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.

Although an interest in electric propulsion has existed in the UK for a considerable time, it is only in the past few years that it has been considered worthwhile to undertake the development of hardware and produce detailed systems studies. The practical developments can be divided into two categories: —

1. 1. The development of comparatively large thrusters (in the region of ½ kW power, giving 10-20 milli Newtons of thrust), for producing major movements in a satellite's position.

2. 2. The development of micro-thruster systems for the maintenance of a satellite's attitude and position against the small disturbing forces and torques it normally experiences in orbit.