The Ruhrstahl Ru 344 X-4 or Ruhrstahl-Kramer RK 344 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.
During 1943, the RAF's Bomber Command and the US Air Force 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 autocannon designs, air-to-air rockets, SAMs, and the X-4.
Work on the X-4 began in June 1943, by Dr Max Kramer at Ruhrstahl AG. 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). The bombers' defensive guns had a maximum effective range of about . The missile was to be 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 and kept the X-4 at this speed during its "cruise", between .
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 thrust initially, declining to 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 (similar to 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.
The missile was spin-stabilized at about 60 rpm, or one rotation a second, so any asymmetrical thrust from the engine or inaccuracies in the control surfaces would be evened out.
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