The F-35 has a fully movable jet exhaust, it can point downward for hover or STOVL operation. (Short Take-Off, Vertical Landing). In hover, doors open on top and bottom and a engine-driven fan provides downward thrust. The fan is dead weight in cruise flight, it's only used in hover and vertical landing. One Lockheed engineer said "The F-35 has three pilots and one observer. In hover, if the pilots determine the human occupant is in danger (if the fan fails) it will eject him automatically, without warning!"