CalTech/Navy Retrorocket (Retrobomb)

By 1942, the U.S. Navy had developed working MAD (Magnetic Anomaly Detection) sets for use aboard anti-submarine aircraft. However, MAD had a very short range, and could effectively detect a submerged submarine only when the aircraft was flying more or less directly over it. But when a conventional depth charge would be dropped at that moment, the aircraft's forward motion would put it way ahead of the target. Therefore the so-called Retrorocket (a.k.a. "Retrobomb" or VAR - Vertical Antisubmarine Rocket) was developed by the NDRC's group at the California Institute of Technology as a derivative of the Mousetrap ASW rocket. The Retrorocket was a depth charge with a rocket motor pointing in the direction of flight. After the drop from the ASW aircraft, the motor rapidly decelerated the Retrorocket to zero forward airspeed so that it fell essentially straight down. The first air-drop of a Retrorocket from a PBY-5A Catalina occurred on 3 July 1942, making it the first ever launch of a rocket from an American combat aircraft.

Operational aircraft equipped for Retrorocket were fitted with multi-rail launchers, from which the rockets could be fired in groups to lay a rectangular pattern of depth charges into the water. Actual service tests began in December 1942, and in January, three new motors optimized for three different firing speeds were designed. The 7V6 for 330 km/h (205 mph), the 7V7 for 320 km/h (200 mph) and the 7V8 for 640 km/h (400 mph). In the designations, the first digit was the diameter in inches, the "V" stood for "Vertical", and the last digit was the modification number. Later, the TNT explosive was replaced by the more powerful "Torpex", creating the 7V11 through 7V13 rockets. This series became the standard Retrorockets in May 1943.

In Summer 1943, several U-boats were destroyed in the Atlantic by the MAD/Retrorocket combination. However, the system was only effective if the launching aircraft was flying relatively low (< 90 m (300 ft)), and when German U-boats began to fight attacking aircraft with guns on the surface, that tactic became increasingly dangerous. Therefore Retrorockets were gradually abandoned in the final months of the war.

Specifications

Note: Data given by several sources show slight variations. Figures given below may therefore be inaccurate!

Data for Retrorocket 7V7:

Length	?
Diameter	18.3 cm (7.2 in)
Speed	320 km/h (200 mph)
Propulsion	Solid-fueled rocket
Warhead	16 kg (35 lb) depth charge

Main Sources

[1] Norman Friedman: "US Naval Weapons", Conway Maritime Press, 1983
[2] Frederick I. Ordway III, Ronald C. Wakeford: "International Missile and Spacecraft Guide", McGraw-Hill, 1960

Back to Directory of U.S. Military Rockets and Missiles, Appendix 4

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Last Updated: 7 March 2005