What proportion of the total gas volume does the volume of the ballonets represent in an airship envelope?

The correct answer is that the volume of the ballonets typically represents 25% of the total gas volume in an airship envelope. Ballonets are air-filled compartments within the envelope that help maintain the shape of the airship and provide pressure stabilization as the altitude changes.

As an airship ascends, the gas inside the envelope expands due to reduced atmospheric pressure, which can lead to the risk of rupture if there is no mechanism to accommodate this change. The ballonets help mitigate that risk by allowing air to be pumped in or out, thus maintaining the envelope's integrity without causing structural stress.

In standard airship design, a balance is maintained between the lifting gas and the air in the ballonets, ensuring that the airship remains correctly inflated and stable during flight. The 25% proportion effectively facilitates the necessary dynamic response to pressure changes while still maximizing the lifting capacity offered by the lighter-than-air gas. Thus, this design choice aligns with operational needs and safety considerations in aircraft engineering.