How come diverterless supersonic inlets haven't been discovered much earlier?
Designing a DSI inlet is not so simple. It required CFD, a field which had existed for a while but was still viewed with some suspicion in the 1990s. CFD is computationally intensive, so the field has benefited from the massive advances in computing power since the invention of the transistor. 


  The DSI traces its roots to work done by Lockheed Martin engineers in the early 1990s as part of an independent research and development project called the Advanced Propulsion Integration project. The concept was developed and refined with Lockheed Martin-proprietary computer modeling tools made possible by advances in Computational Fluid Dynamics, or CFD. CFD is the science of determining a numerical solution to the governing equations of fluid flow and advancing this solution through space or time to describe a complete flow field of interest—in this case, the flow field of a fighter forebody, inlet, and inlet duct.