Depending on the size of the hole, the effect in most cases is, very little! Airships normally operate with very low differential pressure forcing the LTA-gas out, hardly a matter to worry about. Should a hole occur, the resulting gas escape velocity thus is very slow. Punctures made by bullets generally don’t make a round hole; instead causing small plus-sign (+) shape slits, the small flaps of which then close due to material stiffness after the bullet has passed through. The hole made by a missile passing through naturally is bigger. However, it’s still small with respect to the aerostat’s size and the gas volume contained. Gas leakage rate therefore is slow and it takes a long time to vent it all – so not necessarily an immediate concern.
More concerning is the effect of a hole (or slit) causing stress-concentration and then rapid ripping to occur. This generally is safeguarded against from normal design practices using tough materials with high rip strength, tests to prove material properties, high safety factors imposed by regulators and general operations with low super-pressure (reducing stress levels). Airship aerostats therefore don’t pop like party-balloons.
Non-rigid airships are inspected for holes at normal maintenance intervals through the year, generally resulting from irresponsible people shooting at them. Also, the crew (ground and air) continually monitor LTA-gas purity and super-pressure to determine necessary action. Large holes, say 5 ft (1.52 m) in diameter may need the airship’s mission to be aborted and return to base for repair. However, even with a hole this size and, depending on airship size, several hours can pass before the situation becomes critical – although super-pressure may be lost. When super-pressure is lost the aerostat may become limp (unable to hold a stiff form without pumping air directly into the LTA-gas chamber). It also depends on where it’s holed because, without super-pressure, a hole in the aerostat’s underside (opposite to water in a bucket) doesn’t naturally leak downwards. If it did hot air balloons wouldn’t work!
In-flight temporary repair is possible, as has occurred in the past. Today, robotic methods now are available instead. Repair methods also have been developed and tested for use under standard operational procedures.
Repairs within the LTA-gas compartment require maintenance crew to wear breathing apparatus. Special safety arrangements to monitor them in the LTA-gas areas and ability to quickly provide alternative air or get them out also are necessary. Repairs conducted by crew outside the aerostat need suitable climbing facilities to lower themselves from the upper surface, enabling patching with quick setting adhesives.
Depending on circumstances, airships may continue in sustainable flight with say thirty percent LTA-gas loss. However, even if the LTA-gas loss was greater, and depending on ability to keep the aerostat puffed up, which may be with air, they descend at parachute rates. Airships, unlike aeroplanes, thus don’t necessarily fall out of the sky like rocks when they’re hit – only when the aerostat is destroyed or fully collapses (candles).
