In the present study, performance characteristics of a high altitude test facility for testing large area ratio rocket engines have been investigated. Steady-state numerical simulations have been performed initially to highlight the effects of operational parameters on a high altitude test facility operation. Later, the performance of the test facility during the startup phase of the rocket motor has been analyzed. The predicted results show that during the initial high altitude test facility evacuation, the desired vacuum level is attained when the primary jet flow attaches to the ejector duct walls smoothly, thereby arresting any back flow. However, at the fully started condition of the motor, the selfejector action of the rocket plume plays a major role in maintaining the desired vacuum condition and, hence, the ejector flow rate can be reduced significantly. The injection of water as a fine spray cools the hot gas to a sufficiently low temperature (∼600 K) prior to its release into the atmosphere. The transient predictions capture dynamic phenomena, such as the initial back flow of exhaust gas, and the attainment of self-ejection capability by rocket exhaust in the later stage. The predicted results compare well with in-house experimental data measured during cold and hot flow tests. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.