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First, prototype engines are tested in a Rocket engine test facility, where the most promising designs are refined during a period of extensive testing. After an engine has been proven, the complete rocket is assembled. In this second step, the rocket is anchored to a static test stand. With the rocket held down, engineers run the engine at full power and refine the system. The test launch is third, when the missile is fired into the sky. Wernher von Braun and his team used this process to develop the V-2 or A4 missile in Germany during WWII.