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Wingless Wonders

Written by Kevin H. Evans

Lighter-than-air technology is a lot like the game Go. It is easy to learn, but very hard to master. Many countries tried the technology, but only a few managed to master it. By far the largest number of rigid airships were built by Germany. On the other hand, the United States was the country which built the most non-rigid airships. Both countries expended large amounts of resources and effort in perfecting the technology. Other countries, in an effort to keep pace in the lighter-than-air race, created airships that were both technologically inferior and poorly operated. Many of these efforts represent some of the worst accidents in lighter-than-air history.

This technology, although considered by many to be obsolete, is actually quite useful and is coming back into use in modern technology. The following article is mostly about what we would need to do to use this technology in the post-Ring of Fire time frame.

Glossary

Normally I put the definition list at the end of the article, but I have seen such a large variation in terminology, that I want to put the list up front.

Dirigible - any aircraft that is controllable as it flies through the air.

Rigid Airship - any aircraft that has a framework inside the skin which provides shape and support for the aircraft.

Non-rigid Airship - an aircraft that depends on pressurization to maintain its shape.

Semi-rigid Airship - an aircraft that has a keel and framework in the ends to support the load and provide shape to the airship.

Balloon - a non-powered aircraft that depends on lifting gas to make it fly.

Aerostat - any aircraft that depends on lift generated by an internal gas.

Ballonet - an internal balloon used to provide pressure and shape to a non rigid airship.

Gas cell - a container to hold lifting gas in a rigid airship.

Lift - the force that holds the aircraft in the air.

Drag - the force that impedes the aircraft as it moves through the air.

Thrust - the force that propels the aircraft through the air.

Control surface - devices that allow changes in attitude of the aircraft.

Density - the weight of a gas as measured in pounds per cubic inch.

Gross weight - the total weight of the aircraft including cargo and crew.

Tech Work-Arounds

Certainly many of the currently used materials and techniques are not available post-ROF and so work-arounds need to be developed for those items. Further, some items are very expensive and a cheaper item needs to be developed to use in its place. Throughout the text suitable workarounds will be mentioned where possible.

How Lift Works

Lift can be generated dynamically, as in a heaver-than-air aircraft, by moving the aircraft through the air. Indeed many modern aircraft when un-powered, have the flight characteristics of a brick, and depend on a continuous application of thrust to keep the craft in the air.

Lift can also be generated statically, as in aerostats, by using a lifting gas. This gas provides lift by displacing the atmosphere, which is denser and heavy, causing the container of lighter gas to float on top of the thicker air. The lighter the gas, the more it can lift. How much a gas will lift will be described later on.

Captive Balloons

These are balloons that are attached to the ground with an anchor. Such balloons are useful as observation platforms, entertainment devices, advertising, and as a "skyhook" for use as a crane. Captive balloons used as observation points have the advantage of allowing a communication wire to be attached to the tether, making telegraph or voice communications possible.

Balloons can be used for entertainment (rides) or advertising icons. Both are effective as a result of their size, eye catching colors, position overhead, and sense of fantastic unreality.

Balloon cranes have been used into modern times as efficient transportation devices. One of the biggest advantages is the ability to transport bulky items (like logs or large stone blocks) across distances without the need for cutting roads or obtaining access through congested urban areas. This transport is achieved via the use of a cable affixed to a mast or hill top and using a traveling block and tackle mounted on a pulley. The use of a sufficiently large balloon allows lifts of weights in the tens of tons.

Free Balloons

This is the category of balloons that are inflated and released. Such balloons are subject to the wind and go where the wind pushes them. This does not mean that they are uncontrolled or un-guidable. They can be flown to locations by picking the wind layer going in the direction desired.

Free balloons can not "tack" like a ship because they have no counter-drag. A ship has the water it floats through to provide drag or resistance. This drag acts as a modification to the thrust of the wind allowing progress against the wind. Balloons are so large that the size of the envelope makes any secondary sail drag streamer or other passive device irrelevant. The closest a free balloon comes to tacking is when a pilot can balance the balloon on the interface between two wind layers and go in a third direction. This maneuver requires great skill and the existence of suitable wind layers.

