Ruhrstahl X-4 - History

History

During 1943, the USAAF's Eighth Air Force mounted a series of heavy raids against Germany. Despite disastrous 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 to 75mm autocannon designs (often known as Bordkanonen), 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) and kept it 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.

The missile was spin-stabilized at about 60 rpm. Thus any asymmetrical thrust from the engine or inaccuracies in the control surfaces would be evened out. Signals to operate control surfaces on the tail were sent via two wires, which unwound from bobbins housed within long, bullet-shaped fairings, themselves mounted either on the roots of an opposing pair of the larger mid-body fins, or on those same fins' opposing tips. A gyroscope kept track of "up" so that the control inputs from the pilot's joystick in the launch aircraft could be translated into yaw and pitch as the missile spun. Flares attached to two of the midsection wings were used to keep the missile visible through the smoke of its motor.

The warhead consisted of a 20 kg (45 lb) 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 200 Hz sound of the B-17's engines in cruise. The trigger range was 7 m (23 ft).

The first flight test occurred on August 11, 1944 using a Focke-Wulf Fw 190 for 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 use by 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. It is possible some X-4s were used in the closing weeks of World War II, although it was never delivered to the Luftwaffe. The fighter-interceptor designed to use this missile as its primary weapon was the Focke-Wulf Ta 183 Huckebein, which was just a "paper" and wind tunnel prototype.

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