Rebound height

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The impact height of an aerostat such as an airship or gas balloon is the height at which the maximum permissible envelope overpressure is reached.

With increasing altitude, the external pressure in the atmosphere decreases. The lifting gas of a partially filled envelope therefore expands as the aerostat ascends until it completely fills the envelope, i.e. H. until it is full.

A hot air balloon cannot build up a large overpressure because it is open at the bottom. Accordingly, there is no rebound height for hot air balloons .

Safety valves

If the aerostat continues to rise, gas is allowed to escape in order to prevent the envelope from bursting due to the increasing envelope overpressure. For this purpose, safety valves are attached to airships , which open automatically when excess pressure is too high and allow the gas to flow out.

The gas balloon does not have a safety valve, but an open filling attachment at the lowest point of the balloon; this open filling attachment can also be viewed as a safety valve with a response pressure of “zero”.

A failure or insufficient function of the safety valves was blamed for the crash of the airship Erbslöh in 1910 . There it was not the high altitude, but the warming resulting from solar radiation and the associated expansion of the carrier gas that caused the overpressure of the envelope.

The Soviet airship Pobeda crashed in 1947 due to the dropping of too much ballast : its safety valves could not counteract the rise in the lifting gas overpressure, which was just as rapid due to the rapid rise, so the ship rose too quickly and far above its specified impact height, and therefore exceeded it the increased rebound height for the currently reduced amount of gas, which resulted in an envelope tear due to excessive overpressure.

Airships with ballonets

With inflatable airships and semi-rigid airships are within the envelope usually ballonets . These air-inflated pockets are used to compensate for fluctuations in the volume of the carrier gas and to maintain an approximately constant overpressure of the envelope without changing the amount of the carrier gas. The rebound height indicates the height at which the ballonets are emptied and the lifting gas takes up the entire envelope volume.