One new solution was the use of magnetic anomaly detection, or MAD. This device detected variations in the Earth’s magnetic field due to the presence of a large iron object near the detector that caused distortions in the field. A plane flying over a submarine using a MAD device would detect its presence by this distortion. At the time, the U.S. Navy put great faith in the ability of MAD to find submarines. It was the their latest “secret” invention.
But the Navy lacked a weapon that could be used with MAD to attack a submarine after it was found. Division 3, Section L, of the National Defense Research Committee (NDRC) and Office of Scientific Research and Development (OSRD) at Caltech received the task of developing one. Richard C. Tolman, professor of physical chemistry and mathematical physics headed the project with his deputy, Charles C. Lauritsen, professor of physics.
Richard C. Tolman ca. 1945 (Science History Institute)
Because a MAD detection would not occur until the aircraft had flown over its target, the weapon would have to be launched so that it fell absolutely vertical and behind the aircraft. This would allow the bomb to fall directly on the position of a MAD detection and attack the submarine. Using conventional bombs and depth charges meant they fell well ahead of the target instead of on it due to the plane’s motion. The solution Caltech came up with was the retro bomb.
Charles C. Lauritsen
(AIP Emilio Segrč Visual Archives, Physics Today Collection)
Laursiten’s solution was to use the existing model of the Anti-Submarine Rocket (ASR) “Mousetrap,” a 7.2″ rocket developed from the earlier Hedgehog spigot bomb, fired in retrograde. That is, launched backwards from the wing of a naval patrol plane. The Consolidated PBY-5A Catalina, the Navy’s most common patrol plane, would be used.
The Mousetrap rocket had been in design and testing at Caltech since September 1941 and was being tested at Goldstone Dry Lake about thirty miles north of Barstow California (part of Fort Irwin today). This made the modification into the retro bomb quick and simple.
The retro ASR needed a larger motor to match the forward velocity of the Catalina, so this was increased in size from 2.25” diameter to 3.25.” That would give the retro bomb the necessary 400 ft/sec speed to counter forward motion. The explosive warhead remained 60 lbs. and was armed on the operational model by a hydrostatic fuze similar to those used on depth charges. The weapon did not arm until it entered the water and began to submerge increasing its safety while on the carrying aircraft.
Components of the similar Hedgehog bomb. On the retro bomb, the tail tube is larger in diameter due to the use of a rocket motor.
(U.S. Navy)
To test the weapon, engineers from Caltech set up a rig at Goldstone Dry Lake using two telephone poles forty feet apart with a rectangular multiturn coil of heavy wire hung between them powered by car batteries and a DC generator. This simulated the magnetic signature of a submarine and allowed the MAD detector on the PBY to “find” it. On 3 July 1942 several test runs were made before the PBY dropped one its retro bombs directly onto the test rig damaging it. These tests have the distinction of being the first time a rocket was fired from a U.S. combat aircraft.
The aircraft mounting consisted of two dozen retro bombs mounted on the wings of a PBY. A shallow metal trough served to guide the launch and protect the wing of the plane. Later, the mountings were increased to 30 per plane.
Three different motors were developed for the operational rocket, based on the expected speed of the aircraft. The 7V6 for 205 mph, the 7V7 for 200 mph, and the 7V8 for 400 mph. By the time the retro bomb went into operational use, the explosive warhead replaced TNT with Torpex, a more powerful explosive. These retro bombs were designated 7V11 and 7V13 and standardized in May 1943.
VP-63 and the operational use of the retro bomb
With success of the testing, a Patrol Squadron of PBY Catalina was formed to use MAD and retro bombs. Designated VP-63, the squadron was organized at NAS Alameda, California, on 19 September 1942. They were quickly dubbed the “Mad Cats” from their use of the MAD detector and flying PBY Catalina, or “Cat,” patrol planes, a common nickname for this aircraft.
The method VP-63 developed was when a submarine was detected, a smoke/light float would be dropped to mark the sub’s position. Multiple passes established the direction of the sub. This allowed an accurate attack to be made.
A PBY’s crew initiated the attack based on the MAD readings. The retro bombs fired automatically via a circuit connected through the MAD instrumentation. The launch system fired the bombs in a rectangular pattern to increase the likelihood of a hit.
VP-63 moved to the NAS Quonset Point, Rhode Island, on 3 March 1943 where they were assigned to Air Anti-Submarine Development Center Atlantic (AirASDevLant) for operational testing of MAD and the retro bomb.
Parts of the squadron were assigned to separate locations along the U.S. East Coast to begin operational sorties on 21 April, looking for U-boats off the southeastern Atlantic seaboard and Florida.
The squadron quickly discovered their equipment had serious limitations. The operational altitude for MAD was limited to under 400 feet, and frequently the planes were flying at 50 to 150 feet to detect U-boats. Formation flying at very low altitude by four or more planes abreast roughly a quarter mile apart became normal practice. MAD worked, locating a half-dozen shipwrecks, but it was too short-ranged to randomly find a submarine other than by sheer luck.
VP-63 also traded their amphibious PBY 5As for PBY 5s that lacked landing gear. It was found that the extra weight of the landing gear made the plane marginally airworthy when carrying MAD gear and retro bombs.
A smoke/light float in launch position on a PBY. Note the MAD boom under the tail of the plane.
(U.S. Navy)