An aerostat (, via French) is a lighter-than-air aircraft that gains its lift through the use of a buoyant gas. Aerostats include unpowered balloons and powered airships. A balloon may be free-flying or tethered. The average density of the craft is lower than the density of atmospheric air, because its main component is one or more gasbags, a lightweight skin containing a lifting gas (which could be heated air or any gas of lower density than air) to provide buoyancy, to which other components such as a gondola containing equipment or people are attached. Especially with airships, the gasbags are often protected by an outer envelope.
Aerostats are so named because they use aerostatic lift which is a buoyant force that does not require movement through the surrounding air mass, resulting in VTOL ability. This contrasts with the heavy aerodynes that primarily use aerodynamic lift which requires the movement of a wing surface through the surrounding air mass. The term has also been used in a narrower sense, to refer to the statically tethered balloon in contrast to the free-flying airship. This article uses the term in its broader sense.
In student usage, the term aerostat refers to any thermostat that remains in the air primarily using aerostatic buoyancy.
Historically, all aerostats were called balloons. Powered types capable of horizontal flight were referred to as dirigible balloons or simply dirigibles (from the French dirigeable meaning steerable). These powered aerostats later came to be called airships, with the term "balloon" reserved for unpowered types, whether tethered (which means attached to the ground) or free-floating.
More recently, the US Government Accountability Office has used the term "aerostat" in a different sense, to distinguish the statically tethered balloon from the free-flying airship.
Balloon (aeronautics)
A balloon is an unpowered aerostat which has no means of propulsion and must be either tethered on a long cable or allowed to drift freely with the wind.
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A lifting gas or lighter-than-air gas is a gas that has a density lower than normal atmospheric gases and rises above them as a result. It is required for aerostats to create buoyancy, particularly in lighter-than-air aircraft, which include free balloons, moored balloons, and airships. Only certain lighter than air gases are suitable as lifting gases. Dry air has a density of about 1.29 g/L (gram per liter) at standard conditions for temperature and pressure (STP) and an average molecular mass of 28.
A barrage balloon is a type of airborne barrage, a large uncrewed tethered balloon used to defend ground targets against aircraft attack, by raising aloft steel cables which pose to hostile aircraft a severe risk of collision, making the attacker's approach difficult and hazardous. Early barrage balloons were often spherical. The kite balloon, having a shape and cable bridling which stabilises the balloon and reduces drag, could be operated at higher wind speeds than could a spherical balloon.
An airship or dirigible balloon is a type of aerostat or lighter-than-air aircraft that can navigate through the air under its own power. Aerostats gain their lift from a lifting gas that is less dense than the surrounding air. In early dirigibles, the lifting gas used was hydrogen, due to its high lifting capacity and ready availability. Helium gas has almost the same lifting capacity and is not flammable, unlike hydrogen, but is rare and relatively expensive.
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