The F-35's main engine is the Pratt Whitney F135. The General Electric/Rolls-Royce F136 is being developed as an alternate engine.[51] The F135/F136 engines are not designed to supercruise[52] in the F-35. The STOVL versions of both power plants use the Rolls-Royce LiftSystem, patented by Lockheed Martin and built by Rolls-Royce. This system is more like the Russian Yak-141 and German VJ 101D/E[53] than the preceding generation of STOVL designs, such as the Harrier Jump Jet in which all of the lifting air went through the main fan of the Rolls-Royce Pegasus engine.
The Lift System is composed of a lift fan, drive shaft, two roll posts and a "Three Bearing Swivel Module" (3BSM).[54] The 3BSM is a thrust vectoring nozzle which allows the main engine exhaust to be deflected downward at the tail of the aircraft. The lift fan near the front of the aircraft provides a counter-balancing thrust. Somewhat like a vertically mounted turbofan within the forward fuselage, the lift fan is powered by the engine's low-pressure (LP) turbine via a drive shaft and gearbox. Roll control during slow flight is achieved by diverting engine bypass air through wing mounted thrust nozzles called Roll Posts.[55][56]
The F-35B's lift fan achieves the same 'flow multiplier' effect as the Harrier's huge, but supersonically impractical, main fan. Like lift engines, this added machinery is just dead weight during horizontal flight but provides a net increase in payload capacity during vertical flight. The cool exhaust of the fan also reduces the amount of hot, high-velocity air that is projected downward during vertical take off (which can damage runways and aircraft carrier decks). Though complicated and risky, the lift system has been made to work to the satisfaction of DOD officials.