Apron screens came into use shortly before the end of the World War I in response to attacks by German bombers on London. They consisted of three balloons, 500 yards apart, joined together by a heavy steel cable. No enemy aircraft were actually caught in their nets but they did force enemy aircraft to fly higher, reducing their effectiveness as bombers and making them more vulnerable to attack. Although military planners approved a line of 20 aprons — requiring massive support crews of about 30 men per balloon — only about 10 aprons were fully operational by the end of the war.
