The whole process of designing an aero engine, from the original concept to the production of the first of the type, called a prototype, normally takes about two years.

The prototype design may originate from proposals put forward by the engine maker from specifications for military aircraft engines, issued in the United Kingdom by the Ministry of Supply, or from the Aircraft Industry.

It is not usual for engines to be designed specifically for civil aviation requirements. More often they are adaptations of military engines de-rated to increase their reliability, and to extend the overhaul period. With the growth of civil transport and the development of guided missiles, it is possible that this state of affairs may be reversed and engines for civil aircraft may become more important than those for Service use. At the present time, it is difficult to foresee a potential civil or private aircraft engine market that would justify the cost of designing and developing a small jet engine on that score alone.