Early testing is crucial for reducing development costs of cyber-physical systems, such as flight control computers. However, real flight conditions must be simulated to test a flight control computer in the laboratory. In this example, engineers use a dSPACE SCALEXIO hardware-in-the-loop system for real-time preflight simulation to cover the entire mission of a high-altitude research rocket. The objective is to verify the correct behavior of the navigation computer on the ground. In the first step, the HIL simulator is connected only to the navigation computer, because the navigation devices are not available yet. Therefore, they are simulated on the SCALEXIO system together with the flight sequence and the navigation signals. Later, real navigation devices replace the virtual equipment. In this case, the SCALEXIO system continues to simulate the required environment, thus enabling the real navigation devices to provide sensor data directly to the navigation computer.