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The R4M was developed in order to deal with the increasing weight of anti-bomber weapons being deployed by the Luftwaffe fighters. Their designs had started out with the MG151/20 20mm cannons, but it was found that it took an average of 23 rounds from one of these guns in order to down a B-17 Flying Fortress, a number that was difficult to achieve. The 20mm cannons were then replaced, or supplanted, with the MK108 30mm cannon, which could bring down a bomber with only three hits. However the MK108 was much heavier, and the larger ammunition made it difficult to carry more than one or two "passes" worth of ammunition. Worse, the low muzzle velocity of these guns meant that the fighter had to come quite close to the bombers, and therefore place themselves within the range of the defensive guns. The more powerful MK103 had higher muzzle velocity and increased the standoff range, but only at the cost of greatly increased weight and size.
The solution was to replace the gun with a small solid-fuel rocket engine, mounting a warhead similar to that of the cannon shells. The R4M in fact used a much larger warhead of 55mm, one that was selected to guarantee a kill with a single hit. Each R4M weighted 32kg, and was provided with enough fuel to be fired effectively from 1000m, outside the range of the bomber's defensive guns. A battery typically consisted of two groups of 12 rockets, and when all 24 were fired at once they would fill an area about 15m x 30m at 1000m, dense enough that one was almost certain to hit its target. The main body of the rocket consisted of a simple steel tube with flip-out fins on the tail for stabilization. Two warheads were available for the R4M, the common PB-3 with a 4kg shaped charge for anti-aircraft use, and the larger PB-2 for use in the anti-tank role.
Only a small number of aircraft were ever fitted with the R4M, mostly Messerschmitt Me 262's, which mounted them on small wooden racks under the wing just outside of the engines. In service the weapon proved deadly, with one attack in April 1945 downing 30 B-17's for the loss of only three Me 262's. However by this point the war was already over.
After the war the R4M served as the pattern for both the US 2.75" FFAR (Folding-Fin Areal Rocket) and the larger 5" "Zuni".
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