The Ruhrstahl X-4 was developed during World War II, with work beginning in June 1943. During the development of the Ruhrstahl X-1, an air-to-surface glide bomb, Dr Max Kramer began working on an air-to-air missile in 1943.
The objective was to inflict significant damage on Allied bomber formations while minimizing the risk to Luftwaffe pilots. The strategy was to launch the air-to-air missiles from a safe distance, beyond the effective range of the Allied bombers’ machine guns. To circumvent the Allies’ radio jamming capabilities, a wire-guided system was selected for the missile.
Like other Luftwaffe guided missiles or glide bombs, this air-to-air missile utilized a manual command to line of sight (MCLOS) guidance system. The system incorporated a FuG 510 “Düsseldorf” to FuG 238 “Detmold” setup, which included a joystick control in the cockpit for the pilot to guide the missile.
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Dr. Max Kramer
In 1943, the RAF’s Bomber Command and the US Air Force launched a series of intense bombing raids on Germany. Despite suffering significant losses in their bomber fleets, these attacks compelled the Luftwaffe to seek advanced anti-bomber weapons to minimize the loss of their fighter aircraft and aircrew.
This led to an extensive development program that produced various high-caliber autocannon designs, air-to-air rockets, surface-to-air missiles (SAMs), and the X-4.
Development of the X-4 missile commenced in June 1943 under the leadership of Dr. Max Kramer at Ruhrstahl AG. The objective was to create a missile that could be launched from a distance beyond the effective range of the bombers’ defensive guns, essentially making it a stand-off weapon.
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The defensive guns on bombers had an effective range of approximately 1,000 meters (3,300 feet). The missile was designed to be accurate enough to ensure a direct hit. The X-4 not only met these requirements but exceeded them.
Powered by a BMW 109-448 rocket motor, the missile could reach speeds over 1,150 km/h (710 mph) and maintain this velocity during its cruise phase, which ranged between 1.5 and 4 kilometers (0.9 and 2.5 miles).
Ruhrstahl X-4 Cruciform Wings
The X-4 missile featured aerodynamically efficient cruciform wings, which were swept back to minimize drag when mounted beneath a jet fighter. Control of the missile was achieved through a combination of adjustable tabs on the wings and extra control surfaces on the tail fins.
The missile was propelled by a BMW 548 motor, utilizing hypergolic propellants supplied by a sophisticated fuel delivery system. This system was specially designed to prevent fuel starvation during rapid and aggressive maneuvers.
Launched from the same altitude as the target, typically from behind, the X4 needed to be over 1.5 kilometers (0.93 miles) away to be effective. After launching the missile, the pilot would keep the target and missile aligned, using a small joystick within the command link control system.
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The control wires for this system were dispensed from reels located on two of the missile’s four wings. There were plans to equip the X4 with acoustic homing and a fusing system for its 20 kg (40 lb) explosive payload, although most missiles were ultimately fitted with a proximity fuse.
The rocket of the missile utilized a hypergolic propellant composed of S-Stoff, which was nitric acid with a 5% concentration of iron chloride, and R-Stoff, a mixture of organic amines comprising 50% dimethylaminobenzene and 50% triethylamine, known as Tonka 250.
The Ruhrstahl X-4 Bullet-Shaped Fairings
This combination provided an initial thrust of 140 kg (310 lb), which gradually decreased to 30 kg (66 lb) over a 17-second burn duration. Due to space constraints within the missile, there was no room for a traditional fuel pump. Instead, the propellants were forced into the motor by pistons within lengthy, coiled tubes designed to fit inside the missile’s airframe.
Handling the S-Stoff was particularly challenging and hazardous, as it was highly corrosive, capable of dissolving base metals. Plans were in place to replace this motor with a solid fuel design as soon as feasible.
The missile was stabilized through spin at approximately 60 rotations per minute, or one rotation per second. This spin stabilization ensured that any asymmetrical thrust from the engine or inaccuracies in the control surfaces were compensated for. Control signals for the tail surfaces were transmitted via two wires, a choice made to circumvent radio jamming.
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These wires unspooled from bobbins located in long, bullet-shaped fairings mounted on the roots of an opposing pair of the larger mid-body fins (swept at 45°), or on the tips of one pair of these fins. In total, these fairings contained about 5.5 kilometers (3.4 miles) of wire.
The wires of the Ruhrstahl X-4 were operated by a joystick in the cockpit, and a gyroscope was employed to maintain orientation, translating the pilot’s joystick movements into yaw and pitch adjustments as the missile spun. To maintain visibility of the missile through the smoke of its motor, flares were attached to two of the midsection wings.
Focke-Wulf Fw 190
The warhead of the missile was a 20 kg (44 lb) fragmentation device, possessing a lethal radius of about 8 meters (26 feet). The guidance system was designed to enable the pilot to align the missile within this range in terms of pitch and yaw.
However, given the operational distances at which the missile functioned, accurately estimating the range to the target bomber with such precision was nearly impossible. Consequently, the missile was equipped with a proximity fuze named Kranich (Crane), which utilized an acoustical system.
This system was tuned to the 200 Hz sound frequency of the B-17 bombers’ engines during cruising, and activation was triggered by the Doppler shift as the missile approached, with a trigger range of seven meters (23 feet).
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The first flight test of the missile took place on August 11, 1944, using a Focke-Wulf Fw 190 as the launch platform. Later tests involved the Junkers Ju 88 and Messerschmitt Me 262, although launches were not conducted from the latter.
Initially intended for single-seat fighters, including the Me 262 and possibly the Dornier Do 335, the X-4’s complexity in simultaneous guidance of the missile and the aircraft proved too challenging. As a result, it was reassigned for use with multi-seat aircraft like the Ju 88, while the unguided R4M rocket was allocated for single-seaters.
Designed for simple assembly by untrained labor, production of the X-4’s airframe commenced in early 1945, using low-cost, non-strategic materials such as wood for the fins. The production faced setbacks due to Allied bombings targeting the BMW rocket engine factory in Stargard.
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Although as many as 1,000 units of the X-4 may have been completed, it was never officially delivered to the Luftwaffe. The fighter-interceptor intended to primarily deploy this missile, the Focke-Wulf Ta 183 Huckebein, never progressed beyond the project stage.
Post-War
Post-war, French engineers attempted to develop a domestic version of the X-4, named Nord SS.10. Between 1947 and 1950, around 200 units were manufactured.
In the latter part of 1955, Israel placed an order for 36 SS.10 missile launchers, which were delivered in the following year. However, their arrival was too late for use by the Israel Defense Forces (IDF) in the 1956 Suez Crisis. Later, a self-propelled version was developed, featuring four launchers mounted on a Dodge truck. By the early 1960s, the IDF had phased out the SS.10 missiles, replacing them with the SS.11.
The US Army showed early interest in the missile but initially focused on developing their own, known as the SSM-A-23 Dart missile. After the cancellation of the SSM-A-23 in 1958, the US Army began to seriously consider the SS.10 and SS.11 missiles. In February 1959, they opted to procure the SS.10 as an interim solution. The missile was delivered in January 1960 and remained in service until 1963, when it was superseded by the MGM-32 Entac. During its service with the US Army, the missile was designated as the MGM-21A.
The Ruhrstahl X-4 laid the groundwork for contemporary anti-tank missiles, many of which adopted wire guidance systems for their dependability and resistance to countermeasures.
