Airships 101

Continuing the series on Airships:

Some basic comments on airship technical design. I'll be referring to Goodyear's web pages on flying their blimp.

The basic components of the airship are the envelope (outer covering), gondola, fins or aerodynamic controls, engines, and the balloonets.

Lift for the airship is provided by a combination of static (buoyancy) lift provided by the lifting gas - usually helium - and dynamic lift provided by the air moving over the aerodynamic shape of the airship and the thrust provided by the engines. Airships are boyant, like any balloon, by being less dense than the surrounding air. To be specific, the airship weighs less than the amount of air volume it displaces. This difference provides lift. If an airship weighs exactly the same as the air it displaces, then it is neutrally buoyant and will neither rise nor sink. The problem with this is that as you ascend into the atmosphere, the air becomes less dense. Airships are normally trimmed with ballast (added weight) to be slightly heavy at ground level - and to use dynamic lift provided by forward motion to climb.

Balloonets are smaller gas bags inside the main envelope that contain air rather than lifting gas. The Balloonets perform several critical functions. As the airship rises, the helium inside expands because the outside air pressure is less. The Balloonets release air to maintain the proper internal pressure on the blimp's skin. If the airship rises too high (refered to as its Pressure Altitude) the pressure of the internal helium gets too high for the envelope to contain and the excess pressure must be vented out - in the form of helium gas - to prevent the airship from rupturing. Releasing helium is very bad because now as the airship descends, and the helium is compressed again by the surrounding air pressure, there may not be enough lift to keep the airship from crashing into the ground. Two balloonets are used to trim the airship fore and aft to keep the blimp level. Compressed air in the Goodyear blimp is provided by scoops behind the engine propellers and by electric fans to inflate the balloonets.

Some considerations for airship operations:
Thermal Control - particularly for airships that operate for long periods of time, the sun heats the envelope and the gas inside during the day, and at night the helium cools. This changes the density of the gas and the lifting power. This is why many blimps are silver or white in color- to minimize the swings in lift caused by thermal expansion. The Balloonets must be large enough to compensate for this change. A typical airship may be 40% balloonet by volume.

Helium Permeability: The helium atom is the second smallest atom there is, and it is very good at escaping. Airship skins have to be specially designed to trap the helium inside, and have coatings and coverings to prevent the helium from simply leaking out through the spaces between the atoms. The Goodyear blimp uses neoprene-impregnated polyester fabric. Even so, over a long term, airships will have to be "topped off" with helium from time to time.

Ultraviolet Damage: Any fabric exposed to the sun will be damaged by ultraviolet rays. Coatings can be added to minimize this damage, but these also add weight. High altitude airships and balloons also have to contend with atmospheric ozone, which is also damaging.

Fuel Consumption: Look at this picture of the USS Macon. See those black vertical lines on the sides of the airship? Any idea what these are for?

Think about an airship flying along- powered by internal combustion engines driving propellers. Those engines are consuming fuel - gasoline - and turning it into hot gases that are driving the engine and then expelled through the exhaust system - in effect, throwing the fuel overboard and making the airship lighter. So on the USS Macon, they used condensers (a type of air conditioning) to capture water vapor out of the air and replace the expended fuel - keeping the airship in balance on long distance trips. Airships can also compensate for the lost weight by putting more air into the balloonets, compressing the air in them and making the balloonets heavier.