Airships

What we know as airships are aerostats that have the ability to be guided to a desired point regardless of the direction of the wind. The ability to guide an airship depends on the addition of thrust and control surfaces to the airship. This thrust is generally provided by propellers powered by engines attached to the airship. Also of note is the need for the airship to have a means of maintaining its shape. Moving a large object through the air faster than the air is moving causes stress on the object, and as the stress increases the object distorts in shape causing increased drag and unstable movement. Shape distortion can be severe enough to cause the venting of the lifting gasses and loss of lift.

Rigid Airships

Airships that have a system of internal stiffening are known as Rigid Airships. The internal structure provides shape to the airship and support for its equipment. Normally the frame is covered by a skin which is hardened by a "dope" that colors the skin and increases the skin's durability. Lifting gasses are contained in gas cells which are attached to the frame. Power plants, holds, cabins, control cars, and control surfaces are also attached to the frame. This class of airship also tends to be larger as the weight of the frame increases the dead weight which in turn makes the size needed to lift the gross weight larger.

Semi-rigid Airships

These airships are much like their rigid cousins. That is, they have a keel to support equipment. Often this includes a nose-cone frame to resist the forces created by forward movement and a tail cone to support the control surfaces. A semi-rigid design saves much of the weight attached to a rigid design but can make non inflated storage complicated. Semi-rigid designs often include elements of non-rigid designs, notably ballonets to aid in maintaining the shape of the envelope.

Non-rigid Airships

In a non-rigid airship, the skin is the gas-containing device. Shape of the skin is maintained by the use of a ballonet. A ballonet is a cell within the skin that is pressurized to create induced pressure on the skin and so maintain the shape of the airship. It is usually only about five percent of the total envelope capacity, and is only necessary to maintain the desired shape of the airship. It is notable that the ballonet is normally filled with air pumped in from the outside of the skin and thus can be regulated without loss of the lifting gasses. Equipment is usually mounted on the control car, which is hung from a cantenary curtain attached to the top inside of the skin.

Other Classes of Airship

In our time line, near the end of the airship age, (in the 1940s and 50s) developers were experimenting with airships called metal-clads. Metal-clads were airships that had a skin composed of aluminum and had elements of rigid and non-rigid design. The greatest advantage was that the metal skin almost completely stopped leakage of the lifting gasses. The best known of these was the US ZMC-2 called the "Tin Bubble." The Tin Bubble was perhaps the most successful of the U.S. Navy's airships. This airship was so reliable that it used up two sets of engines before it was retired from service.

In addition were classes where a significant portion of the lift was provided by the shape of the airship, and acted much like more standard aircraft.

Lifting Gasses

Flight in LTA ( lighter than air) is a result of static lift. That is lift that exists whether the aircraft is moving or not. This lift is generated by the difference in weight of the contained gas compared to the atmospheric gas the aircraft is immersed in. By and large there are three gasses in use for lift and a few more gasses that can work but are marginal in application. In our time line, we use hydrogen, helium, and hot air as lifting gasses. Some commercial city gasses such as natural gas and ammonia are also lighter than air and have been used as lifting gasses. But they are not a lot lighter than air and require a much larger volume to be effective. Of the big three, hydrogen is the lightest and provides the most lift, approximately 66 lb per 1000 cubic feet. Helium will lift around 44 lb per 1000 cubic feet. And hot air will lift around 20 lb per 1000 cubic feet.

