February 17, 2010 > TechKnow Talk: Lighter than Air: Ships in the Sky
TechKnow Talk: Lighter than Air: Ships in the Sky
Following the invention of the hot air balloon in the 1780s, people began to seek ways to make it less reliant on the whims of the wind. This eventually led to the airship, which enjoyed its heyday in the first few decades of the 20th century, and was in turn rendered obsolete by the more maneuverable, and much faster, airplane. The hot air balloon has staged a comeback in the last 50 years, and there is a glimmer of hope for renewed popularity for the airship as well.
First, let's clarify the terminology. While both are "lighter than air," an airship is powered and steerable; a balloon is neither. Airships, also called dirigibles, may be broken into three categories: rigid, semi-rigid, and non-rigid. Rigid airships are often referred to as zeppelins, though this is actually a brand name. Non-rigid airships are known as blimps.
Airships operate on the same principle of buoyancy discussed in last month's column on hot air balloons. Instead of hot air, modern airships use helium, a gas of much lower density than air, to provide lift. Helium provides sufficient buoyancy to enable a large airship to lift a far heavier load than is possible with the largest hot air balloons.
Depending on temperature and altitude, a cubic foot of helium has a lifting capacity of somewhat more than an ounce; an equal volume of hot air can lift about a quarter of an ounce. Doing the arithmetic, a 100,000 cubic foot airship can lift in excess of three tons. Modern airships such as the Goodyear blimps contain about twice that volume.
Airships are typically powered by gasoline or diesel engines turning two or more large propellers. Steering is accomplished with horizontal fins or "elevators" that are rotated to point the nose up or down and vertical fins or "rudders" rotated to turn the craft left or right.
To maintain proper buoyancy as the airship ascends and descends, the gas envelope contains large bags of air called ballonets that can "breathe" outside air. Since air is heavier than helium, the ballonets may be deflated or inflated to increase or decrease buoyancy, respectively. This clever design is analogous to ballast tanks on a ship or submarine.
Modern blimps often lift off slightly heavier than air by pointing the nose or propellers up and powering the engines to generate upward thrust. Ballonets and flying heavier than air are both intended to preclude the need to exhaust expensive helium to return to earth.
Rigid airships were constructed with a metal framework over which layers of fabric material were laid. The shape was defined by this framework, not by the pressure of the gas within. They also had a large metal hull or keel under the gas envelope, thus these were truly ships of the air.
Since rigid airships did not rely on a pressurized gas envelope they did not use ballonets. Instead, they often carried ballast such as water, which could be dumped to compensate for gas leakage or to increase the rate of ascent.
Advantages of a rigid airship derive from the strength of the metal substructure, enabling the support of huge gas envelopes, large engines and massive loads. Disadvantages are the need for sizable on-board and ground crews and, because it cannot be deflated, a huge storage hangar is required.
These huge airships were used by several countries in the early 20th century, including a fleet of four operated by the U.S. military for scouting, surveillance, and rescue missions, as well as, surprisingly, aerial aircraft carriers capable of carrying, launching, and retrieving several small airplanes. One of these, the USS Macon, was housed in "Hangar One" at Moffett Field, the former Naval Air Station in Mountain View.
But Germany was the most successful country in building rigid airships, fielding dozens used as bombers in World War I. Germany also produced by far the largest and most far-ranging airships, and used them for trans-Atlantic passenger transportation, offering luxury amenities similar to cruise ships.
The Graf Zeppelin, powered by five engines and buoyed by three million cubic feet of hydrogen, completed the first lighter-than-air voyage around the world in 1929 in a mere 21 days. The Germans also built the biggest airship ever, the Hindenburg. Filled with an amazing seven million cubic feet of hydrogen, this monstrous airship was capable of lifting 112 tons of cargo in addition to its own 130 ton weight.
The Hindenburg was designed to use safe, non-flammable helium, but the U.S. had the world's only capability to produce helium, a component of some natural gas deposits. In the 1930s, amidst escalating Nazi aggression, the U.S. refused to sell helium to Germany, forcing the use of dangerously flammable hydrogen. This led to the well-publicized loss of the Hindenburg to fire as it landed in New Jersey following a trans-Atlantic voyage in 1937.
The Hindenburg disaster, along with improved aircraft designs, spelled the end of the largest crafts ever to sail the skies, and by the end of World War II the last of the rigid airships had been destroyed or dismantled.
Semi-rigid airships also have a metal keel structure, but lack the framework shaping the gas envelope. Thus they must be pressurized to take their shape. This allows them to be deflated after use and eliminates the need for huge hangars. But compared to the full metal framework of a rigid design, semi-rigid airships cannot be made nearly as large or strong and thus cannot generate as much lift.
It was an Italian-built semi-rigid airship that first flew over the North Pole in 1926. Like their rigid contemporaries, semi-rigid airships were used extensively prior to and during World War I, but had yielded to the airplane by the 1930s.
Non-rigid airships, or blimps, have fared better. These are characterized by the lack of either a keel or framework, and a more aerodynamic form than their cigar-shaped zeppelin cousins. Without a frame, they too must be pressurized to take shape and can be deflated for easy storage and transportation. Modern blimps can be operated by an on-board crew of two supported by a small ground crew. Because blimps lack any metal support structure, the engines and propellers are typically attached directly to the gondola. The propellers are often rotatable for improved maneuverability.
The best known example of blimp is probably the fleet of Goodyear blimps used primarily to cover sporting events. The advantages these airships maintain over airplanes are the ability to stay airborne for very long periods of time and to hover over the same area with little expenditure of fuel. They also offer stable platforms for cameras and, being highly visible, are ideal for advertising.
These benefits are likely to ensure the continued presence of blimps overhead. But recently a semi-rigid airship has joined the blimps. A German company has built several keeled airships dubbed the Zeppelin NT. At less than 300,000 cubic feet of gas, these are small compared to their gargantuan zeppelin ancestors, but utilize high-tech composite materials providing high strength and reduced weight.
The NT can cruise at up to 70 mph at an altitude of 1000-5000 feet, carrying two pilots and 12 paying passengers, and is being used for "flightseeing" tours in several locations around the world. At the time of this writing, an NT is based at Moffett Field and another in Oakland. Operating under the trade name Airship Ventures, they offer scenic flights over bay and coastal landmarks.