http://en.wikipedia.org/wiki/V-2_rocket#Technical_de...

"The V-2 was guided by a gyroscopic inertial navigation system controlling four external rudders on the tail fins, and four internal rudders, made of graphite, at the exit of the motor. The LEV-3 guidance system consisted of two free gyroscopes (a horizon and a verticant) for lateral stabilization, and a gyroscopic accelerometer connected to an electrolytic integrator (engine cut-off occurred when a thin coating of silver was electrochemically eroded off of a poorly conducting base). Some later V-2s used "guide beams" (i.e. radio signals transmitted from the ground), to navigate the missile toward its target, but the first models used a simple analog computer that would adjust the azimuth for the rocket, and the flying distance was controlled by the moment of engine cut-off,"Brennschluss", ground controlled by a Doppler system or by different types of on-board integrating accelerometers. The rocket would stop accelerating and soon reach the top of the (approximately parabolic) flight curve."