The direct lift system of the Harrier is simple but not robust. Turbines hate ingesting hot gas, and a Harrier in a descending hover is descending straight into

The direct lift system of the Harrier is simple but not robust. Turbines hate ingesting hot gas, and a Harrier in a descending hover is descending straight into its own hot exhaust. This can and did cause "pop stalls" which led to an abrupt loss of thrust just when you want consistent thrust, near the ground in a hover. The LiftFan of the F-35B serves to shield the front intakes of the engine from ingesting hot gas, plus there is an auxiliary air inlet on the top of the plane that opens up and allows cold air from above to get sucked straight into the front face of the engine. The 3 Bearing Swivel Nozzle (main engine nozzle pivoting down and somewhat steerable) combined with roll posts in the wings that can give translational thrust/roll control, and a Variable Aperture Vane Box Nozzle (VAVBN) under the lift fan all combine to give high control authority in the hover, and the flight control system has special modes to make controlling the F-35B simpler. Harriers required constant adjustment in the hover to keep from losing control. In the F-35B, once the altitude and attitude in the hover are set, the jet will maintain that as long as there's fuel remaining, without corrective pilot inputs required.