Most 1950s fighter design chased speed and altitude at any price. Northrop’s designers went the other way. They wrapped two small General Electric J85 turbojets — originally developed for a decoy drone — in the smallest, lightest airframe that could still go supersonic. The result weighed a fraction of its contemporaries, cost far less to buy and to run, and could operate from short, rough runways with minimal support.

That frugality is exactly why the F-5 succeeded where flashier jets did not. It could be given or sold cheaply to dozens of allied air forces under the Military Assistance Program, kept flying by modest maintenance crews, and later operated in large numbers as a training adversary at a fraction of the cost of a real frontline fighter. The “compromise” fighter became one of the most widely operated jets of the Cold War.

The heart of the F-5 is the General Electric J85, a compact afterburning turbojet that began life as the powerplant for the McDonnell ADM-20 Quail decoy missile. Northrop scaled its fighter around a pair of them. In the F-5E Tiger II the uprated J85-GE-21 produced roughly 22 kN (about 5,000 lbf) of thrust in afterburner each. Two of them in such a light airframe gave the Tiger II a healthy thrust-to-weight ratio and crisp acceleration — and the same engine family went on to power the T-38 Talon and a range of other aircraft.

The original F-5A/B Freedom Fighter was a day fighter with a simple gunsight and little radar. When the USAF ran its International Fighter Aircraft competition in 1970 to find a MiG-21 rival for export, Northrop reworked the design into the F-5E Tiger II. It gained uprated J85-GE-21 engines, a wider fuselage, extended and manoeuvre-flapped wings for better turning, an Emerson radar, and improved avionics. First flown in 1972, the Tiger II became the definitive F-5 and the version most widely used as both a frontline fighter and an aggressor.

Exact dollar figures for the F-5 vary by variant, year and buyer, so any single number is an estimate. What is consistently reported is that the F-5 was cheap to acquire and cheap to fly relative to frontline fighters. That economy is precisely why the US Navy and Air Force adopted it as an aggressor: reporting around the adversary role has cited the F-5 as costing on the order of one-third as much per flight hour to operate as a fast jet like the F/A-18. Whatever the precise figures, low cost is the thread that runs through the entire F-5 story — from Military Assistance Program exports to red-air training today.