What is a gas balloon ?

A gas balloon, or Charlière, contains a gas lighter than air, usually hydrogen or helium. When the pilot wants to rise, some of the extra weight (usually sand) that was brought at take-off, is released.

When the decision has been made to go down, the pilot opens the valve and releases some of the gas.

At the base of the envelope, an appendage insures the inflation and the evacuation of the gas when it expands at higher altitudes.

A rip panel ensures a brutal landing as quickly as possible to avoid being dragged and to immediately evacuate the gas held inside the envelope.
Where the average hot air balloon flight only lasts two hours (due to the perserverence of the propane bottles) a flight in a gas balloon can last a few days.