RP-3

The RP-3 (from Rocket Projectile 3 inch) was a British rocket projectile used during and after the Second World War. Though primarily an air-to-ground weapon, it saw limited use in other roles. Its 60 lb (27 kg ) warhead gave rise to the alternative name of the "60 lb rocket"; the 25 lb (11.3 kg) solid-shot armour piercing variant was referred to as the "25 lb rocket". They were generally used by British fighter-bomber aircraft against targets such as tanks, trains, motor transport and buildings, and by Coastal Command and Royal Navy aircraft against U-boats and shipping.

History
The first use of rockets fired from aircraft was during World War I. The "Unrotated Projectiles" were Le Prieur rockets which were mounted on the interplane struts of Nieuport fighters. These were used to attack observation balloons and were reasonably successful. Sopwith Baby and Pup and Home Defence B.E.2 fighters also carried rockets. With the end of the war the Royal Air Force, intent on retrenching, forgot about the potential uses for rockets fired from aircraft. The British Army, however, did see a use for rockets against low flying aircraft; from late 1940 parts of Britain were defended by increasing numbers of "Z-Batteries" 2 inch (51 mm) rockets supplementing the conventional anti-aircraft guns.

When German forces under the command of Rommel intervened in the Western Desert from early 1941 it became clear that the Desert Air Force did not have the weapons capable of damaging or destroying the large numbers of armoured fighting vehicles, particularly the heavier Panzer III and Panzer IV tanks possessed by the Germans. Something needed to be done and in April 1941 Henry Tizard, the Chief Scientist, 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 40mm Bofors and the US 37 mm T9 cannon fitted to the Bell P-39 Airacobra: however, it was already recognised that these weapons were only capable of dealing with light tanks and motor transport and 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 as a means of delivering a large warhead capable of destroying or disabling heavy tanks. Information was sought from the Russians who had just started using unguided RS-82 rockets against German ground forces in the opening phases of Operation Barbarossa.

By September 1941 it was decided that two models of UP (Unrotated Projectiles) would be developed:
 * A 23 lb plastic explosive on a standard 2 inch (51 mm) UP.
 * A 20 lb solid armour piercing head on a 3 inch (76 mm) 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 in version which could be developed from the 2 inch rocket used in the Z-Batteries.

Design
The rocket body was a steel tube 3 in in diameter, hence the name. The tube was filled with 11 pounds (5 kg) of cordite which was the propellant; this was fired electrically. A warhead was screwed into the forward end, initially a solid 25 lb, 3.44 in armour piercing shell which was quickly supplemented by a 6 in diameter, 60 lb high explosive head. Another type of head was a 25 lb (11 kg) mild steel (later concrete) practice head. Once the rocket had been mounted on the rails, an electrical lead (nicknamed "pigtail") was plugged into the exhaust of the rocket.

Four large tailfins were fitted which gave enough spin to stabilize the rocket, though it was unguided and targeting was a matter of judgment and experience. For a start the approach to the target needed to be precise, with no sideslip or yaw which could throw the RP off line. Aircraft speed also had to be precise at the moment of launch, and because the launch rails were a fixture, the angle of attack also required precision. Trajectory drop was also a problem, especially at longer ranges.

On the plus side the rocket was less complicated and more reliable than a gun firing a shell and there was no recoil on firing. It was found to be a demoralising form of attack against ground troops, and the 60 lb warhead could be devastating. The rocket installations were light enough to be carried by single seat fighters, giving them the punch of a cruiser. Against slow-moving large targets like shipping and U-boats the rocket was a formidable weapon.

The weight and drag of the all-steel rails as initially fitted to British aircraft had a detrimental effect on the aircraft's performance. Some aircraft such as the Fairey Swordfish had steel "anti-blast" panels fitted under the rails to protect the wing, which further increased weight and drag. Aluminium Mark III rails, introduced from late 1944, reduced the effect. American experience with their own rockets (the USAAF's 3.5-Inch Forward Firing Aircraft Rocket (FFAR) and the USN's 5-inch FFAR & HVAR ) showed that the long rails and anti-blast panels were unnecessary. British aircraft only started being fitted with "Zero-Point" mounting pylons in the post-war years.

