Rurhstahl X-4 Guided Missile

The Rurhstahl X-4 was a German air-to-air missile which began development in 1943.  The Luftwaffe sought a missile capable of downing American B-17s while keeping the launching aircraft safely beyond the range of the bombers’ defensive fire.  The missile was wire-guided, and had a maximum range of 2.2 miles (3.5 km) and a maximum speed of 708 mph (1,124 kph).  It carried a 44 pound (20 kg) fragmentation warhead.  This missile is on display at the National Museum of the United States Air Force in Dayton, Ohio.
The weapon was armed seven seconds after launch, and self-destructed thirty seconds after launch.  The warhead was detonated upon contact or by a unique acoustical proximity fuse housed in the nose.  The fuse was tuned to the sound of the engines of a B-17, and actuated when the target entered lethal range.
The missile was controlled by inputs from the launching aircraft using wire guidance.  Bobbins of wire were housed at the tips of two of the missiles fins, the other two housed flares so the controller could track the missile visually.  Since it was not radio guided it was not susceptible to jamming.  The missile was stabilized by rotating, spinning at the rate of one revolution per second.  Control inputs were normalized using a gyroscope.  A Joystick was used, but directing the missile while simultaneously piloting an aircraft proved difficult.
Propulsion was via a liquid-fueled rocket motor using S-Stoff and R-Stoff.  Fuel was fed to the combustion chamber by forcing pistons through coiled reservoirs.  The fuels were volatile and extremely corrosive but solid-fueled rocket motors were not available.  Motor production ended after the BMW factory at Stargard was destroyed by Allied bombing, which effectively ended any possibility of the X-4 being deployed in numbers.
The X-4 was launched from Ju 88G-1, Ju 388, and Fw 190 aircraft during the testing program, and was expected to be deployed operationally using the Me 262.  The first air launch was from a Fw 190 on 11AUG44.  Here two X-4s are seen under the wings of a Focke-Wulf Fw 190.
The X-4 was never used operationally as an air-to-air weapon.  After the war several nations developed similar weapons for the anti-tank role.  Wire-guided anti tank missiles are still in use today.