Ruhrstahl X-4

The Ruhrstahl Ru 344 X-4 was a wire guided air-to-air missile designed by Germany during World War II. The X-4 did not see operational service and thus was not proven in combat but inspired considerable post-war work around the world and was the basis for the development of several ground-launched anti-tank missiles, including the Malkara.

During 1943, the RAF's Bomber Command mounted a series of heavy raids against Germany. Despite heavy bomber losses, these prompted Luftwaffe research into considerably more powerful anti-bomber weaponry in order to reduce the cost in lost fighter aircraft and aircrew. A massive development effort resulted in a number of heavy-calibre 30mm designs like the MK 103 and MK 108 cannon and the even heavier-calibre 37, 50 and 75 mm calibre Bordkanonen which were converted, auto-loading anti-tank and AFV weapons for use on twin-engined bomber destroyers, air-to-air rockets, SAMs, and the X-4.

Work on the X-4 began in June 1943, by Dr Max Kramer at Ruhrstahl. The idea was to build a missile with enough range to allow it to be fired from outside the range of the bombers' guns (what is now called a stand-off weapon), while being guided with enough accuracy to guarantee a "kill". The X-4 met these specifications and more; its BMW 109-448 rocket motor accelerated the missile to over 1,150 km/h (715 mph) - about the same speed as both the earlier Werfer-Granate 21 and folding-fin R4M rockets - and kept the X-4 there during its "cruise", between 1.5 and 4 km (0.9-2.5 mi), while the defensive guns had a maximum effective range of about 1000 m (1,094 yd). The rocket burned a hypergolic mixture of S-Stoff (nitric acid with 5% iron(III) chloride) and R-Stoff (an organic amine-mixture of 50% dimethylaminobenzene and 50% triethylamine called Tonka 250) as propellant, delivering 140 kg (310 lb) thrust initially, declining to 30 kg (66 lb) over the 17-second burn. As there was no room for a fuel pump, the fuels were forced into the motor by pistons inside long tubes, the tubes being coiled (as with a coil spring) to fit inside the airframe. S-Stoff was so corrosive, it dissolved all base metals and was extremely difficult and dangerous to handle. The Germans planned to replace the motor with a solid fuel design as soon as possible.

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