In the present work, a second throat ejector using nitrogen as the primary fluid is considered for the creation of a low vacuum in a high-altitude testing facility for large-area-ratio rocket motors. Detailed numerical investigations have been carried out to evaluate the performance of the ejector for various operational conditions and geometric parameters during the nonpumping and pumping modesof operation.In the nonpumping mode, the lowest vacuum chamber pressure is attained when the primary jet just expands up to the mixer throat and the resulting single shock cell seals the throat against any backflow. The study illustrates how each geometric and operational parameter of the ejector can be optimized to meet the test requirements in a high-altitude testing facility by ensuring that the primary jet completely expands without a strong impact on the duct wall. When the rocket motor is fully started, due to the self-pumping action, the required vacuum is almost maintained by the exhaust flow itself and the external nitrogen ejector plays only a supplementary role. Numerical predictions for both nonpumping and pumping modes of operation have been validated with experimental data obtained from a scaled-down model of a high-altitude testing facility.