The purpose of this research is to evaluate a ground test bed of an orbital transmission engine with pre-evacuation of the engine's INTERNAL space. In the usual tests on the ground, the initial pressure of the engine is atmospheric pressure. While during the orbital mission, the INTERNAL space of the engine may be in the vacuum pressure. Therefore, to ensure the proper performance of INTERNAL BALLISTICS, it is necessary to test the performance by pre-evacuating its INTERNAL space. In this research, first, the suitability of an exhaust diffuser for this type of test is investigated numerically. Then, the unsteady numerical SIMULATIONs have been done by applying the pressure-time profiles of the engine as the boundary condition of the inlet pressure. Investigations show that the two phenomena of flow being supersonic in the diffuser at very low engine pressures and the discharge of the return flow to the vacuum chamber prevent the significant influence of environmental conditions on the flow inside the nozzle. So, from the initial moment to the stable working of the diffuser, the flow in the first half of the nozzle is in the supersonic state. Therefore, the INTERNAL BALLISTICS of the engine is evaluated independently of the conditions of the outside environment.