"A common intermediate solution is to develop a
“derivative” engine. A derivative development starts with an existing
engine and changes components and controls to “derive” an engine
that meets the new requirements. In some cases, derivative engines
are simply “growth” versions of their predecessors that are intended
for use in the same aircraft to accommodate increased mission requirements
or to compensate for increasing aircraft weight. In other
cases, a derivative engine may be so different from the original engine
that its commonality with the original is outwardly indiscernible.
Derivative engines are built around a previously designed engine’s
core (sometimes these cores are also enhanced over their original"