Superpressure balloon

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 ambient pressure outside the balloon, and 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. 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 ambient pressure outside the balloon, and 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 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 maneuvers. 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. As a result, the balloon will be stable in a finite equilibrium altitude range for long periods.