Aviation History, Part III
NASA has proposed the High Altitude Venus Operational Concept, which comprises a series of five missions including manned missions to the atmosphere of Venus in airships. Pressures on the surface of the planet are too high for human habitation, but at a specific altitude the pressure is equal to that found on Earth and this makes Venus a potential target for human colonization.
The advantage of airships over airplanes is that static lift sufficient for flight is generated by the lifting gas and requires no engine power. This was an immense advantage before the middle of World War I and remained an advantage for long-distance or long-duration operations until World War II. Modern concepts for high-altitude airships include photovoltaic cells to reduce the need to land to refuel, thus they can remain in the air until consumables expire.
The disadvantages are that an airship has a very large reference area and comparatively large drag coefficient, thus a larger drag force compared to that of airplanes and even helicopters. Given the large frontal area and wetted surface of an airship, a practical limit is reached around 130–160 kilometers per hour (80–100 mph). Thus airships are used where speed is not critical.
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The lift capability of an airship is equal to the buoyant force minus the weight of the airship. This assumes standard air-temperature and pressure conditions. Corrections are usually made for water vapor and impurity of lifting gas, as well as a percentage of inflation of the gas cells at liftoff. Based on specific lift (lifting force per unit volume of gas), the greatest static lift is provided by hydrogen (11.15 N/m3 or 71 lbf/1000 cu ft) with helium (10.37 N/m3 or 66 lbf/1000 cu ft) a close second. At 6.13 N/m3 (39 lbf/1000 cu ft), steam is a distant third. Other cheap gases, such as methane, carbon monoxide, ammonia and natural gas have even less lifting capacity and are flammable, toxic, corrosive, or all three (neon is even more costly than helium, with less lifting capacity). Operational considerations such as whether the lift gas can be economically vented and produced in flight for control of buoyancy (as with hydrogen) or even produced as a byproduct of propulsion (as with steam) affect the practical choice of lift gas in airship designs.
In addition to the static lift, an airship can obtain a certain amount of dynamic lift from its engines. Dynamic lift in past airships has been about 10% of the static lift. Dynamic lift allows an airship to “take off heavy” from a runway similar to fixed-wing and rotary-wing aircraft. However, this requires additional weight in engines, fuel and landing gear, negating some of the static lift capacity.