The Battle Balloon is a type of fixed defensive installation, consisting of a helium- or hydrogen-filled dirigible, moored by steel line, carrying a .50-cal machine gun. The Battle Balloon is deployed to provide anti-aircraft cover of fixed installations. However, the balloon itself is unarmored and vulnerable to enemy fighter attack. Battle balloons, therefore, work best when supported by other anti-aircraft defences. However, battle balloons can be deployed much more rapidly than ground turrets, and are therefore common at temporary or new facilities.