R4M (rocket) - Development

Development

The R4M was developed in order to deal with the increasing weight of anti-bomber weapons being deployed by Luftwaffe fighters. Their design had started out with the 20 mm MG 151/20 cannons, compact enough to be mounted in an internal wing bay mounting in the Focke-Wulf Fw 190, but it was found that it took an average of twenty 20 mm hits to shoot down a typical four engined bomber. The 20 mm cannons, also fitted to the Bf 109 in drag-producing underwing gun pods, were then supplemented with or replaced by the 30 mm MK 108 cannon, in slightly larger underwing pods, which could bring down a bomber with an average of one to three hits. However the MK 108 was much heavier and the larger ammunition made it difficult to carry more than one or two "passes" worth. Worse, the low muzzle velocity of this gun meant it had a very short range and extreme curvature of trajectory, of over 41 meters at 1,000 meters range. In approaching close enough to get hits, the fighters placed themselves within the range of the bomber's defensive guns. The more powerful MK 103 cannon had higher muzzle velocity and increased range, at the cost of greatly increased weight, size and much lower rate of fire: 380-420 RPM vs. 600-650 RPM for the MK 108.

Also, the Nebelwerfer 42-derived Werfer-Granate 21 (Wfr. Gr. 21) rockets fitted to Messerschmitt Bf 109 and Bf 110, and Focke-Wulf Fw 190 fighters, used to break up the USAAF combat box bomber formations, had launch tubes that were not only drag-producing, from their exposed five-strut under-wing mounting setup, but also from the fact that the launch tubes needed to be aimed upwards at some 15° from level flight, to counter the rocket's considerable ballistic drop after firing, adding to the already considerable drag the launch tube mountings created, as well as the Wfr. Gr 21's relatively slow projectile velocity of 1,150 km/h (715 mph).

The solution was to replace the underwing gun pods, and draggy large-calibre underwing rocket launch tubes, with a small diameter solid-fuel rocket engine-propelled projectile, mounting a warhead similar to that of the cannon shell. Although each "round" was heavier than the corresponding gun-fired shell, the lack of a gun reduced the overall weight considerably. The weight difference was so great that even a much larger and longer ranged rocket was still lighter than the guns it could replace.

The anti-aircraft version of the R4M used a large warhead of 55 mm with 520 g (17.6 ounces) of Hexogen explosive charge, nearly guaranteeing a kill with one hit. Each R4M weighed 3.2 kg and was provided with enough fuel to be fired from 1000 m, outside the range of the bomber's defensive guns. The main body of the rocket consisted of a simple steel tube with flip-out fins on the tail for stabilization. A battery typically consisted of two groups of 12 rockets and when all 24 were fired at once they would fill an area about 15 by 30 m at 1000 m, a density that made it almost certain that the target would be hit. The R4Ms were usually fired in four salvos of six missiles at intervals of 0.07 seconds from a range of 600 m, and would streak towards their target at a sixty percent higher velocity than the Wfr. Gr. 21's rockets would, at a speed of roughly 1,890 km/h (1,175 mph). Two warheads were available for the R4M, the common PB-3 with a 0.4 kg charge for anti-aircraft use and the larger shaped charge, similar in construction to the Panzerschreck, the Panzerblitz II/III (PB-2/3), for anti-tank use. The Panzerblitz III, mounting a gigantic 210mm hollow charge warhead, can be seen as the ultimate development of the basic Orkan rocket. It was intended to be carried (six or eight rockets per plane) by the tank-busting B model of the Henschel Hs 132 jet dive-bomber - however, neither the missile nor the warplane it was exclusively intended for got beyond the prototype stage before the end of the war.