In Part I of this series of papers, the development was described of a kW ion thruster for use as one of a set of primary propulsion units to be used for executing an orbit transfer manoeuvre. Both during the expansion phase, and when the final orbit is attained, micro-thrusters are necessary to provide attitude and position control of the satellite. This paper will describe two types of micro-thruster, each of which has particular merits and areas of use, so that the choice of thruster for a particular mission depends upon a fairly detailed specification of the mission. This consideration and broader system concepts will be dealt with in the final paper of this series (Part III).

The complicated time-dependent flow in the wake of a blunt based body at low speeds is a well known phenomenon, but drastic changes in the wake structure occur as speeds increase. Somewhere in the transonic regime, the periodic vortex shedding ceases, and at supersonic Mach numbers a steady flow pattern is established. The boundary layer on the body separates to form a free shear layer, the two layers from opposite sides of the body meeting at the so-called neck of the wake. Downstream of this point, the viscous wake persists for many hundreds of body diameters, and the term “far wake” is usually taken to refer to downstream distances greater than about ten body diameters. The neck of a hypersonic wake occurs typically one or two diameters from the base, and marks the downstream end of what is often known as the base flow region; this region includes a low speed, recirculating flow bounded by the shear layers and the base wall. Between the base and somewhere a few body diameters beyond the neck, the flow field is known as the near wake, and forms the subject of this survey.