High altitude simulation on a ground testing facility is carried out for studying various aspects such as the vacuum ignition characteristics, flow pattern in a large expansion ratio nozzle, on- off characteristics of an engine, thermal characteristics of the nozzle/thrust chamber and the thrust variation of a rocket engine. This paper demonstrates the variation of pressure inside the vacuum chamber during the testing of large expansion ratio rocket engine in a simulated high altitude environment. When large expansion ratio rocket engines are tested at sea level conditions (ambient pressure), the boundary layer flow does not have sufficient momentum to overcome the resistance offered by the nozzle wall. Hence, the flow separates in the divergent portion of the nozzle. In such a situation, the design thrust of the rocket engine cannot be realized. In order to realize the full thrust of the engine, the low pressure value (about 40 milli bar) corresponding to the flight environment has to be created in the ground testing facility. Therefore, in the current study, a transient numerical analysis has been conducted to investigate the variation of pressure in the vicinity of a large expansion ratio rocket engine. The rocket nozzle coupled with a vacuum chamber and a second throat ejectordiffuser system has been considered for the computational analysis. Simulations have been carried out from no flow to full flow condition of the rocket engine. The static pressure variation inside the vacuum chamber, expansion of exhaust flow inside the nozzle, flow pattern, shock location, and shock structure are investigated in detail. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.