A sky anchor is a system of two balloons in tandem, with a "zero-pressure" lifting

gas balloon A gas balloon is a balloon that rises and floats in the air because it is filled with a gas lighter than air (such as helium or hydrogen). When not in flight, it is tethered to prevent it from flying away and is sealed at the bottom to prevent t ...

tethered to a

superpressure balloon A superpressure balloon (SPB) is a style of aerostatic balloon where the volume of the balloon is kept relatively constant in the face of changes in ambient pressure outside the balloon, and the temperature of the contained lifting gas. This all ...

"anchor". The gas balloon is filled with a

lifting gas 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 ballo ...

and provides the

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...

, while the superpressure balloon is filled with air, and pressurized to provide the desired ballast weight. In a passive sky anchor, the superpressure balloon is sealed, while in an active system, its pressure can be varied. Both versions have been tested in flight, but have had frequent failures with only occasional successful outcomes. The tandem arrangement makes launching difficult, and this complexity can lead to mission failure. The tandem balloon system is intended to increase the flight time of the zero-pressure balloon by damping the diurnal altitude variations caused by solar heating of the lifting gas and its subsequent expansion. As the balloons descend, the superpressure balloon's constant volume displaces a larger mass of the denser air, and becomes more buoyant. Likewise as they rise, the system displaces less air mass, limiting the ascent to prevent the lifting gas from expanding too far and escaping from the zero pressure balloon. The Sky Anchor system, originally developed at

Texas A&M University Texas A&M University (Texas A&M, A&M, or TAMU) is a public, land-grant, research university in College Station, Texas. It was founded in 1876 and became the flagship institution of the Texas A&M University System in 1948. As of late 2021, T ...

in 1976, is usually applied to unmanned balloons, but the concept was also applied to a piloted balloon. The "Earthwinds" balloon system used a tandem balloon arrangement for a planned

circumnavigation Circumnavigation is the complete navigation around an entire island, continent, or astronomical object, astronomical body (e.g. a planet or natural satellite, moon). This article focuses on the circumnavigation of Earth. The first recorded circ ...

record attempt using the

jet stream Jet streams are fast flowing, narrow, meandering thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are west ...

winds. Five attempts at the circumnavigation in the early 1990s all failed early in the flight, with the fifth abort on January 1, 1995 blamed on a failure of the lower ballast balloon.

References

* * * *{{cite web , url = https://query.nytimes.com/gst/fullpage.html?res=990CE2D6113FF932A35752C0A963958260 , title = Earthwinds Balloon Takes Off But Is Back on Ground in Hours , accessdate = 2007-10-18 , date = 1995-01-01 , format = , work = New York Times Balloons (aeronautics)