A simple analytical model is developed for optimum ejector steady-state performance under the assumption that the driver gas mass flow rate exceeds the suction side mass flow rate. This requires the mixing to be at constant pressure and the suction side inlet Mach number to be small. Since the model is formulated in terms of enthalpy, it is applicable to arbitrary motive and suction gases. For purposes of verification, the model is compared with exact numerical results, and with the experimentally determined entrainment ratio used to correct the suction side mass flow rate for changes in temperature or molecular weight. The effect on ejector performance of the pertinent dimensionless parameters is considered. For example, it is shown that the use of a motive fluid with a ratio of specific heats near unity is detrimental to performance. I. Introduction O VER the past few decades there have been many fluid mechanic studies of ejector pumps. Currently, there is renewed interest in ejectors for pumping the subatmospheric effluent of cw chemical lasers,1-4 with primary emphasis on optimized ejector performance. Since an ejector is a pump, performance is in terms of a compression ratio, in particular, the ratio of the exhaust gas stagnation pressure divided by the stagnature pressure of the inlet suction gas. (The suction gas is referred to as the secondary flow; the motive gas is referred to as the primary flow.) Optimum performance is defined here as the largest compression ratio for given primary and secondary flow rates and for a given secondary stagnation pressure. An objective of this work is to derive simple, approximate formulas for the optimum compression ratio in terms of a minimum number of key dimensionless parameters. This approach provides a simple estimate of the maximum possible compression ratio, given the primary and secondary fluids, their stagnation conditions, and their flow rates. Conversely, it does not determine such fundamental characteristics as the transition1 between the mixed and super sonic regimes. Some generality is retained in the choice of parameters for the thermodynamic properties of both primary and secondary gases. This is important for laser application where the-composition of both gases often differs considerably from steam or air. Because of its analytical simplicity, the derivation provides useful insight into the dominant process in an ejector, e.g., a natural consequence of the analysis is an ejector figure of merit. The assumptions underlying the derivation, the derivation itself, and its method of application are presented in Sec. II. The effect on ejector performance of changes in primary gas stagnation enthalpy, ejector nozzle Mach number, etc., are discussed in Sec. III. Of particular interest are the sections which show that a value near unity for the ratio of specific heats of the primary gas leads to a small compression ratio, and that a low subsonic Mach number is required for the secondary inlet flow for optimum compression. To establish the validity of the analysis, a comparison is provided in Sec. Ill with exact computer solutions for a steam and air ejector pumping a chemical laser flow. A simple derivation is also given for the entrainment ratio used by the Heat Exchange In