The 3-inch rocket motors (less warhead) were used in the bunker buster Disney bomb; 19 of them propelling the 4,500 lb bomb to 990 mph at impact with the target.

Air to ground use
Before the new weapon was released for service extensive tests were carried out by the Instrument, Armament and Defence Flight (I.A.D.F) at Royal Aircraft Establishment, Farnborough. Hurricanes were fitted with rockets and rails and flown during June and July 1942. Further tests were undertaken from 28 September to 30 November to develop rocket firing tactics. Other aircraft used were a Hudson, a Swordfish, a Boston II and a Sea Hurricane. At the same time the Aeroplane and Armament Experimental Establishment (A&AEE) had to develop tactics for all the individual aircraft types which were to be armed with the RPs. Aiming was through a standard GM.II reflector gunsight. A later modification enabled the reflector to be tilted with the aid of a graduated scale, depressing the line of sight, the GM.IIL. For rockets only the Mk IIIA was the most successful - it was used on the Ventura and Hudson.

The first operational use of the RP was in the Western Desert as a "tank-busting" weapon on Hawker Hurricane Mk. IIEs and IVs. The 25 lb armour-piercing heads were found to be ineffective against the Tiger I tanks coming into German service. With the example of the success of Royal Artillery gunners using high-explosive shells from the 25 pdr gun-howitzer, it was decided to design a new 60-pound semi-armour-piercing (SAP) head. These were capable of knocking turrets off tanks.

A typical RP-3 installation was 4 projectiles on launching rails under each wing. A selector switch was fitted to allow the pilot to fire them singly (later omitted), in pairs, or as a full salvo. Towards the end of the war some RAF Second Tactical Air Force Hawker Typhoons had their installation adapted to carry an additional four rockets doubled up under the eight already fitted.

Possibly the best known action involving RP-3s was that of the Falaise pocket of mid-August 1944. During the battle German forces, retreating to avoid being trapped in a pincer movement by Allied ground forces, came under air attack. Amongst the waves of light, medium and fighter bombers attacking the German columns the Typhoons of 2 TAF attacked with their rockets, claiming hundreds of tanks and "Mechanised Enemy Transport". After the battle Army and 2nd TAF Operational Research Sections studying the battleground came to the conclusion that far fewer vehicles (17 in total) had been destroyed by rocket strike alone. What was clear was that in the heat of battle it was far harder for pilots to launch the weapons while meeting the conditions needed for accuracy. Smoke, dust and debris in the target areas made accurate assessment of the damage caused almost impossible.

It was also clear that the rocket attacks were devastating to the morale of the enemy troops: many vehicles were abandoned intact or with superficial damage and interrogation of captured prisoners showed that even the prospect of rocket attack was extremely unnerving.

Anti-submarine
Soon after some encouraging results from the initial deployment, trials of the weapon were conducted against targets representing U-boats. It was discovered that if the rockets were fired at a shallow angle, near misses resulted in the rockets curving upwards in seawater and piercing the targets below the waterline. Soon Coastal Command and the Royal Navy's Fleet Air Arm aircraft were using the rockets extensively.

The first U-Boat destroyed with the assistance of a rocket attack was U-752 (Kapitän-Leutnant Schroeter), on May 23, 1943 by a Swordfish of 819 NAS. The rockets used on this occasion had solid, cast-iron heads and were known as Rocket Spears. One of these punched right through the submarine's pressure hull and rendered it incapable of diving; the U–boat was scuttled by its crew. On May 28, 1943 a 608 Squadron Hudson destroyed a U-boat in the Mediterranean, the first destroyed solely by rocket. These rockets were, among other factors, credited with making it too dangerous for the Germans to continue operating their Flak U-Boats, which were initially designed with heavy anti-aircraft weaponry to hold off air attacks.

From then until the end of the Second World War in Europe, Coastal Command and the Fleet Air Arm used the rockets as one of their primary weapons (alongside torpedoes, which, to a certain extent they replaced) against shipping and surfaced U-Boats.



