MAD (Magnetic Anomaly Detectors) Cats used against submarines were equipped with CalTech 7.2 in ASW rockets "Mousetraps"
On 3 July 1942, in the first successful firing of an American rocket from a plane in flight, Lieutenant Commander J. H. Hean, Gunnery Officer of Transition Training Squadron, Pacific Fleet, fired a retro-rocket from a PBY-5A in flight at Goldstone Lake, Calif. The rocket, designed to be fired aft with a velocity equal to the forward velocity of the airplane, and thus to fall vertically, was designed at the California Institute of Technology. Following successful tests, the retro-rocket became a weapon complementary to the magnetic airborne detector with Patrol Squadron 63 receiving the first service installation in February 1943. On 24 April 1944, the squadron scored the first kill with the retro-rockets.
The "Mousetrap" was an air-launched rocket based on the ship-launched Hedgehog antisubmarine rocket. The retrorocket weight 65 lbs with a 35 lb warhead and solid rocket motor in the tail with a ring fin assembly for stability. The retrorocket was mounted under the Catalina's wings pointed aft. Once the MAD sensor detected the U-boat, the rockets fired aft to fall vertically at the point where it had been fired as the forward facing rocket canceled out the forward speed of the launching aircraft.
Retrorockets brought answer to the operational problem met when using regular depth bombs and anti-submarine bombs: As the Catalinas had to fly low over the surface of the water (100 m) to detect the magnetic signature of a submerged U-boat, by the time a depth bomb was released, the aircraft's forward motion would carry the bomb ahead of the intended target.
