It should be fairly obvious that, in contemplating the design of a future combat aircraft, it is important at the outset to relate the effort to some forecast definition of what is needed. This may take the form of an official or an unofficial requirement; however, its precise form is immaterial to the central argument. Either way, it is necessary to recognise several limitations to a so-called requirement:

1. i) the chances of it being ‘right’, some 10-20 years before the realisation of its potential wartime application, is fairly remote. This is particularly true with today’s rapid advancement in, for example, weapons and sensor technology. These developments make it doubly difficult to predict the nature and strength of the threat force;

2. ii) the postulated scenarios will change from day to day, and indeed, during the day–according to the course of the main battle; therefore the military commander's requirements of the forces at his disposal can change by the hour;

3. iii) the environment, both militarily and climatically, plays a large part in the definition of a design. The variability of the environment is neglected only with grave risk to the eventual utility of the weapon system.