The rate of progress in most technical fields, and propulsion is no exception, has increased so rapidly during the past 50 years that it is becoming difficult to predict the trend of development for more than a few years ahead. On the other hand, there are fundamental limits to progress in certain directions and it is within these limits that this paper is written. One must, of course, devote some thought to the validity of the limit. Not so long ago it was thought that the speed of sound would be a limiting speed for aircraft, for instance. One cannot go far wrong with the basic laws of physics and the intention, at least, of this paper is not to transgress such laws.