Talk:Superpressure balloon
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Dropping a redraft here... Can't figure why I keep breaking references.
[edit]Edit summary: changed from pressure change to volume change as cause of change in buoyancy, added types of variable-volume balloons, links to greenhouse effect - which causes big difference in _relative_ changes of temp betw. atmosphere and lift gas, ref to Venus SP balloons in Vega program, ref to upper atmosphere temperature stability.
A superpressure balloon is a style of aerostatic balloon where the volume of the balloon is kept constant in the face of changes in the temperature of the contained lifting gas. This allows the balloon to keep a relatively stable altitude for long periods. Such ballooons are made with inelastic material with a high tensile strength, and tightly filled with gas, to achieve a stable shape.
This is in contrast with much more common variable-volume balloons, which are either only partially filled with lifting gas, or made with elastic materials. In a variable-volume balloon, the volume of the lifting gas changes as its temperature varies due to the heating and cooling of 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 ad drops, it becomes necessary to drop ballast. Since both ballast and gas are finite, there is a limit to how long a variable-volume baloon can compensate in order to stabilize its altitude. In contrast, a superpressure balloon will change altitude much less without any compensation maneuvre.[1]
Since the volume of the balloon is constant, so is the volume of air displaced by the balloon. 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.[2] 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 at that equilibrium altitude for long periods, except for the comparatively modest movements solely due to changes in external temperature and pressure, and vertical drafts. The disadvantage is that they require much stronger materials than non-pressurized balloons.
Superpressure balloons are typically used for extremely long duration flights of unmanned scientific experiments in the upper atmosphere[1], where atmospheric gas temperature is quite stable through the diurnal cycle. <ref [1] Seidel, Free, and Wang: Air Resources LaborDiurnal cycle of upper-air temperature estimated from radiosondes; Journal of Geophysical Research, 2005 /ref>. 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. — Preceding unsigned comment added by Spamhog (talk • contribs) 15:38, 29 January 2011 (UTC)