Ruhrstahl X-4

Ruhrstahl X-4

The Ruhrstahl 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. The X-4 was the basis for the development of experimental, ground launched anti-tank missiles that became the basis for considerable post-war work around the world, including for example the Malkara missile.


During 1943, the USAAF's 8th Air Force mounted a series of heavy raids against Germany that, while resulting in disastrous bomber losses, made the Luftwaffe research for 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 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 bombers' guns (what we call now 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 got the missile up to a speed of over 1,152 kilometres per hour and kept it there during its "cruise", which lasted between 1.5 and 4 kilometres, while the defensive guns had a maximum effective range of about 1 kilometre. 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 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 corrosive, that it dissolved all base metals and was extremely difficult and dangerous to handle. The Germans 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 fuze 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 11, 1944 using a Focke-Wulf Fw 190 to as the launch platform. Subsequent tests used the Junkers Ju 88 and Messerschmitt Me 262, 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 rocket was to be used on the single-seaters.

The X-4 was designed to be easily assembled by unskilled labour. By early 1945, Ruhrstahl's Brackwede factory had produced over 1,000 airframes (the number 1,300 is typically quoted), 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.

After the war, French engineers tried to develop a domestic version of the X-4, called AA-10. 200 units were manufactured in series between 1947 and 1950. However, the program was disbanded due to the dangerous pre-flight refuelling that was involved (the nitric acid and Tonka combination was highly explosive).


X-4 air-to-air missile

* Primary Function: short-range air-to-air missile
* Power Plant: BMW 109-448 liquid rocket motor giving 16 kN thrust for 33 s

* Length: 2.01 m
* Diameter: 0.22 m (max)
* Wing Span: 0.726 m
* Launch Weight: 60 kg

* Speed: 325 m/s
* Warhead: 20 kg
* Range: 1.5 - 3.5 km
* Fuzes: Kranich acoustic proximity fuze, tuned to the pitch of a bomber's propellers
* Guidance system: FuG 510/238 "Düsseldorf/Detmold" MCLOS visual guidance with wire control

* Unit Cost:
* Date Deployed: never

X-7 anti-tank missile

* Primary Function: anti-tank guided missile
* Power Plant: solid rocket motor

* Length:
* Diameter:
* Wing Span: 0.726 m
* Launch Weight: 9 kg

* Speed: 245 m/s
* Warhead: 2.5 kg hollow charge.
* Range: 1,000 m
* Fuzes: impact (?)
* Guidance system: MCLOS visual guidance with wire control

* Unit Cost:
* Date Deployed: never

See also

* List of missiles
* List of World War II guided missiles of Germany

External links

* [ Ruhrstahl Kramer X4 - Royal Air Force Museum, Cosford (UK)]
* [ Luft '46]

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