A defence used by merchant ships against German aircraft was a battery of rockets carrying aerial mines. These consisted of a parachute with a warhead below, beneath which was a long cable with a drogue chute attached. When a plane encountered the cable the warhead detached from its chute and began to fall. The pull of the drogue chute ensured that the warhead hit the aircraft rather than falling behind it. Aerial mines could be deployed by artillery as a hazard to attack helicopters. Fitting a hydrogen balloon in place of the upper parachute would keep the mine aloft longer. Mines might also be deployed by aircraft in combat.
