 The F-104 was designed to use the General Electric J79 turbojet engine, fed by side-mounted intakes with fixed inlet cones optimized for performance at Mach 1.7 (increased to Mach 2 for later F-104s equipped with more powerful J79-GE-19 engines).Unlike some supersonic aircraft, the F-104 did not have variable-geometry inlets; instead at high Mach numbers excess air was bypassed around the engine.This bypass air also helped cool the engine.