OVERVIEW: The Mk 20 “Mousetrap” is an unguided, surface-launched anti-submarine rocket depth charge system developed by the U.S. Navy and used during World War II. It launches 7.2‑inch rockets in salvos of four or eight rounds to deliver a forward-throwing pattern of explosives against submarines.

DETAILS: “Mousetrap” refers to the ASW Projector Mark 20 (four-rail) and Mark 22 (eight-rail) systems, which fire 7.2‑inch (183 mm) rockets each carrying a 15 kg (33 lb) Torpex warhead with a contact pistol—equivalent to the Hedgehog projectile—using rocket propulsion instead of spigot launchers. Development began in 1941–42 at Caltech under the NDRC to create a recoil-free launcher suitable for small ships. The Mk 20 enters service in 1942, followed by the Mk 22 by 1943; over 100 Mk 22 units were deployed across U.S. Navy destroyers and submarine chasers by late war. Rockets had a range of approximately 280 m and could be fired at up to one round every three seconds per rail.

FUNCTION: Mousetrap’s role was to engage submerged submarines detected ahead of the ship by sonar, firing a salvo to create a lethal pattern of contact-warhead rockets. Upon water impact, rockets sink and detonate upon striking the hull of a submarine.

NOTE:
IOC: 1942 (Mk 20), ~1943 (Mk 22)
Operators: United States Navy, U.S. Coast Guard
Platforms: Destroyers (e.g., Benson-class), submarine chasers (SC-class)
Conflict used in: World War II

SOURCE:
Norman Friedman: US Naval Weapons, Conway; Andreas Parsch: “Surface‑Launched 7.2 inch Rockets”; Phoebus Publishing: Mousetrap entry; U.S. National Archives / NDRC Caltech project files