In the late 1930s the British saw a need for an intermediate, close-range AA weapon to supplement their 2-pdr AA guns. The UP Mark I was one of the small rocket weapons designed to meet this need and was employed on many large British warships early in World War II.
UP stands for "Unrotated Projector." "Unrotated" meant that the barrel did not have any rifling, i.e., the projectile was not spin-stabilized. Each emplacement was a set of twenty smooth-bore tubes, usually fired ten at a time. Cordite was used to ignite ("Project") a 3-inch (7.62 cm) rocket motor which propelled a fin-stabilized 7-inch (17.8 cm) diameter Parachute and Cable (PAC) rocket which carried a 8.4 oz (238 g) mine. When the rocket reached approximately 1,000 feet (330 m), it exploded and put out the mine which was attached to three parachutes by 400 feet (122 m) of wire. The design concept was that if a plane hit the parachutes or the wire, it would then pull the mine into itself.
These UP projectiles were kept in ready lockers close to the projectors. The sinking of HMS Hood showed that these stored weapons were rather flammable. They were also found to be an almost totally ineffective weapon, as the barrage took too long to establish and was easily avoided. In addition, reloading was slow and the mines showed an alarming tendency to drift back onto the firing ship. For these reasons, the UP was gradually replaced on surviving ships with either the British 2-pdr or the Bofors 40 mm heavy AA machine gun. For instance, shortly before she was sunk by the Japanese, HMS Prince of Wales had all three of her UP emplacements replaced with 2-pdr mounts.
Other information: The rockets were 32 inches (81.3 cm) long and weighed 35 lbs. (15.9 kg). The effective horizontal range was 3,000 feet (910 m) and the parachute sinking speed was 16 to 23 fps (5 to 7 mps). The 20-tube mounting weighed about 4 tons (4 mt).