Unrotated Projectile

The 7-inch unrotated projectile, or UP, was a short range anti-aircraft rocket, developed for the Royal Navy. It was used extensively by British ships during the early days of the Second World War, but proved unreliable and ineffective in operation, prompting the withdrawal of the system during 1941.

The name "unrotated projectile" comes from the fact that the projectile was not spin-stabilized. The weapon had 20 smoothbore tubes, fired ten at a time. A small cordite charge was used to ignite a rocket motor which propelled the fin-stabilized 7-inch (18 cm) diameter rocket out of the tube to a distance of about 1,000 feet (300 m), where it exploded and released an 8.4 ounces (240 g) mine attached to three parachutes by 400 feet (120 m) of wire. The idea was that an aeroplane hitting the wire would draw the mine towards itself where it would detonate.

The UP was developed by Sir Alwyn Crow who was the director of the Projectile Development Establishment at Fort Halstead. In November 1939, Winston Churchill as First Lord of the Admiralty asked Dr. Crow to produce urgently a means of laying an aerial minefield and to consider other methods of protecting ships against aircraft. It is likely that Churchill was influenced in his request by his friend and advisor Frederick Lindemann who had previously advocated a scheme for "dropping bombs hanging by wires in the path of attacking aircraft". A high-altitude barrage was developed: an aerial minefield up to 19,000 feet (5,800 m), the fast aerial mine up to 2,000 feet (610 m), the PE fuse up to 18,000 feet (5,500 m) and the UP up to 20,000 feet (6,100 m).

The 7-inch UP was developed simultaneously with the 2-inch and 3-inch UP systems, the latter being successfully deployed in the anti-aircraft Z Batteries which were operated by the Home Guard and was the basis of the RP-3 air-to-surface rocket and the Mattress surface-to-surface multiple rocket launcher.

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