A N EJECTOR is a flow device in which the momentum of the primary flow is transferred to the secondary flow. The momentum transfer takes place as the primary flow is injected into the secondary flow that is stagnant or moving. Ejectors are classified as subsonic, transonic, and supersonic depending on the flow speed at the exit of the primary nozzle. Typically the flow channel of an ejector is axis symmetric. Depending on the configuration of the primary flow inlet, the ejectors are either a central injection type or an annular injection type. Primary flow is injected either annularly or centrally based on the nature of the secondary flow. Ejectors have many advantages over other devices in pumping fluids [1–9]. Although ejectors with central injection primary flow are widely used, annular injection of primary flow is indispensable to certain applications: pumping chemical lasers and high speed/high altitude test facilities. To pump a high power chemical laser, central injection of primary flow cannot be used, as the supply tubing of the primary flow is exposed in the stream of high temperature secondary flow [10]. For a test facility to simulate a high speed/high altitude environment, the central injection with protrusion of the primary supply into the secondary flow would cause significant loss in the momentum of the secondary flow. The starting behavior of a supersonic ejector with annular injection of primary flow is different from one with central injection [11,12]. In the present study, the starting behavior of a supersonic ejector with an annular injection of primary flow was investigated. To recover stagnation pressure, supersonic ejectors are generally equipped with a mixing tube downstream of the injection of the primary flow. A sequence of the starting operation of such a supersonic ejector with annular injection of primary flow is illustrated in Fig. 1. The plot begins at a point where both primary stagnation pressure and secondary static pressure are equal to ambient pressure. As the stagnation pressure of the primary flow injection increases, the secondary flow accelerates from a stagnant ambient state and its static pressure decreases as shown in region (1) of Fig. 1. In this region, the flow is supersonic only within the primary nozzle. The whole flowfield outside of the primary nozzle is still subsonic. As the primary pressure further increases, the shock wave moves out of the primary nozzle to form an oblique shock as in region (2) of Fig. 1. When the primary pressure increases beyond the starting point, the oblique shockwave is abruptly swallowed by themixing tube and the whole flowfield inside the ejector becomes supersonic; the ejector is started and the static pressure of the secondary flow drops abruptly in region (3). Once the ejector is started, the static pressure of the secondary flow is not sensitive to the variation of the stagnation pressure of the primary flow. The ejector maintains supersonic operation, even when the stagnation pressure of the primary flow decreases below the value at which the ejector started. The unstarting of the ejector occurswhen the stagnation pressure of the primaryflow is noticeably less than the starting pressure. The discrepancy between the starting and unstarting stagnation pressures of the primary flow is due to the hysteresis of the ejector operation. To take advantage of this hysteresis, the stagnation pressure of the primary flow is lowered to a value that is slightly higher than the unstarting pressure, once the ejector enters the supersonic operationmode [13,14]. In this way, the design requirement on the primary flow that drives the ejector can be reduced. In the present study, a new procedure to start a supersonic ejector is proposed to reduce the burden on the stagnation pressure of the primary flow further. The new starting procedure was tested and validated by varying the secondary inlet condition. It was discovered that a significantly less primary mass flow rate was required to start a supersonic ejector when the new starting procedure was used.