4-missile launcher "anti-submarine projector Mark 20"
Mousetrap (ASW Marks 20 and 22) was an anti-submarinerocket used mainly during the Second World War by the U.S. Navy and the U.S. Coast Guard. Its development was begun in 1941 as a replacement for Hedgehog, a British-made projector, which was the first ahead-throwing ASW weapon. These, however, were spigot-launched, placing considerable strain on the launching vessel's deck, whereas Mousetrap was rocket-propelled. As a result, Mousetrap's four or eight rails for 7.2-inch (183 mm) rockets saved weight and were easier to install.