The Williams F121 (company designation WR36-1[1]) is a small turbofan engine designed for use in the AGM-136 Tacit Rainbow anti-radiation cruise missile.

As a cruise missile engine, it was designed to have a long shelf life (be able to sit around unused for long periods of time) and then operate when needed for several hours.

[2] It was designed to power the AGM-136 Tacit Rainbow, which was to be a stand-off anti radiation missile.

Another unique feature of the engine is that it was started with an explosive cartridge because it couldn't start while still mated to its aircraft.

[2] In the late 2000s, the engine was being used by the Naval Air Warfare Center at Naval Air Weapons Station China Lake to test fuel performance and additives.