Ruhrstahl/Kramer X-4
          In early 1943, work began on the X-4 air-to-air wire-guided missile by Dr. Kramer at Ruhrstahl. The missile received a development order in the summer of 1943 and was given the number 8-344 by the RLM, and was developed to give fighters a chance to down the ever increasing number of Allied bombers from outside of their defensive gun range.
          The X-4 featured a tapering, cigar-shaped fuselage, with four small swept wings and four smaller tail fins. At the ends of two of the opposing wings were small pods which held the wires that unwound during the X-4's flight. On the wing tips of the other two main wings were simple flares to aid the pilot in keeping the X-4 on it's intended path. The tail unit contained small spoilers which could control the missiles pitch and yaw. Power was supplied by the BMW 109-548 rocket engine. The fuel was held in a cleverly designed spiral fuel tank (to save space). A piston was fitted into each coiled fuel tank, and the fuel was pushed into the combustion chamber at the rear of the missile by discharging compressed air to force the piston against the fuel. The two fuels (R-Stoff or Tonka and SV-Stoff or Salbei) were hypergolic, which means they ignited upon contact with each other. A warhead weighing 20 kg (44.1 lbs) with a destructive blast radius of 7.6 meters (25 feet) was mounted in the nose of the missile, being detonated by the pilot, impact or by an acoustical proximity fuse, tuned to the pitch of the bomber's propellers.
          A typical flight would have the carrier aircraft reaching the same altitude or slightly higher than the target. The X-4 would be released from the ETC 70 or 71 bomb rack, and would spin at approximately one revolution per second (this was achieved by having the wings slightly offset from the missile center line), the gyro being used mainly for  line of flight only. The pilot can then steer the missile (FuG 510/238 "Düsseldorf/Detmold" system) by the use of a small joystick in the cockpit. Seven seconds after launching, the acoustic proximity and impact fuses are armed. A self-destruction fuse is also actuated about 30 seconds after launch. The Kranich acoustical fuse (tuned to the pitch of the bombers propellers) would activate the firing mechanism within 40 meters (131 feet), with an additional slight delay allowed for the distance to close to within five meters (16 feet) before detonation of the warhead. A maximum speed of 1152 km/h (716 mph) could be reached; the range of attack was to be between 1.5 km and 3.5 km (.93 mile and 2.2 miles), although there was 5.5 km (3.4 miles) of wire on the spools.
          By August 1944, 225 prototype X-4s had been completed, with the first air launched test occurring on August 11, 1944 by an Fw 190. Tests continued through early February 1945, also by Ju 88s. Test flights were also undertaken by a Me 262 jet fighter with two X-4 missiles under the wings outboard of the jet nacelle, but were not launched.
          The production of the X-4 was simple, the fuselage being made up in three sections; a turned steel nose which contained the warhead, a cast aluminum center section and a tail section made from sheet aluminum. The design of the missile was set up so that unskilled labor could assemble the missiles, indeed, the sheet metal sections assembled by tabs in one section being pushed into slots of another section. The plywood wings were secured to the missile's center section with simple nuts and bolts. Approximately 1000-1300 airframes had been finished at Ruhrstahl's Brackwede factory by early 1945, and were awaiting their rocket motors, when the BMW facility at Stargard was bombed, destroying all the finished BMW 109-548 rocket engines. This missile was intended to be issued in numbers to the Luftwaffe by the early Spring of 1945, but the bombing of the BMW rocket engine factory, and the war's end prevented the combat use of the world's first guided air-to-air missile.
Ruhrstahl/Kramer X-4 Dimensions
(across wings)
Length Height 
(across wings)
Max Diameter 
.575 m 
1' 11"
1.907 m 
6' 3"
.575 m 
1' 11"
.222 m 
Ruhrstahl/Kramer X-4 Weights
Operational R-Stoff RV-Stoff Warhead Engine Wings Bobbin & Wire Battery Gyro & Relays
60 kg 
132 lbs
1.7 kg 
3.75 lbs
6.3 kg 
13.9 lbs
20 kg 
44 lbs
25 kg 
55.1 lbs
1.5 kg 
3.3 lbs
5 kg 
11 lbs
1 kg 
2.2 lbs
1.5 kg 
3.3 lbs
Ruhrstahl/Kramer X-4 Performance
Max Speed Thrust Duration Acceleration Max Range Glide Ratio
1152 km/h @ 6500 meters 
716 mph @ 21325'
1600 kg 33 sec. 3.5 G 5.5 km 
3.4 miles
1:5 - 1:6
Ruhrstahl/Kramer X-4 Models
Manufacturer Scale Material Notes
Airmodel 1/72 Resin w/launch racks, included in Luftwaffe Weapons Set # 1
DML/Dragon 1/72 Injected w/launch racks, included in Me P.1101 kit
RS 1/72 Resin included in BV Ae 607 & Me P.1106
Resin Master 1/72 Resin w/launch racks 
Trimaster 1/48 Injected w/launch racks, seperate acoustic or impact noses, 
included in Fw 190F-8 kit

Ruhrstahl/Kramer X-4 exploded view
Assembly Procedure of the 8-344 (X-4) air-to-air wire-guided missile

Left: The Kranich Acoustical Fuse.

The Plastic Diaphragm is centered in the missile, and is connected by means of a lever arrangement to a needle-like pointer whose mass is much smaller than that of the diaphragm. Two opposing openings (covered by a fine mesh screen) in the missiles body allow sound to enter to the diaphragm, and when the missile passes through the the transition region of the Doppler frequency shift, the vibrations of the diaphragm excite resonances in the needle, the needle touches an electric contact, completes the circuit and fires the igniter.



      Note: the X-4 could be fired at
                targets up to 56° away
      Abwurfbereich = Release Range
      H - 6000 m = Height of 19685 feet
      Vo - 180 m/s = Starting Speed of
                              648 km/h (402 mph)


A wartime photo of the Ruhrstahr/Kramer 8-344 (X-4)
X-4 Details 
    A wingtip bobbin is exposed, revealing the wire spool 
    One of the coiled fuel tanks, split open to see the piston 
A US soldier examines the rear housing of an X-4 


     X-4 center sections (complete w/BMW 548 rocket engines) 
found at the Hovelhofer facility 
More component close-up photos from Ray Corridon's webpage
Three stills from a filmed rocket engine bench test
 Fw 190A-8, November 1944, during testing
A closeup photo of the small joystick used 
to control the X-4 in flight. An elbow rest was 
also provided for the pilot on the starboard side 
of the cockpit to allow better control and comfort 
while guiding the X-4.
The X-4 mounted under a Ju 88 during tests at Karlshagen, August 1944
The X-4 is released, the rocket fires and disappears in the distance in these stills from a filmed test launch
The X-4 Today
Cosford Royal Air Force Museum in Shropshire, England 
Photo by Graham Causer, May 1998 
NASM Paul E. Garber Facility 
Photo by Dan Johnson, 1993 
Cosford Aerospace Museum, Shropshire, Great Britain 
Photo by Matthew Hopson, June 1985 
U.S. Airforce Museum, Dayton, Ohio, USA 
Photo by Rick Geithmann