During a recent visit to the Krigsmuseet in Copenhagen, I came across something I didn’t expect to see: a World War 2 “X-4” wire-guided air-to-air missile. At first, I almost walked past it—the missile hangs from the ceiling just as you leave the kiosk at the start of the exhibition. I couldn’t find anything about this specimen online, so I asked the museum; they confirmed it’s authentic.
This X-4 is part of the special exhibition Game Changer? Drones in War (18 Nov 2024 – 18 Nov 2025), which traces the evolution of unmanned and remotely operated systems—from early concepts through the guided innovation of the X-4 to today’s nano-drones and AI-enabled weapons.
Work on the X-4 began in Germany in 1943 under Ruhrstahl engineer Max Kramer. The concept was ambitious: using this weapons, a single fighter would be able to attack Allied bomber formations from outside the reach of their defensive perimeter. Early tests were carried out in August 1944, with a single-seater Fw 190 plane.
In practice, however, the missile’s manual guidance made it almost impossible for a single pilot to both fly the aircraft and steer the weapon. It was therefore far better suited to two-seat aircraft, such as the Ju 88 or the Me 262B night fighter.
The missile carried a 20 kg fragmentation warhead intended to detonate near a bomber via the “Kranich” acoustic proximity fuse. Post-war Allied evaluations, however, were sceptical: with an effective triggering distance of about 15 meters, coupled with the modest warhead, it offered little destructive effect against heavy bombers, causing minimal, if any, serious damage.
Propulsion came from a BMW 109-548 liquid-fuel rocket engine, fuelled by hypergolic propellants— R-Stoff (Tonka-250) and S-Stoff (nitric acid). Guidance was manual: the pilot steered with a joystick while commands travelled down two thin wires unwinding to roughly 5.5–6 km. Unlike the radio-controlled Fritz-X, this wire-guidance system was immune to jamming, but it was no less demanding for the operator.
To balance strength and weight under wartime shortages, the X-4 combined an aluminium/steel fuselage with wooden mid-body wings and metal tail fins. On the Copenhagen specimen, the laminated wood surface of the wing is still visible through the paint.
Although around 1,000 rockets were produced in late 1944, Allied bombing of BMW’s rocket-motor facilities meant the missile never reached combat service. Yet the X-4 remains a remarkable technological precursor to post-war guided-weapon systems
Today, including the one in Copenhagen, only a handful of X-4s are known to survive, some of which were re-assembled from multiple loose parts:
- Deutsches Museum, Munich, Germany
- RAF Museum, Cosford, UK
- National Museum of the U.S. Air Force, Dayton, Ohio, US
- Smithsonian’s National Air and Space Museum, Chantilly, Virginia, US
One X-4 was also auctioned in August 2021, described as “an original example … made up of original and later restored parts,” and sold for $7,260.
Further Reading
- https://airandspace.si.edu/collection-objects/missile-air-air-ruhrstahl-x-4/nasm_A19510067000
- https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/196222/ruhrstahl-x-4-air-to-air-missile
- https://www.rafmuseum.org.uk/research/collections/ruhrstahl-kramer-x4/
- https://www.youtube.com/watch?v=zXan1fo6qi0
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