As early as 1942 the RLM was looking at more complex ways to develop ATA rockets. The 55mm R4M was chosen first as unguided and was used with the Me-262 and rocket fighter Me-163. Although successful under intense fire, only 12,000 were ever made and were used late in the war. The world's first guided AAM was proposed by Dr. Kramer (of Fritz-X fame) as the X-4 which used a BMW rocket engine burning a combo of Salbeik and Tonka 250. The missile carried a 20 kg HE charge and had 4 wings and 4 tail- fins. Guidance was achieved through a command line wire of 5500 m length that unspooled when the weapon was fired. Systems Dusseldorf radio and Detmond receiver helped guide it and once it was near a target the Dogge system locked-on. Then, the Miese acoustic sensor switched on during the last 7 m- listening for the enemy engines and detonated within range. Test firings and some claimed actual war use came from Fw-190F-8 aircraft. But because the Reich was shrinking under enemy troops and armor, the delivery of the X-4's critical systems for guidance could not be guaranteed and so with hundreds stockpiled, the X-4 was cancelled. Another AAM, the Hs-298 was also stockpiled with 450 of them sitting around unused. That was a larger AAM meant to be carried on multiple rails from Dornier and Junkers attackers. From the X-4 came the X-7 AT missile that was used against the Red Army near Stolberg.