago. But then it was all over in just a few minutes, the pictures having all been taken, the airship slowly walked back into its hangar absent any fanfare, just quiet sadness for the once fervent hopes of youth now put away for another interminable period, awaiting either demolition or completion, of which I remain unwilling or unable, held in tension between childhood's dream and adulthood's practicalities.


6. Aftermath:
Rigid airships appear at the present time to be a technological dead-end, whose time came and went, and that from the vantage point of the present appear to be too large, too expensive and too slow for today's demands. Yet their proponents live on, fueled by hopes of ultra-inexpensive air transport, or the revival of the huge passenger ships or, pending the consequence of global climate change and peak oil, an idea whose future is yet to be. In an act of extreme irony, the one means by which rigid airships could reasonably be seen to fly at the present time - in the form of radio controlled models - appears to be out of practical reach by one of my means and ability. And so the era of the rigid airship lives on only within dusty books, grainy videos or corny cinematic productions, more the stuff of steam-punk fiction than historic fact.

Dreams seldom completely die, though their fervor may be dampened by the rigors of time. Though our lives are lived best when buoyed by dreams, this project has taught me that some dreams are best left as fantasy, they seldom make the transition to reality without significant loss, and that it remains of paramount importance to know which to pursue and which to leave untouched, if only to ponder in the privacy of one's own mind, such as during some quiet spell, when glancing skyward, permitting one's imagination to go wild with the thought of a behemoth floating by, wingless, weightless, itself the thing of dreams.

This is a personal account, of my life's interest in airships and my fledgling attempts at model making. Others have also trodden this same ground, to better success than I. Mention must be made specifically of Jack Clemens successful flying model of the US Navy rigid airship U.S.S. Macon, the model flying in its hangar on the anniversary of its loss at sea.


Blogger Blank said...

Wonderful writing, Joe. I thoroughly empathize with this article and I'm saving it for future re-reading. Thanks.
== Michael Höhne

10:08 AM  

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