Superpressure balloon
Encyclopedia
A superpressure balloon is a style of aerostatic
balloon
where the volume of the balloon is kept relatively constant in the face of changes in the temperature of the contained lifting gas
. This allows the balloon to keep a stable altitude for long periods. This is in contrast with much more common variable-volume balloons, which are either only partially filled with lifting gas, or made with more elastic materials.
In a variable-volume balloon, the volume
of the lifting gas changes due to heating and cooling in the diurnal
cycle. The cycle is magnified by a greenhouse effect
inside the balloon, while the surrounding atmospheric gas is subject to a much more limited cyclical temperature change. As the lift gas e.g. heats and expands, the displacement
of atmospheric gas increases, while the balloon weight remains constant. Its buoyancy
increases, and this leads to a rise in altitude
unless it is compensated by venting gas. Conversely, if the balloon cools and drops, it becomes necessary to release ballast
. Since both ballast and gas are finite, there is a limit to how long a variable-volume balloon can compensate in order to stabilize its altitude. In contrast, a superpressure balloon will change altitude much less without compensation maneuvres.
Since the volume of the balloon is more constrained, so is the volume of air displaced by it. In accordance with the Principle of Archimedes, the upwards force
on the balloon is equal to the weight
of the displaced ambient gas. But the weight
of the atmospheric gas is reduced as the balloon rises, because its density diminishes with increasing altitude. So the force pushing the balloon upwards diminishes with altitude and at some particular altitude the upwards force will equal the weight of the balloon: the balloon will be stable in a finite equilibrium altitude range for long periods.
The disadvantage is that such balloons require much stronger materials than non-pressurized types.
Superpressure balloons are typically used for extremely long duration flights of unmanned scientific experiments in the upper atmosphere, where atmospheric gas temperature is quite stable through the diurnal cycle. . In 1985 such balloons were used for aerobots
flying at an altitude of approximately 50km in the atmosphere of Venus
, in the international, Soviet-led Vega program.
In February 1974, Colonel Thomas L. Gatch Jr, USAR attempted to make the first crossing of the Atlantic by balloon in a superpressure balloon named Light Heart
. Following the loss of at least two of the ten balloons which provided lift and after deviating substantially from the course that Colonel Gatch had plotted to take advantage of the jet stream
, the last reported sighting of the Light Heart was 1,610km west of the Canary Islands
; no further trace of the aircraft was ever found.
Aerostat
An aerostat is a craft that remains aloft primarily through the use of buoyant lighter than air gases, which impart lift to a vehicle with nearly the same overall density as air. Aerostats include free balloons, airships, and moored balloons...
balloon
Balloon (aircraft)
A balloon is a type of aircraft that remains aloft due to its buoyancy. A balloon travels by moving with the wind. It is distinct from an airship, which is a buoyant aircraft that can be propelled through the air in a controlled manner....
where the volume of the balloon is kept relatively constant in the face of changes in the temperature of the contained lifting gas
Lifting gas
Because of the Archimedes' principle, a lifting gas is required for aerostats to create buoyancy. Its density is lower than that of air . Only certain lighter than air gases are suitable as lifting gases.- Hot Air :...
. This allows the balloon to keep a stable altitude for long periods. This is in contrast with much more common variable-volume balloons, which are either only partially filled with lifting gas, or made with more elastic materials.
In a variable-volume balloon, the volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of the lifting gas changes due to heating and cooling in the diurnal
Diurnal temperature variation
Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights.-Temperature lag:Temperature lag is an important factor in diurnal temperature variation...
cycle. The cycle is magnified by a greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...
inside the balloon, while the surrounding atmospheric gas is subject to a much more limited cyclical temperature change. As the lift gas e.g. heats and expands, the displacement
Displacement (fluid)
In fluid mechanics, displacement occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the fluid displaced can then be measured, as in the illustration, and from this the volume of the immersed object can be deduced .An object that sinks...
of atmospheric gas increases, while the balloon weight remains constant. Its buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
increases, and this leads to a rise in altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
unless it is compensated by venting gas. Conversely, if the balloon cools and drops, it becomes necessary to release ballast
Buoyancy compensator (aviation)
The static buoyancy of airships during a trip is not constant. It is therefore necessary to take measures to control the buoyancy and thus the altitude, the so-called buoyancy compensation.-Changes which have an effect on buoyancy:...
. Since both ballast and gas are finite, there is a limit to how long a variable-volume balloon can compensate in order to stabilize its altitude. In contrast, a superpressure balloon will change altitude much less without compensation maneuvres.
Since the volume of the balloon is more constrained, so is the volume of air displaced by it. In accordance with the Principle of Archimedes, the upwards force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
on the balloon is equal to the weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
of the displaced ambient gas. But the weight
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of the atmospheric gas is reduced as the balloon rises, because its density diminishes with increasing altitude. So the force pushing the balloon upwards diminishes with altitude and at some particular altitude the upwards force will equal the weight of the balloon: the balloon will be stable in a finite equilibrium altitude range for long periods.
The disadvantage is that such balloons require much stronger materials than non-pressurized types.
Superpressure balloons are typically used for extremely long duration flights of unmanned scientific experiments in the upper atmosphere, where atmospheric gas temperature is quite stable through the diurnal cycle. . In 1985 such balloons were used for aerobots
Aerobot
An aerobot is an aerial robot, usually used in the context of an unmanned space probe or unmanned aerial vehicle.While work has been done since the 1960s on robot "rovers" to explore the Moon and other worlds in the Solar system, such machines have limitations...
flying at an altitude of approximately 50km in the atmosphere of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, in the international, Soviet-led Vega program.
In February 1974, Colonel Thomas L. Gatch Jr, USAR attempted to make the first crossing of the Atlantic by balloon in a superpressure balloon named Light Heart
Light Heart (balloon)
Light Heart was a balloon constructed by Colonel Thomas Leigh Gatch, Jr., USAR for an unsuccessful attempt at the first crossing of the Atlantic by balloon....
. Following the loss of at least two of the ten balloons which provided lift and after deviating substantially from the course that Colonel Gatch had plotted to take advantage of the jet stream
Jet stream
Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere and the stratosphere . The major jet streams on Earth are westerly winds...
, the last reported sighting of the Light Heart was 1,610km west of the Canary Islands
Canary Islands
The Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...
; no further trace of the aircraft was ever found.
See also
- Global horizontal sounding techniqueGlobal horizontal sounding techniqueThe Global horizontal sounding technique program was an atmospheric field research project for gathering weather data with balloons. Unlike radiosonde balloons which collect vertical atmospheric sounding data over the release point during a relatively short ascent lasting a few minutes, horizontal...
- Sky anchorSky anchorA sky anchor is a system of two balloons in tandem, with a "zero-pressure" lifting gas balloon tethered to a superpressure balloon "anchor". The gas balloon is filled with a lifting gas and provides the buoyancy, while the superpressure balloon is filled with air, and pressurized to provide the...
- NASA Image of the Day "Aloft" showing a superpressure balloon over Antarctica.