The RP-3 (from Rocket Projectile 3 inch) was a British air to ground rocket projectile introduced during the Second World War. The "3 inch" designation referred to the nominal diameter of the rocket motor tube. The use of a warhead gave rise to the alternative name of the "60-pound rocket". Though an air-to-ground weapon, it saw limited use in other roles. They were generally used by British fighter-bomber aircraft against targets such as tanks, trains, motor transport and buildings, as well as by Coastal Command and Royal Navy aircraft against U-boats and ships.
Use continued post-war, with the last known major operational use being during the Aden Emergency in 1964, where Hawker Hunters flew 642 sorties and fired 2,508 RP-3s in support of Radforce. Use continued until the withdrawal from Aden Protectorate in November 1967, at which point the RP-3 was withdrawn from service in favour of the newer SNEB. Concerned about the possibility of shipboard radar setting off the SNEB's electrical ignition, the Royal Navy replaced their RP-3s with a new design, sometimes known as the 2-inch RP.
History
The first use of rockets fired from aircraft was during the First World War. The "unrotated projectiles" (UPs) were Le Prieur rockets mounted on the interplane struts of Nieuport fighters. These were used to attack observation balloons and were reasonably successful. Sopwith Baby, Sopwith Pup and Home Defence B.E.2 fighters also carried rockets.
With the war ended the Royal Air Force, intent on retrenching, forgot about firing rockets from aircraft. The British Army did see a use for rockets against low-flying aircraft; from late 1940 parts of Britain were defended by increasing numbers of "Z-Batteries" rockets supplementing the conventional anti-aircraft guns.
When German forces under the command of Erwin Rommel intervened in the Western Desert campaign from early 1941, it became clear that the Desert Air Force lacked weapons capable of damaging or destroying the large numbers of armoured fighting vehicles, particularly the heavier Panzer III and Panzer IV medium tanks used by the Germans. In April 1941 Henry Tizard, the Chief Scientist of the British government, called together a panel to study "Methods of Attacking Armoured Vehicles".
The types of weapons investigated included the 40 mm Vickers S gun and related weapons manufactured by the Coventry Ordnance Works, as well as the Bofors 40 mm and the US 37 mm T9 cannon fitted to the Bell P-39 Airacobra; it was already recognised that these weapons were only capable of dealing with light tanks and motor transport. Using larger weapons on fighter-bombers was ruled out because of weight and difficulties handling recoil. The chairman of the panel, Mr. Ivor Bowen (Assistant Director of Armament Research) turned to the idea of using rocket projectiles to deliver a large warhead capable of destroying or disabling heavy tanks. Information was sought from the Soviets, who had been using unguided RS-82 rockets since 1937.
By September 1941 it was decided that two models of UP would be developed: A 23 lb plastic explosive on a standard 2-inch UP A 20 lb solid armour-piercing head on a 3-inch UP
When it was realised that the 2-inch version would be less effective than the Vickers S cannon, it was decided to concentrate on development of the 3-inch version, which could be developed from the 2-inch rocket used in the Z-Batteries.
Design
The rocket body was a steel tube in diameter and long filled with of cordite propellant, fired electrically. The warhead was screwed into the forward end, and was initially a solid , diameter and long (with adaptor) armour-piercing warhead which was quickly supplanted by a , high explosive head. For practice there were also a mild steel semi-armour-piercing warhead, a concrete practice head and a concrete practice head. Once the rocket had been mounted on the rails, an electrical lead (or "pigtail") connected the rocket to the firing controls.
Four large tailfins induced enough spin to stabilize the rocket, but as it was unguided, aiming was a matter of judgment and experience. Approach to the target needed to be precise, with no sideslip or yaw, which could throw the RP off line. Aircraft speed had to be precise at the moment of launch, and the angle of attack required precision. Trajectory drop was also a problem, especially at longer ranges.