In 1983, Northrop unveiled a Mach 2.1 fighter that cost $9 million — one quarter the price of an F-14 Tomcat — built on private money and powered by General Electric’s new F404 engine. The F-20 Tigershark could hit 1,611 miles per hour, fire AIM-9 Sidewinders from its wingtips, and operate from austere airfields with the maintenance footprint of a Cold War light fighter. Forty years later, not a single foreign country ever bought one, and the Un…