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In 1943 the increasing tempo of operations by the USAAF's 8th Air Force were proving to be difficult for the Luftwaffe to counter. Although by the end of the year they had managed to do serious damage to the bombing raids, the cost in fighter aircraft lost to the bomber's guns was enough to start a massive development effort to equip their fighters with considerably more powerful armament. Out of these projects came a number of heavy 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. The idea was to build a missile with enough range to allow it to be fired from well outside the range of the bomber's guns, while being guided with enough accuracy to guarantee a "kill". The X-4 met these specifications and more; its BMW 109-448 rocket motor got the missile up to a speed of over 1152 km/h and kept it there during its "cruise", which lasted up to 4 km (although 1.5 to 3.5 km was more typical) while the defensive guns had a maximum effective range of about 1 km. The rocket burned a hypergolic mixture of S-Stoff ( nitric acid with 5% iron(3)chloride) and R-Stoff (an organic amine-mixture of 50% dimethylaminobenzene and 50% triethylamine called Tonka 250) as propellant, delivering 1.4 kN thrust. There was no room for a fuel pump, so instead the fuels were forced into the engine by pistons inside long tubes, the tubes being coiled to fit inside the airframe. S-Stoff was so aggressive that it dissolved all base metals and was difficult to handle, and it was planned to replace the engine with a solid fuel design as soon as possible.
The missile was stabilized by spinning it slowly in flight, at about 60 RPM. This meant that any asymmetrical thrust from the engine, or inaccuracies in the control surfaces, would be evened out as the missile spun. Signals were sent to the missile over two wires that were wound onto spools on the missile body, and corrected the direction of flight by operating control surfaces on the tail. A gyroscope kept track of "up" so that the control inputs from the pilot's joystick in the launch aircraft would be translated into yaw and pitch even as the missile rolled. Flares attached to two of the mid-section wings were used to keep the missile visible through the smoke of its engine.
The warhead consisted of a 20 kg fragmentation device that had a lethal radius of about 25 feet (8 m). It was thought that the guidance system would allow the pilot to get the missile into this range in terms of pitch and yaw, but at the ranges that the missile could operate at it would be almost impossible to judge range to anywhere near this accuracy. For this reason the missile mounted a proximity fuse known as Kranich, an acoustical system that was tuned to the sound of the B-17's engines in cruise. While approaching at high speed from the rear the Doppler effect would mean that the sound would be shifted to a higher frequency, but as the missile passed the bomber the shift would suddenly drop to zero and the warhead would be triggered.
The first flight test occurred on August 11August 11 is the 223rd day of the year (224th in leap years) in the Gregorian Calendar. There are 142 days remaining. Events 3114 BC beginning of our current era in the Maya Long Count Calendar 480 BC Persians under Xerxes defeat Spartans under King Leoni, 1944Events World War II January January 4 The Battle of Monte Cassino begins. January 5 Murder of Danish playwright Kaj Munck January 17 British forces, in Italy, cross the Garigliano River. January 20 The Royal Air Force drops 2,300 tons of bombs on Berlin; using a Focke-Wulf Fw 190The Focke-Wulf Fw 190 was a single-seater, single-engine fighter aircraft of the Luftwaffe. Used extensively during WW II from 1941 over 20,000 were manufactured including around 6,000 fighter-bomber models. Designed around the radial air-cooled BMW 139 e to as the launch platform. Subsequent tests used the Junkers Ju 88The Junkers Ju 88 was a WW2 Luftwaffe twin-engine multi-role aircraft. Among the most versatile planes of the war, it was used as a bomber, close-support aircraft, nightfighter, torpedo bomber and reconnaissance aircraft. A solid aircraft with great perfo and Messerschmitt Me 262The Messerschmitt Me 262 ''Schwalbe (Swallow) was the first operational jet powered aircraft. It was mass-produced in World War II and saw action from late 1944 in bomber/reconnaissance and fighter/interceptor roles. German pilots nicknamed it the Turbo,, although they were not launched from the latter. The X-4 had originally been intended for launch from single-seat fighters, but the problems in guiding both the missile and the aircraft at the same time proved this to be unworkable. Instead the X-4 was re-directed to multi-seat aircraft like the Ju 88, while the R4M rocketR4M rockets ''This is the starboard launcher on a Me 262 The R4M rocket Rakete 4 kg Minenkopf was the first practical anti-aircraft rocket, developed by the Luftwaffe during World War II, and used operational for a very brief time just prior to the end of was to be used on the single-seaters.
The X-4 was designed to be easily assembled by unskilled labour. By early 1945Events January January 5 The Soviet Union recognizes the new pro-Soviet government of Poland. January 7 British General Bernard Montgomery holds a press conference in which he claims credit for victory in the Battle of the Bulge. January 12 World War II:, Ruhrstahl's Brackwede factory had produced over 1000 airframes (the number 1300 is typical), and were waiting for the rocket motors when the factory that produced them (BMW's Stargard factory) was bombed. It is possible that some X-4s were used in the closing weeks of World War II, although it was never delivered to the Luftwaffe.