As early as 1943 it was becoming clear that Messerschmitt's super-interceptor, the Me 163, would be difficult to use in combat. After flying to the 25,000 ft altitude of the US bombers, it had only a few minutes to track them down before running out of fuel. Even if it did manage to find the bombers, it had the equally prickly problem of lacking a weapon that could be aimed effectively while closing on the target at some 400 mph difference in speeds.