If we look at contemporary space vehicles which start their journeys from the surface of this planet, there is no doubt that the propulsion plant is the dominant feature. Physically it represents a major proportion of the total vehicle weight while the technical effort in attaining the necessary standards of performance and reliability has been of commensurate magnitude. It is clear also that the rocket motor has, at present, an unchallenged position as the chosen form of power unit. The question may well be asked in these circumstances what place there could be for an air-breathing power unit in such vehicles specifically intended to travel for most of their journey through the empty regions of space. Clearly the answer must be that such air-breathing units must function during the boost phase of space missions when there is air for them to breath so that the engines can add their quota of energy (kinetic plus potential) which is to be given to the vehicle in fulfilment of its task.