The F-35B, a variant of the F-35 Lightning II, is the Short Takeoff and Vertical Landing (STOVL) aircraft. This takeoff and landing operation succeeds through a very innovative technology known as the shaft-driven lift fan propulsion system. Besides the propulsion system, the STOVL variant differs only slightly from the US Air Force variant. The short takeoff/vertical landing (STOVL) F-35B will replace the aging AV-8B Harrier STOVL attack jets (which have also proven increasingly difficult to support) of the US Marine Corps, as well as its F/A-18s. The F-35B will have a stealthy, belly-mounted 25 mm missionized gun pod and a combat radius of more than 450 NM- nearly two times that of legacy STOVL strike fighters. A shaft-driven lift fan, in combination with a vectoring rear exhaust nozzle, gives this fighter the ability to take off in short distances, accelerate to supersonic speeds in level flight, and land vertically. Thanks to the lift-fan system, the F-35Bs total vertical lifting thrust is about 39,700 pounds (the aircraft weighs about 30,000 pounds)- more than 14,000 pounds greater than the engine alone would produce without the lift fan. The F-35B will be the worlds first operational supersonic STOVL aircraft. The F-35B is similar in size to the Air Force F-35A, trading fuel volume for vertical flight systems. Like the AV-8 Harrier II, guns will be carried in a ventral pod. Vertical flight is by far the riskiest, and in the end, a decisive factor in design.