The solid-fuel 3 inchrocket used by the Z Batteries was known as the UP-3 and had been developed in the late 1930s by the Projectile Development Establishment at Fort Halstead in Kent under the direction of Alwyn Crow and was related to the Royal Navy's 7 inch Unrotated Projectile. The naval weapon had been enthusiastically backed by Winston Churchill when he was First Lord of the Admiralty at the outbreak of war. By June 1940, Churchill was Prime Minister and he requested "large supplies of projectors" for the anti-aircraft defence of the mainland. The development of all British rockets was under the control of Professor F A Lindemann and he enthusiastically backed Churchill's suggestion. The naval weapon was intended to bring down low flying aircraft with a trailing wire to the end of which was attached an explosive mine; however, the land based system was intended to have a high explosivewarhead, detonated by a specially designed photoelectricproximity fuse. The rocket itself was propelled by special solvent-free cordite, which was initially manufactured at Bishopton in Scotland; in December 1940, a new propellant factory was commissioned at Ranskill, which was in production by the start of 1942. The metal working firm G. A. Harvey and Co of Greenwich was given the contract to manufacture the rocket bodies and over 1,000 had been made by September 1940. In October 1940, an experimental Z Battery became operational at Cardiff in South Wales under the command of Major Duncan Sandys, Churchill's son-in-law. Trials against a radio controlledQueen Bee target aircraft were successful, although the Director of Artillery at the Ministry of Supply suspected that the results were "fixed". Despite this Churchill and Lindemann drove the project forward, and by 1942, 2.4 million rockets were being produced annually.
Service
The first Z Batteries were equipped with a single-rocket launcher, the Projector, 3-inch, Mark 1. It was soon found that the rockets did not perform as accurately as the trials had suggested and that the proximity fuses were rarely effective. Therefore, the technique of firing the rockets in large salvos was introduced, and projectors capable of firing an ever-larger number of rockets were developed. The Projector, 3-inch, No 2, Mk 1 was a twin launcher and the No 4 Mk 1 and Mk 2 fired 36 rockets at a time in a "ripple" firing sequence. During the Battle of Britain in an attack on RAF Kenley by Dornier 17 bombers, AC2 David Roberts, RAF Regiment, downed one of the two attacking aircraft destroyed using the RAF's newest anti-aircraft weapon, a line of twenty-five rockets that deployed a barrage of 500 foot cables suspended by parachutes. This weapon - the naval version of the Z Barrage - was symptomatic of the hodge-podge of weapons issued to the RAF Regiment in the early war years, The other Dornier 17 was shot down by Corporal John Miller of the Scots Guards, using a Lewis gun. For this action both men received the Military Medal. From early 1942, the manning of Z Batteries began to be transferred to the Home Guard, as the equipment was comparatively simple to operate and the rounds were lighter.
Use as a ground attack weapon
Both the No 2 and No 4 projectors were used in the North African Campaign, mounted on converted 3-inch AA gun trailers. The emergency use of a No 4 projector against an enemy infantry attack in that theatre provided the inspiration for the No 8 Projector, better known as the "Land Mattress", a surface-to-surface rocket system, used in action by the Canadian Army in 1945. The UP-3 rocket was also developed into the RP-3 air-to-ground anti-tank rocket.
Bethnal Green disaster
On 3 March 1943, civilians queueing to enter Bethnal Green Underground station in East London, which was being used at night as an air raid shelter, were panicked by the noise of a newly installed Z Battery firing in nearby Victoria Park. After somebody tripped on the stairs leading down to the ticket office, some three hundred people were crushed in the stairwell. 173 were killed and 90 needed hospital treatment.
