had been recognized by the British and trials began. Hydrophones had been developed and were becoming effective as detection and location devices. Also, aircraft and airships had flown with depth bombs (aerial depth charges), albeit quite small ones with poor explosives. In addition, the specialist hunter-killer submarine had appeared, HMS R-1. Inter-war The main developments in this period were in detection, with both active sonar (ASDIC) and radar becoming effective. The British integrated the sonar with fire control and weapons to form an integrated system for warships. Germany was banned from having a submarine fleet but began construction in secret during the 1930s. When war broke out it had 21 submarines at sea. In the inter-war period Britain and France had experimented with several novel types of submarine. New sonars and weapons were developed for them. World War II Atlantic/Mediterranean Theatre By the time of World War II, anti-submarine weapons had been developed somewhat, but during that war, there was a renewal of all-out submarine warfare by Germany as well as widespread use of submarines by most of the other combatants. The effective use of depth charges required the combined resources and skills of many individuals during an attack. Sonar information, helmsmen, depth charge crews and the movement of other ships had to be carefully coordinated in order to deliver a successful depth charge attack. As the Battle of the Atlantic wore on, British and Commonwealth forces in particular proved particularly adept at depth charge tactics, and formed some of the first destroyer hunter-killer groups to actively seek out and destroy German U-boats. Air-dropped depth bombs were normally set to explode at a shallow depth, while the submarine was crash-diving to escape attack. Aircraft were very successful in not only attacking U-boats, but also in disrupting U-boats from carrying out attacks against ships. Some were fitted with a searchlight as well as bombs. A host of new anti-submarine weapons were developed. Forward-throwing anti-submarine mortars were introduced in 1942 to prevent loss of sonar contact. These mortars, the first being Hedgehog, fired a pattern of small depth charges. One type of charge was used to create entire patterns of explosions underwater around a potential enemy, while the second type of round was fitted with contact detonators, meaning the warhead exploded only upon contact with the submarine. A later design enabled a pursuing destroyer or destroyer escort to maintain continual sonar contact