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 "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, making it useful in lifting lighter-than-air aircraft. Only certain lighter-than-air gases are suitable as lift ...

and provides the

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

, 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, TA&M, or TAMU) is a public university, public, Land-grant university, land-grant, research university in College Station, Texas, United States. It was founded in 1876 and became the flagship institution of ...

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 circumnaviga ...

record attempt using the

jet stream Jet streams are fast flowing, narrow thermal wind, air currents in the Earth's Atmosphere of Earth, atmosphere. The main jet streams are located near the altitude of the tropopause and are westerly winds, flowing west to east around the gl ...

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)