Ground to ground use
RP-3 rockets were fired by British troops as part of artillery bombardments, eg during Operation Veritable in 1945. In 1945, some British Shermans were fitted with two rails, one either side of the turret, to carry two 60 lb (27 kg) headed rockets. These were used at the Rhine Crossing by tanks of the 1st Coldstream Guards. The tanks were called "Sherman Tulips". The tanks fitted included both conventional Shermans and the more heavily armed Sherman Fireflies.

The rockets were highly inaccurate when fired from a tank as they were being fired from a stationary point and had little slipstream over the fins. Despite this, the RP-3 was valued by tank crews for the destructive effect of its 60 pound warhead.

Shore bombardment
RP-3s were used to arm the Landing Craft Rocket, also referred to as the Landing Craft Tank (Rocket) to denote that it was a modified Landing Craft Tank. These were used for shore bombardment during amphibious landings.

LCT(R)s carried 1,000 launchers and 5,000 rockets. The firepower was claimed to be equivalent to 80 light cruisers or 200 destroyers.

The method of operation was to anchor off the target beach, broadside to the shore. The range to the shore was then measured by radar and the elevation of the launchers set accordingly. The crew then went below, with the exception of the commanding officer who retreated to a special cubby hole to control the firing. The launch could comprise either the entire set or individual ranks of rockets.

A full reload was a very labour-intensive operation, and at least one LCT(R) went alongside a cruiser and got a working party from the larger ship to assist in the process.

Specification

 * Length: 55 in (1.4 metres)
 * Propelling charge: 11 lb (5 kg) cordite, electrically ignited.
 * Max speed 1,200 mph (480 m/s)
 * Range: 1 mile (1,600 m)
 * Weight: 47 lb (21 kg) with 25 lb (11 kg) AP head


 * Warhead
 * 60 lb Shell, HE/SAP (Semi-armour piercing)(27 kg)
 * 60 lb Shell, HE/GP, Hollow Charge
 * 18 lb Shell, HE (8 kg)
 * 25 lb Shot, AP (11 kg)
 * 25 lb Head, Solid, A/S (anti-submarine) (11 kg)
 * 60 lb Shell, Practice (training only) (27 kg)
 * 12 lb Head, Practice (training only)(5 kg)

Aircraft using the RP-3 in the Second World War
These are aircraft that used the RP-3 operationally, a number of aircraft types were fitted with RP-3s on an experimental basis.

RAF and Commonwealth Air Forces

 * Boeing Fortress Mk. II and IIA: (Coastal Command)
 * Bristol Beaufighter Mk. VI, VIC, X and 20: (Coastal Command, South East Asia Command and Royal Australian Air Force, Pacific Theatre.)
 * Consolidated Liberator B. Mk. III, VI: (Coastal Command.)
 * de Havilland Mosquito F.B. Mk. VI: (Coastal Command, SEAC and RAAF, Pacific Theatre.)
 * Hawker Hurricane Mk. IIE & IV: (DAF, 2 TAF, SEAC.)
 * Hawker Typhoon Mk. Ib: (2 TAF.)
 * Republic Thunderbolt
 * Vickers Wellington GR Mk. XIV: (Coastal Command)

Royal Navy Fleet Air Arm

 * Fairey Firefly Mk. I
 * Fairey Swordfish Mk. II, III
 * Grumman Tarpon/Avenger Mk. I, II, III

Post WW2
The 3" RP continued to be used on RAF and RN aircraft in the ground attack role until replaced by the SNEB podded rocket (RAF) and the 2" podded RP (RN).
 * Aircraft types included:
 * Royal Navy: Hawker Sea Fury and Sea Hawk; Supermarine Attacker and Scimitar;
 * RAF: Bristol Brigand, de Havilland Hornet, Vampire and Venom, Gloster Meteor, and the Hawker Hunter.


 * Operational use included the Malayan emergency, the Korean War, the Suez crisis, and the Radfan campaign.