Originally Posted by Rainboe
It needs a major design change switching engines on an aeroplane. For example, the RR engine with 3 spools is heavier than the GE and P&W equivalents. The mountings are different, the thrustlines are different, the whole interaction of the engines on the wing change, the ducted airflow supplies, hydraulics and electrics are different, and then you have to carry out all the development flying and manual and performance manual testing! Big Job. I don't think there was enough of a delay by RR to make swapping engines worthwhile.
But why was RR trusted with developing engines for Tristar to begin with - why not some other engine easier to develop and from a more reliable manufacturer?