Each of these gasses have advantages and disadvantages. Hydrogen will burn, helium is extremely hard to find, and lift from hot air depends on the air pressure, temperature and humidity present during its use. On the other hand, hydrogen can lift a lot, helium is non-flammable, and hot air is easy to get and can be used with minimal crews and facilities. For example, to lift one ton hydrogen needs 30,304 cubic feet of gas, with a sphere of 39 feet in diameter. Helium needs a sphere 44 feet in diameter with a volume of 45,454.4 cubic feet. Hot air varies (18-24 lb per 1000 cubic feet) but for design purposes is centered at 20 lb per 1000 cubic feet. This results in a sphere of 57 feet in diameter with a volume of 100,000 cubic feet.

Of the lifting gasses now used, helium is right out for the Ring of Fire. The only known source of helium in usable quantities is a set of gas wells in the western half of the North American continent. Both the location and the technology needed will make this gas impractical.

This leaves hydrogen and hot air as usable alternatives. Hydrogen will lift 30% more than helium and is much easier to get. Hot air will lift just less than half of helium, but is even more simple to get. The disadvantages are that hydrogen burns with great enthusiasm, and hot air needs the frequent application of heat to keep its lift.

Hydrogen has had a bad reputation since the 1930s, but has become much more favored in the last ten or so years. Much of the reputation was due to a number of accidents caused by an imperfect understanding of the gas and electricity. New practices and designs have significantly lowered the hazards of hydrogen. Long distance gas balloon racing has switched more and more to hydrogen due to its substantially lower cost and greater lifting capacity.

Most important among the new practices for hydrogen use is that the aircraft must be a single entity in relation to conducting electricity. This oneness of structure prevents arcing from one section to another section of the aircraft and denies any ambient hydrogen an ignition source. Also the envelope must be adequately vented so as to allow any leakage of gas to immediately exit the aircraft. And finally the gas cells must be frequently emptied and refilled with pure hydrogen, as oxygen has a tendency to migrate into the gas cell, creating what is called a rotten cell. That is a cell that is easily combustible due to the availability of oxygen in the mix.

Hot air has a lower lifting capacity and requires an aircraft of roughly three times the size for an equivalent amount of lift. Also significant allowance must be made for fuel to maintain the heat in the air, this fuel is in addition to the fuel for used for motive power if any. Currently in our timeline, fuel requirements have been going down with the use of redesigned materials. A standard hot air balloon usually gets about an hour and a half of flight time from twenty gallons of fuel. New materials have allowed as much as thirty hours of flight from the same amount of fuel. Surprisingly the biggest modification has been a multiple layer approach that reduces the heat transfer out of the envelope. Hot air also has another great advantage, because the typical balloon or airship is non-rigid it stores in a much smaller space and can be handled and crewed by significantly smaller numbers of people.

Power Plants

Many types of aerostats need power plants. Airships need them to move through the air and hot air balloons need them to heat the air inside to provide lift. A power plant should be light. That is, they need to have a good power-to-weight ratio and should be dependable. Traditionally, diesel and gasoline have been the fuels of choice, but kerosene and propane have also been used. Due to the ability of an airship to provide static lift, lower horsepower engines are usable. Lower horsepower engines provide economy in fuel and cost of the engines. Additionally, other types of power plants have been used, with steam and hot air (Carnot cycle) engines being the most common. The lifting gas used also affects the power plant, with the power being mounted inside the envelope when using nonflammable gasses (allowing easy engine maintenance) and mounting the plants outboard when flammable gasses are used. Power plants for hot air balloons are the burners used to heat the air inside the balloon. Such burners normally provide 2 to 6 million BTUs to the air inside the envelope depending on the size of the air mass to be heated.

Modern airships are powered by a variety of means, most commonly the internal combustion engine. Such engines are in limited supply in the immediately post-ROF world, but will become more common as knowledge and tooling spread out from Grantville. Many internal combustion engines need tight tolerances and advanced lubricants, however there are large numbers of engines possible at a lower technical expertise.

In 1900 the "gnome rotary" (an engine where the cylinder block spun and the pistons were attached to the frame) was invented. This engine was a single valve (per cylinder) with the fuel fed from the center crankshaft along with the lubrication, all of which was exhausted from the cylinder each rotation of the block. This is the engine that made all early airplanes possible. Their major disadvantage was that they were a single-pass lubrication. That is, the oil is used once, and ejected from the engine. This oil was castor oil, and the engine moved in a constant cloud of oil vapor.

