Parts I and II of this series of papers have dealt with the development of ion thrusters for use in an orbit transfer manoeuvre, and of various types of micro thruster for position and attitude control. The present paper deals with the actual satellite electric propulsion system and configuration needed for expansion manoeuvres. The control problems both during the expansion phase and subsequently in the required orbit are discussed. Short analyses for two specific cases (ELDO launcher and Black Arrow) are presented in which payloads and missions are defined.

Burt has shown that all the orbital elements of a spacecraft can be changed by means of a small continuous thrust. He gave particular attention to the problem of expanding from a low orbit to a synchronous one, and the change of orbit inclination and orbit eccentricity was also considered. Such a type of manoeuvre is eminently suitable to electric propulsion because the high exhaust velocities possible lead to a small propellant mass; this mass saving is achieved at the expense of the time taken for the manoeuvre.