The German X-4 was a small air-to-air missile of World War II that could be fired at heavily armed Allied bombers from a distance. To prevent jamming, guidance was by wires running between the missile and launch aircraft, rather than by radio. Slated for use on the Me 262 jet fighter, the X-4 could also have been fired from such piston-engine aircraft as the Ju 88, Ju 388, and Fw 190, all of which launched test missiles beginning in August 1944.
A BMW 109-548 liquid-fuel rocket engine powered the missile. Ruhrstahl produced 1,000 X-4 airframes in late 1944, but an Allied air raid destroyed the BMW engines and production lines, a blow from which the program never recovered. The Smithsonian obtained this missle from the U.S. Navy in 1948.
The X-4 was a small wire-guided, air-to-air missile designed as a standoff weapon against heavily armed Allied bombers. It was slated for use with the Me 262 jet fighter, but could also have been fired from a number of piston-engine aircraft such as the Ju 88, Ju 388 and Fw 190, all of which launched test missiles beginning in August 1944. The missile was powered by a BMW 109-548 liquid-fuel rocket motor with a thrust of 140 kg declining to 30 kg, and used Tonka 250 (a hydrocarbon fuel) and SV-Stoff (nitric acid with additives) as propellants. In the latter half of 1944, 1000 airframes were made by Ruhrstahl, but the BMW motors and production lines were destroyed in an Allied air raid--a blow from which the program never recovered. This artifact was given to the Smithsonian by the U.S. Navy in October 1948.
The X-4 has a small, ogival body made of aluminum, with four wooden mid-body wings and four metal tail fins set at 45 degrees to the wings. Trim tabs on the wings rolled the missile slowly at about 1-1.5 revolutions per second (rps) for stabilization. Control was effected by the four spoilers on the tail fins, commanded by the pilot of the launch aircraft, who used a joystick to keep the missile heading directly for the target. Launch would take place on a level altitude with the target and preferably astern. The spoilers vibrated at a velocity of 5-20 rps (the secondary sources used in this essay vary), and commands altered the rate of movement of each of the two pairs of spoilers. A single gyro and a commutator sent the commands to the right pair of spoilers on the spinning missile to pitch it up or down, or yaw it right or left, according to the pilot input. The gyro was spun up before launch by electric power from the aircraft, but was not powered in flight. The wire bobbins for the command wires were contained within two ogival fairings on two of the four wings; the pilot's commands were sent by a Duesseldorf (FuG 510) transmitter and received by Detmold (FuG 238) receiver in the missile. There was 5500-6000 meters of wire, i.e., the maximum controlled range would have been 6 km (3.7 miles).
Two unique spiral tanks contained the Salbei, which was nitric acid with additives, and Tonka 250, a mixture usually of 57% m-xylidine and 43% triethylamine. The outer tank contained the Salbei, the inner one the Tonka 250. The propellants were expelled with pistons that travelled down the spiral tubes pushed by compressed air from bottles that were opened when small charges broke membranes. The propellants were hypergolic, that is, they ignited on contact in the combustion chamber.
The nose has a 270 mm acoustic proximity fuze extension connected to a 450 mm steel or plastic-case warhead, which was to contain 20 kg (44 lb.) of high explosive. The "Kranich" or "Meise" acoustic proximity fuzes, for the detection of the engine and propeller noise of Anglo-American four-engine bombers, were to be fitted to operational missiles. There was also to be an impact, a graze, and a self-destruct fuze; the latter would operate after 35 seconds. The warhead would have been very effective at ranges of 6-7 m (c. 20 ft), but not very effective at 15 m (50 ft.). Some experimental work was done on an acoustic homing system to improve effectiveness.
The X-series of Ruhrstahl missiles began as a result of the work of Dr. Max Kramer of the German aeronautical establishment DVL (Deutsche Versuchanstalt) in Berlin-Adlershof in the late 1930s. Dispensing with the usual aircraft-like controls for guided weapons, he developed the use of spoilers as a control method. In 1940, the German Air Ministry (RLM) became interested in the method, leading to the Fritz-X or X-1 bomb, with Ruhrstahl (Ruhr Steel) in Brackwede near Bielefield as the contractor. The Fritz-X was deployed with great effectiveness against Allied shipping in the Mediterranean theater in late summer 1943.
Meanwhile, the deteriorating air defense situation over Germany as a result of the Allied bomber offensive belatedly spurred the RLM in 1942 to push the development of ground-based anti-aircraft missiles, and aircraft-launched rockets and guided missiles. Kramer began work on a small air-to-air missile that Ruhrstahl designated X-4. In early 1943 the project received the RLM number 8-344, and the rocket engine assigned to the project was the 109-548, a motor already under development by the rocket division of engineer Helmut v. Zborowski at BMW in Berlin-Spandau. This propulsion system, with its unique spiral tanks, was capable of sustaining thrust even during violent maneuvers. With swept-back wings, and higher performance than the Henschel Hs 298 air-to-air missile, the X-4 would be more suitable for high-performance jet fighters like the Messerschmitt Me 262, its primary intended launch platform.
Development was delayed by problems when early versions of the rocket motor did not work or had low thrust. One report indicates that the first flight tests used a Schmidding solid-fuel dyglycol motor as a substitute. One source asserts that the first air launch was made on 11 August 1944 by a Focke-Wulf Fw 190 at Karlshagen (Peenemünde), and test firings continued until early 1945, also on Junkers Ju 88s and Ju 388s. The destruction of the BMW's Stargard plant in early 1945 severely upset the production program so that no X-4s were deployed, although it is possible that a few were fired at Allied aircraft in tests in the last phase of the war. The missile was influential after the war, as Allied air forces in East and West began to look more seriously at the air-to-air missile as a weapon of the future.
Nothing is presently known about how the Smithsonian's artifact was acquired by the Navy, but it was shipped to the Smithsonian by the U.S. Naval Air Development Station in Johnsville, Pennyslvania, in October 1948.
Bill Gunston. The Illustrated Encyclopedia of the World's Rockets & Missiles (New York: Crescent Books, 1979), p. 212-13.
Fritz Hahn. Deutsche Geheimwaffen 1939-1945: Flugzeugbewaffnungen. Heidenheim: Erich Hoffmann, 1963.
United States Navy, Bureau of Ordnance. German Explosive Ordnance, OP 1666 (11 June 1946), 214-219.
J. R. Smith and Anthony L. Kay, German Aircraft of the Second World War (London: Putnam, 1972), pp. 700-702.