The recognition of the political importance of energy supply, inefficient energy usage and its adverse ecological impact combined to produce rapidly inflating energy prices which in turn contributed to an industrial slump. With this platform of a decade of uncertainty, predicting future requirements is, to say the least, somewhat problematical. One thing, however, that seems certain is that the future will offer more free time for the population generally which, together with a continuously improving standard of living, will promote cost effective travel as an essential ingredient to satisfying leisure demand. The most cost effective and energy efficient means of travel over significant distances, and certainly over water, is the 0·70/0·85 Mach No subsonic aircraft. We have, therefore, to speculate about the relative operational priorities of the powerplant to be utilised.