By the way, this accounts for the drinking tradition of fighter pilots. Since the pilot was bathed in a cloud of castor oil, they ingested large amounts of it. In an effort to absorb some kind of food value that was not "cleansed away" by the qualities of castor oil, they took in vast quantities of wine and beer as the alcohol metabolized quickly, before the colonic took everything else away. At least, that was the excuse. If this sort of engine is used in an airship, since it would be mounted below or behind the cabin, airship pilots would be "beyond" this sort of problem.

In 1903 the Wright brothers made their engine in a bicycle shop. This was a standard internal combustion engine of four cylinders using the Otto cycle. Such engines are not high compression, efficient, or even very powerful. But they work, dependably and every time (mostly). Further, such engines can be made with low tolerances and primitive machine tools.

Steam power is also an option. A steam generator (a flash boiler), a light weight engine, and a condensing coil can be made well within the weight limits available.

Last, a Carnot cycle engine removes even the need for water as a working fluid, but does so at a need for much higher tolerances. So much so that the internal combustion engine, with its low tolerances claimed the position of first choice among engines, and so received almost all the research and development in our culture.

Envelope Construction

Envelopes are constructed from materials that are impervious or resistant to passage of the lifting gas. Of note is that the rigid frame airship has gas-containing cells inside the frame with a cloth covering over the frame that provides a smooth surface to the outside environment.

Traditionally the great airships of the 1930s used a material called goldbeaters skin to form the gas-containing area. Goldbeaters skin is made from the lining of an ox stomach and had the dual properties of being impervious to hydrogen gas and of making gas-tight seals when the edges are properly treated and placed together. The problem with this material is that the total amount of "skin" per oxen is very small (not much larger than a sheet of paper), and thus needs a lot of dead oxen, with over 200,000 used for a ship like the Hindenburg.

Currently most airships use a layered fabric made from cloth treated with latex or Mylar. Mylar is also used solo as a gas-containing material in some airships. Gas balloons are made from treated nylon, because the lifting gas is vented in flight as part of the control process. Hot air balloons are normally made from treated rip stop nylon. It is important to note that the use of the nylon imposes a maximum usable temperature, as too much heat will melt the envelope.

Physically the envelopes are normally made from a set of segments called gores. The gores are sewn together using "French seams" which are double sewn and leave no loose ends. Additionally some seams may have a load-bearing tape or wire enclosed to provide strength to the envelope and give places where the load can be attached.

The best envelope ever constructed was that of the ZMC-2, a semi-rigid airship called by its crew the "Tin Bubble." This envelope was a three layer sandwich of aluminum that allowed gas leakage only at the valves and outlasted two sets of engines. Alas, large quantities of aluminum are probably out of reach for the near- and mid-future in the 1630s.

By far, the largest number of envelopes were made from latex-impregnated fabric. Over 150 such aircraft envelopes were made for the US Navy alone. Nylon, polyester, and rayon are also popular materials for envelopes. Of all of these, the cotton and latex fabric is the most feasible for the ROF. Cotton cloth is available in large quantities from India, and latex is found in usable quantities in a number of common plants notably dandelions, ragweed, and milkweed. And so the manufacture of this fabric is possible in the post ROF time line.

Structure

The material of choice for airships is aluminum. As previously mentioned, aluminum will not be available in large quantities for some time. Rigid airships will need something else. A substitute, actually used by the German navy in World War I, was wood. Split and laminated spruce is light, strong, and provides many of the properties of aluminum. The downfall of wood is its slightly greater weight by volume for the same strength, and its tendency to absorb moisture. Moisture makes the wood heavy and can cause degradation of the lamination in the frame. One of the cures is to coat the frame in varnish. This excludes the moisture but adds to the overall weight.

Applications

Aerostats have both civil and ...