Its true that computer simulation cannot entirely replace physical nuke tests ... how ever the parameters recorded in a series of tests may form the basis to have entirely comp simulated design improvements and to calculate the destructive power. The P5 have stopped testing since the last decade ( france and China were the last ones before pokhran 2) and are entirely relying on computer simulations. As per indian nuke scientists they managed to gather enough inputs for computer based simulations from the pokhran 2 .
Developing a 4th generation fighter is a different ball game all together where the major aspects of avionics , armaments etc depends on numerous sub technologies and their inter compatibility.
